**Aromatic-naphthene hydrocarbons**

Fig. 1. Step I of oxidation of hydrocarbons according to the peroxide theory [2].

**Figure 1.** Step I of oxidation of hydrocarbons according to the peroxide theory [2].

The theory was based on a chain-like pathway of the oxidation process. In addition to final products, the reactions produce active or unstable bonds which initiate further changes; as a result, the reaction is repeated without participation of external factors, as shown below:

**a.** Backbone chain

**Paraffin hydrocarbons** 

106 Storage Stability of Fuels

**Isoparaffin hydrocarbons**

**Naphthene hydrocarbons** 

**Aromatic hydrocarbons** 

**Aromatic-naphthene hydrocarbons** 

Fig. 1. Step I of oxidation of hydrocarbons according to the peroxide theory [2].

**Figure 1.** Step I of oxidation of hydrocarbons according to the peroxide theory [2].

$$\rm CH\_4O + O\_2 \rightarrow CH\_4O\_2 + O\tag{14}$$

$$\rm{CH}\_4\rm{O}\_2 \rightarrow \rm{CH}\_2\rm{O} + \rm{H}\_2\rm{O} \tag{15}$$

$$\text{CH}\_4 + \text{O} \rightarrow \text{CH}\_4\text{O etc.} \tag{16}$$

#### **b.** Chain scission

$$\rm CH\_2O \rightleftharpoons \rm O\_2 \rightarrow \rm CH\_2O\_2 + O \tag{17}$$

$$\rm CH\_2O\_2 \rightarrow H\_2O + CO \tag{18}$$

#### **c.** Chain breaking

$$\rm CH\_2O + O\_2 \rightarrow H\_2O + CO\_2 \tag{19}$$

$$2\text{CH}\_2\text{O} + \text{O}\_2 \rightarrow \text{H}\_2\text{O} + 2\text{CO} \tag{20}$$

$$\text{O} + \text{wall} \rightarrow \text{1/2O}\_2 \tag{21}$$

Other authors maintain that free radicals (R\*) are formed in the first step of oxidation of hydrocarbons according to the following reaction (22):

$$\text{R-H} \xrightarrow{\text{energy}} \text{R\*} \tag{22}$$

The resulting free alkyl radical reacts with atmospheric oxygen, forming a peroxide radical (ROO\*), as in the previous theories. The peroxide radical reacts with another hydrocarbon molecule. The reaction products at this step are acid and one more alkyl radical, as shown in the reaction below (23, 24). These are chain-like reactions.

$$\rm R^\* + O2 \rightarrow ROO^\* \tag{23}$$

$$\rm ROO^\* + \rm R-H \rightarrow \rm ROOH + \rm R^\* \tag{24}$$

In addition to reactions which accelerate the oxidation of hydrocarbons, there occur also ones that slow down the process. Alkyl radicals are able to combine, forming hydrocarbon mole‐ cules with different properties, compared with hydrocarbons in a fresh fuel.

$$\text{R1}^\* + \text{R2}^\* \to \text{R1} - \text{R2} \tag{25}$$

Some of the products of oxidation have acidic properties or readily become acidic compounds (the fuel's acid number is higher). Moreover, hydrolysis of esters (the biocomponent of the fuel) may occur, leading to higher acid content, as may be indicated by the ester value.

In addition to the peroxide theories, also proposed in the available literature is Bone's [7] hydroxylation theory. It says that alcohols are formed in the first step of oxidation, the process runs via a number of hydroxylation steps and peroxides are products of secondary reactions of aldehydes. However, the theory was criticized by others, who propose that the reaction starting from alcohols is not much probable because aldehydes are formed by dehydrogena‐ tion more readily than by hydroxylation. In addition, if alcohols actually were the first products of oxidation, then oxygen would dissociate into atoms. Objectors also maintained that alcohols may be formed by decomposition of peroxides. Speaking of energy, it is more probable that breaking the C-C and C-H bonds leads to peroxides rather than alcohols, since the latter require dissociation of oxygen.

Wiegand proposed a different theory, saying that the process of oxidation is activated by molecular hydrogen rather than by oxygen and that the process is reverse to hydrogenation [2].

Generally, the mechanisms governing fuel degradation and deposit formation include the following [8]:


The following are typical forms of degradation for different fuel types [8]:

	- **•** self-oxidation during storage,
	- **•** rapid self-oxidation and thermal oxidation in the vehicle's fuel system.

R\* + O2 ROO\* ® (23)

ROO\* + R–H ROOH + R\* ® (24)

R1\* + R2\* R1–R2 ® (25)

In addition to reactions which accelerate the oxidation of hydrocarbons, there occur also ones that slow down the process. Alkyl radicals are able to combine, forming hydrocarbon mole‐

Some of the products of oxidation have acidic properties or readily become acidic compounds (the fuel's acid number is higher). Moreover, hydrolysis of esters (the biocomponent of the fuel) may occur, leading to higher acid content, as may be indicated by the ester value.

In addition to the peroxide theories, also proposed in the available literature is Bone's [7] hydroxylation theory. It says that alcohols are formed in the first step of oxidation, the process runs via a number of hydroxylation steps and peroxides are products of secondary reactions of aldehydes. However, the theory was criticized by others, who propose that the reaction starting from alcohols is not much probable because aldehydes are formed by dehydrogena‐ tion more readily than by hydroxylation. In addition, if alcohols actually were the first products of oxidation, then oxygen would dissociate into atoms. Objectors also maintained that alcohols may be formed by decomposition of peroxides. Speaking of energy, it is more probable that breaking the C-C and C-H bonds leads to peroxides rather than alcohols, since the latter require

Wiegand proposed a different theory, saying that the process of oxidation is activated by molecular hydrogen rather than by oxygen and that the process is reverse to hydrogenation [2]. Generally, the mechanisms governing fuel degradation and deposit formation include the

**•** self-oxidation: a spontaneously catalyzed reaction of oxidation (typical process accompa‐

**•** thermal oxidation (at temperatures higher than 200°C): a typical process for fuels which are

**•** pyrolysis: decomposition of the structure of fuels and the formation of deposits due to thermal oxidation on very hot surfaces (deposits are formed in nozzles and sprayers).

The following are typical forms of degradation for different fuel types [8]:

**•** rapid self-oxidation and thermal oxidation in the vehicle's fuel system.

nying long-term storage of fuels in storage tanks),

in contact with hot surfaces in aircraft fuel systems,

**•** self-oxidation during storage,

cules with different properties, compared with hydrocarbons in a fresh fuel.

dissociation of oxygen.

following [8]:

108 Storage Stability of Fuels

**a.** for gasolines:

**b.** for diesel fuel:


Shawn P., Heneghan and Long P. Chin proposed in [9] the radical-based mechanism of oxidation of hydrocarbons present in aircraft fuel:

**a.** initiation:

$$R-R \xrightarrow{k\_{\perp}} R\cdot + R\cdot \tag{26}$$

**b.** propagation:

$$R \cdot + O\_2 \xrightarrow{k\_2} RO\_2 \cdot \tag{27}$$

$$RO\_2 \cdot + RH \xrightarrow{k\_1} RO\_2H + R \tag{28}$$

**c.** termination:

$$RO\_2 \cdot + RO\_2 \cdot \xrightarrow{k\_4} productivity \tag{29}$$

#### **d.** chain transfer

$$RO\_2 \cdot + AH \xrightarrow{k\_i} RO\_2H + A \cdot \tag{30}$$

**e.** autocatalysis

$$RO\_2H + RO\_2H \xrightarrow{k\_s} ROH + RO\_2\cdot + H\_2O \tag{31}$$

$$RO\_2H \xrightarrow{k\_2} RO\cdot + OH\cdot \tag{32}$$

where:

RH – hydrocarbons contained in the fuel,

R , RO2 – radicals,

AH – compounds with antioxidant properties, present in the fuel,

A – radical with low reactivity.

Considering the mechanism, Jones's research team proposed two possible pathways of the autocatalysis step, presented in reactions (31, 32). In the first case, the general reaction rate for oxidation of organic compounds which are present in fuels is expressed in Equation (33) [10]:

$$\frac{-d\left[O\_2\right]}{dt} = k\_3 \left(\frac{R\_i}{2k\_4}\right)^{0.5} \cdot \left[RH\right] \tag{33}$$

where:

*Ri* – reaction rate of initiation step;

*k3*, *k4* – reaction rate constants for (3) and (4);

[RH]-concentration of hydrocarbons.

Jones's research team demonstrated that, in the case of fuels, the rate of the initiation process (*Ri* ) increases with time and is proportional to the squared, decreasing concentration of oxygen, as shown in Equation (34):

$$R\_i = k\_6 \left( [O\_2]\_0 - [O\_2]\_t \right)^2 + k\_1 \tag{34}$$

where:

*k1*-reaction rate constant for formation of radicals in the initiation step,

*k6*-reaction rate constant for formation of radicals *RO2* from oxidation products.

According to Jones, radicals *RO2*, which are responsible for catalyzing the oxidation process, are formed in a reaction which involves two hydroperoxide molecules – reaction (31), as suggested earlier by Halling [11].

Heneghan and Zabarnick [12] demonstrated that, where the course of reaction (4) is limited by the presence of antioxidants, the oxidation reaction rate may be described by Equation (35): Liquid Fuel Ageing Processes in Long-term Storage Conditions http://dx.doi.org/10.5772/59799 111

$$\frac{-d[O\_2]}{dt} = \frac{k\_3[RH]R\_i}{k\_5[AH]}\tag{35}$$

In this case, the reaction rate of initiation is described by Equation (36):

$$R\_i = k\_\gamma \left( [O\_2]\_0 - [O\_2]\_t \right) + k\_1,\tag{36}$$

where:

(33)

7

Considering the mechanism, Jones's research team proposed two possible pathways of the autocatalysis step, presented in reactions (31, 32). In the first case, the general reaction rate for oxidation of organic compounds which are present in fuels is expressed in Equation (33) [10]:

> 4 [ ] [ ] <sup>2</sup> *Ri d O k RH*

Jones's research team demonstrated that, in the case of fuels, the rate of the initiation process

( )

According to Jones, radicals *RO2*, which are responsible for catalyzing the oxidation process, are formed in a reaction which involves two hydroperoxide molecules – reaction (31), as

Heneghan and Zabarnick [12] demonstrated that, where the course of reaction (4) is limited by the presence of antioxidants, the oxidation reaction rate may be described by Equation (35):

*k1*-reaction rate constant for formation of radicals in the initiation step,

*k6*-reaction rate constant for formation of radicals *RO2* from oxidation products.

) increases with time and is proportional to the squared, decreasing concentration of oxygen,

2

6 20 2 1 [][] *RkO O k i t* = -+ (34)

è ø

0,5

*<sup>k</sup> RO H RO OH* ¾¾® × + × (32)

2

AH – compounds with antioxidant properties, present in the fuel,

2

3

*dt k* - æ ö = × ç ÷

RH – hydrocarbons contained in the fuel,

A – radical with low reactivity.

– reaction rate of initiation step;

[RH]-concentration of hydrocarbons.

as shown in Equation (34):

suggested earlier by Halling [11].

*k3*, *k4* – reaction rate constants for (3) and (4);

where:

110 Storage Stability of Fuels

where:

*Ri*

(*Ri*

where:

R , RO2 – radicals,

*k7-*reaction rate constant.

Jones presumes that, owing to the limited availability of oxygen in the storage tanks, the concentration of the peroxides being formed in the fuel is not high, which strongly favors the occurrence of single-molecule decomposition, as in Equation (32). The reaction produces highly reactive radicals *RO* and *OH* from products of oxidation.

In addition, as found by other researchers, the rate of fuel oxidation (especially at high temperatures) is affected by the presence of metal ions [13]. According to Clark, Cu2+ cations catalyze the self-oxidation process, thus accelerating the initiation step, that is, the formation of free radicals [13, 14]:

$$RH(fuel) + O\_2 \xrightarrow{Catalyst(Cu}^{2+}) \longrightarrow RO\_2 \cdot R \cdot \text{ etc.} \tag{37}$$

According to Walling [11], copper cations dissolved in the fuel tend to react directly with hydroperoxides which have resulted from oxidation of the fuel components and which form highly reactive radicals *ROO , RO,* responsible for the fuel's further degradation:

$$\text{Cu}^{2+} + \text{ROOH} \rightarrow \text{Cu}^{+} + \text{ROO} \cdot + \text{H}^{+} \tag{38}$$

$$\text{Cu}^+ + \text{ROOH} \rightarrow \text{Cu}^{2+} + \text{RO} \cdot + \text{OH} \cdot \tag{39}$$

On the other hand, one cannot say without hesitation that the amount of deposit depends solely on the concentration of hydroperoxides [13]. Harde's research team has studied [15] fuels which, in spite of a high content of *ROOH*, demonstrated higher thermal stabilities in comparison with the fuels in which a lower amount of hydroperoxides was formed. The different reactivity of hydroperoxides depends probably on the fuel's composition. Nonethe‐ less, high-temperature deposits in the fuel are formed as the result of the reaction between ageing precursors, which are generated as the result of self-oxidation of the fuel, and polar compounds of sulfur and nitrogen which are present in the fuel [3]:

$$RH + O\_2 \rightarrow \text{reactive ROOH} \rightarrow \text{preursors} \xrightarrow{\text{polar compounds}} \text{thermal deposits} \tag{40}$$

Another, less popular potential mechanism of oxidation of fuel components is the Electron Transfer Initiated Oxygenation (ETIO) [3] where, in the step of initiation, electrons are transferred from electron-rich molecules of the fuel to oxygen molecules. ETIO is to be considered as a whole group of oxidation mechanisms which have the common feature of the rate-determining step of electron transfer. A majority of antioxidants, presently used in petroleum products, are designed to control oxidation of fuels, which follow according to the radical mechanism discussed earlier. The ETIO mechanism is an alternative model of oxidation and may be a rational explanation of oxidative degradation of fuels which takes place in spite of the presence of antioxidants. radical mechanism discussed earlier. The ETIO mechanism is an alternative model of

The simplified model shown below illustrates the principle of ETIO. The oxidation mechanism is explained using dibenzopyrrole (THC), Fig. 2: oxidation and may be a rational explanation of oxidative degradation of fuels which takes place in spite of the presence of antioxidants. The simplified model shown below illustrates the principle of ETIO. The oxidation

mechanism is explained using dibenzopyrrole (THC), Fig. 2:

In that mechanism, reaction a) (electron transfer mechanism) is the slowest reaction, thus **Figure 2.** ETIO – simplified mechanism [3]

limiting the rate of the entire process. The total rate of the process may be expressed by means of rate equation for a second-order reaction (41) [3]: [ ][ ] *THC O*<sup>2</sup> *k dt d[THC]* (41) In that mechanism, reaction a) (electron transfer mechanism) is the slowest reaction, thus limiting the rate of the entire process. The total rate of the process may be expressed by means of rate equation for a second-order reaction (41) [3]:

> *[O2]* – concentration of oxygen, *k* – reaction rate constant.

which are in contact with hot surfaces in aircraft fuel systems,

rapid self-oxidation and thermal oxidation in the vehicle's fuel system.

acids and bases, to form insoluble microparticles and deposits in the fuel, thermal oxidation, during which deposits are formed in fuel injectors,

The following are typical forms of degradation for different fuel types [8]:

surfaces and on moist surfaces of the combustion chamber.

$$\frac{-d\langle \text{THC} \rangle}{dt} = k[\text{THC}][O\_2] \tag{41}$$

 pyrolysis: decomposition of the structure of fuels and the formation of deposits due to thermal oxidation on very hot surfaces (deposits are formed in nozzles and sprayers).

self-oxidation during storage, condensation, esterification, and reactions involving

pyrolysis, during which deposits are formed and accumulated over the injector

**Fig. 2. ETIO – simplified mechanism [3]** 

where:

Generally, the mechanisms governing fuel degradation include the following [8]: [THC] – concentration of dibenzopyrrole,

d) for gasolines:

e) for diesel fuel:

self-oxidation during storage,

 self-oxidation: a spontaneously catalyzed reaction of oxidation (typical process accompanying long-term storage of fuels in storage tanks), [O2] – concentration of oxygen,

where:

 thermal oxidation (at temperatures higher than 200°C): a typical process for fuels *k* – reaction rate constant.

Generally, the mechanisms governing fuel degradation include the following [8]:


The following are typical forms of degradation for different fuel types [8]:

**a.** for gasolines:

2

112 Storage Stability of Fuels

of the presence of antioxidants.

a)

b)

c)

where:

[O2] – concentration of oxygen,

*k* – reaction rate constant.

where:

of rate equation for a second-order reaction (41) [3]:

**Figure 2.** ETIO – simplified mechanism [3]

d) for gasolines:

[THC] – concentration of dibenzopyrrole,

e) for diesel fuel:

self-oxidation during storage,

is explained using dibenzopyrrole (THC), Fig. 2:

place in spite of the presence of antioxidants.

polar compounds *RH O*+ ® reactive precursors *ROOH* ® ¾¾¾¾¾¾®thermal deposits (40)

Another, less popular potential mechanism of oxidation of fuel components is the Electron Transfer Initiated Oxygenation (ETIO) [3] where, in the step of initiation, electrons are transferred from electron-rich molecules of the fuel to oxygen molecules. ETIO is to be considered as a whole group of oxidation mechanisms which have the common feature of the rate-determining step of electron transfer. A majority of antioxidants, presently used in petroleum products, are designed to control oxidation of fuels, which follow according to the radical mechanism discussed earlier. The ETIO mechanism is an alternative model of oxidation and may be a rational explanation of oxidative degradation of fuels which takes place in spite

The simplified model shown below illustrates the principle of ETIO. The oxidation mechanism

mechanism is explained using dibenzopyrrole (THC), Fig. 2:

means of rate equation for a second-order reaction (41) [3]:

[ ][ ] *THC O*<sup>2</sup> *k dt*

*[THC]* – concentration of dibenzopyrrole,

Generally, the mechanisms governing fuel degradation include the following [8]:

accompanying long-term storage of fuels in storage tanks),

The following are typical forms of degradation for different fuel types [8]:

surfaces and on moist surfaces of the combustion chamber.

which are in contact with hot surfaces in aircraft fuel systems,

rapid self-oxidation and thermal oxidation in the vehicle's fuel system.

acids and bases, to form insoluble microparticles and deposits in the fuel, thermal oxidation, during which deposits are formed in fuel injectors,

*[O2]* – concentration of oxygen, *k* – reaction rate constant.

<sup>2</sup> [ ][ ] *d[THC]*

In that mechanism, reaction a) (electron transfer mechanism) is the slowest reaction, thus limiting the rate of the entire process. The total rate of the process may be expressed by means

radical mechanism discussed earlier. The ETIO mechanism is an alternative model of oxidation and may be a rational explanation of oxidative degradation of fuels which takes

The simplified model shown below illustrates the principle of ETIO. The oxidation

**Fig. 2. ETIO – simplified mechanism [3]**  In that mechanism, reaction a) (electron transfer mechanism) is the slowest reaction, thus limiting the rate of the entire process. The total rate of the process may be expressed by

*d[THC]* (41)

*dt k THC O* - = (41)

self-oxidation: a spontaneously catalyzed reaction of oxidation (typical process

thermal oxidation (at temperatures higher than 200°C): a typical process for fuels

 pyrolysis: decomposition of the structure of fuels and the formation of deposits due to thermal oxidation on very hot surfaces (deposits are formed in nozzles and sprayers).

self-oxidation during storage, condensation, esterification, and reactions involving

pyrolysis, during which deposits are formed and accumulated over the injector

	- **•** self-oxidation during storage, condensation, esterification, and reactions involving acids and bases, to form insoluble microparticles and deposits in the fuel,
	- **•** thermal oxidation, during which deposits are formed in fuel injectors,
	- **•** pyrolysis, during which deposits are formed and accumulated over the injector surfaces and on moist surfaces of the combustion chamber.
	- **•** self-oxidation, during which resins and peroxides are formed,
	- **•** thermal oxidation, which leads to the formation of deposits over fuel-wetted, hot surfaces of heat exchangers, in valves and nozzles.

It is extremely difficult to establish a single, universal model of oxidation of fuels based on available literature data. This is caused by the chemical structure of the fuel, which is a mixture of many compounds, their different susceptibilities to degradation processes, and their different rates of oxidation. The formation of various deposits and resins in the fuel may follow a variety of mechanisms and take place at different times – during storage and use.

Numerous reports, concerning identification of the mechanisms of degradation of petroleum products indicate that the rate of oxidation depends, first of all, on the following factors:


The effect of such factors on the quality of the fuel depends on its storage conditions. To minimize their potential adverse influence, efforts are made to reduce the impact of external conditions as much as possible, for instance, by using hermetic tanks, addition of antioxidants and corrosion inhibitors.

#### *3.1.1. Composition of fuel*

Every group of hydrocarbons which are the components of a fuel have a different susceptibility to oxidation due to their different structure. Based on the works of N.I. Czernożukow and S.E. Krejn [4, 16] resistance of hydrocarbons to self-oxidation was arranged in the following order, starting from those least susceptible to oxidation:

#### aromatic hydrocarbons > naphthenes > paraffins

Aromatic hydrocarbons are characterized by the highest oxidation stability due to the absence or small number of side chains. During the oxidation of aromatic hydrocarbons a molecule of atmospheric oxygen locates itself between the carbon and hydrogen atoms, which is accompanied by the formation of low-molecular volatile acids, phenols, qui‐ nones and high-molecular products of polymerization, tars, and asphaltenes. C-H bonds in the side chains are attacked first, therefore, susceptibility to oxidation increases in propor‐ tion to the number of side chains and to the number of carbon atoms per molecule. Moreover, the larger the number and the length of the side chains, the more acids are formed by decomposition, while the products of condensation are formed in a lesser amount. After decomposition of the side chains into peroxides, aldehydes and acids, oxygen attacks the ring the aromatic hydrocarbon is made of.

Oxidation of naphthene hydrocarbons is much faster and simpler. As in the case of aromatic hydrocarbons, susceptibility to self-oxidation is higher for larger molecules, higher number of side chains, and more complex structures as well as higher temperature. The oxygen molecule will attack, first of all, tertiary carbon atoms leading to the breaking of the hydrocarbon chain with generation of carboxylic acids, hydroxy acids which precipitate in the form deposits, esters, tar and asphaltenes.

Paraffin hydrocarbons were found to be quite resistant at moderate temperatures, although their susceptibility to oxidation is higher at higher temperatures. The process produces carboxylic acids, hydroxy acids and asphaltene-type of products.

Fuels are blends, made of hydrocarbons. Therefore, the compounds interact during the process, thus affecting susceptibility of the fuel to oxidation, providing final products which are a combination of those described above. A mixture of aromatic hydrocarbons without any side chains and naphthene hydrocarbons has a higher oxidation stability, compared with naphthenes alone. It was found that the concentration of aromatic hydrocarbons of around (20...30)% has an anti-oxidative effect when mixed with paraffins and naphthenes, in which case less tars and less deposits are formed as products of oxidation. Concentrations of aromatic hydrocarbons below and above that range tend to inhibit oxidation only to a certain extent, producing more asphaltenes and more high-molecular compounds, which do not dissolve in the fuel. The optimum effect is observed for polycyclic aromatic hydrocarbons with short side chains.

A slightly different mechanism of oxidation was found for biocomponents added to fuels (FAME is added to diesel fuel and bioethanol is added to gasoline). FAME oxidation mecha‐ nism is a sequence of reactions involving free alkyls. In the first phase of oxidation, oxygen breaks double bonds of polyunsaturated fatty acids which have low stability, leading to peroxides. In the next step of the process, the peroxides undergo scission (into alcohols, aldehydes and free acids), dehydration (into ketones) and the formation of free radicals which initiate further reactions forming oxidized monomers, dimers and polymers. Oxidation of FAME is a self-oxidative reaction of which the rate is higher as the reaction continues (the products of oxidation tend to catalyze the processes that follow). By oxidation of FAME, acids with strong anti-corrosive properties and insoluble high-molecular compounds (polymers, resins) are formed, leading to deposits on components of the injection system and engine combustion chambers. FAME ageing processes run much faster, compared with changes in diesel fuels [17]. They do not only depend on the quality of the raw material but also on the choice of production technology and method of purification of vegetable oil [18]. The chemical stability of FAME during storage is also affected by temperature and the construction material for the storage tank. Addition of FAME as a component to a fuel results in its lower stability, which may drop dramatically below standard requirements just within a month, affecting other parameters as well. The parameters are not exactly established. Different criteria apply in different countries, resulting in a lack of universal methods for evaluation of this type of fuel. As a general rule, it is possible to extend the safe storage time of FAME by using improv‐ ers. Moreover, it is not recommended to store them for more than 12 months in typical fuel storage conditions (closed tanks, absence of oxygen, dark). Stability of FAME is indicated by such parameters as: acid value (represents the amount of free fatty acids), iodine number (measure of the amount of unsaturated compounds C=C), and oxidation stability at 110°C.

In gasolines with a content of bioethanol, fuel ageing processes may run faster as well. Ethanol accelerates the formation of reactive compounds such as free peroxides, which leads to polymerization. In effect, the fuel will change color and resinous compounds will be formed. After addition of the biocomponent to the fuel, its safe storage time is shorter while quality is unaffected. In the case of ethyl alcohol, this results from its susceptibility to form mixtures with water at any chosen ratio, including azeotropic mixtures, as well as susceptibility to oxidation and corrosive effect, while for FAME it is caused by its low chemical stability and susceptibility to oxidation.

#### *3.1.2. Atmospheric conditions*

conditions as much as possible, for instance, by using hermetic tanks, addition of antioxidants

Every group of hydrocarbons which are the components of a fuel have a different susceptibility to oxidation due to their different structure. Based on the works of N.I. Czernożukow and S.E. Krejn [4, 16] resistance of hydrocarbons to self-oxidation was arranged in the following order,

aromatic hydrocarbons > naphthenes > paraffins

Aromatic hydrocarbons are characterized by the highest oxidation stability due to the absence or small number of side chains. During the oxidation of aromatic hydrocarbons a molecule of atmospheric oxygen locates itself between the carbon and hydrogen atoms, which is accompanied by the formation of low-molecular volatile acids, phenols, qui‐ nones and high-molecular products of polymerization, tars, and asphaltenes. C-H bonds in the side chains are attacked first, therefore, susceptibility to oxidation increases in propor‐ tion to the number of side chains and to the number of carbon atoms per molecule. Moreover, the larger the number and the length of the side chains, the more acids are formed by decomposition, while the products of condensation are formed in a lesser amount. After decomposition of the side chains into peroxides, aldehydes and acids, oxygen

Oxidation of naphthene hydrocarbons is much faster and simpler. As in the case of aromatic hydrocarbons, susceptibility to self-oxidation is higher for larger molecules, higher number of side chains, and more complex structures as well as higher temperature. The oxygen molecule will attack, first of all, tertiary carbon atoms leading to the breaking of the hydrocarbon chain with generation of carboxylic acids, hydroxy acids which precipitate in the form deposits,

Paraffin hydrocarbons were found to be quite resistant at moderate temperatures, although their susceptibility to oxidation is higher at higher temperatures. The process produces

Fuels are blends, made of hydrocarbons. Therefore, the compounds interact during the process, thus affecting susceptibility of the fuel to oxidation, providing final products which are a combination of those described above. A mixture of aromatic hydrocarbons without any side chains and naphthene hydrocarbons has a higher oxidation stability, compared with naphthenes alone. It was found that the concentration of aromatic hydrocarbons of around (20...30)% has an anti-oxidative effect when mixed with paraffins and naphthenes, in which case less tars and less deposits are formed as products of oxidation. Concentrations of aromatic hydrocarbons below and above that range tend to inhibit oxidation only to a certain extent, producing more asphaltenes and more high-molecular compounds, which do not dissolve in the fuel. The optimum effect is observed for polycyclic aromatic hydrocarbons with short side

and corrosion inhibitors.

114 Storage Stability of Fuels

*3.1.1. Composition of fuel*

esters, tar and asphaltenes.

chains.

starting from those least susceptible to oxidation:

attacks the ring the aromatic hydrocarbon is made of.

carboxylic acids, hydroxy acids and asphaltene-type of products.

The rate and course of oxidation of petroleum products depend on a number of factors which have nothing in common with the chemical structure of fuels. The essential external factors, in addition to the presence of atmospheric oxygen, include temperature, pressure, and humidity.

The susceptibility of fuels to oxidation is higher for higher temperatures, since more products of oxidation are formed, with different conversions.

A change in the fuel storage temperature from –30 to 50°C leads to the following:


The rate and frequency of temperature fluctuations favors accumulation of higher amounts of oxygen and water in storage tanks. A significant drop or temporary fluctuations in the fuel's temperature cause changes which are not always reversible (any generated deposits or highly dispersed phases do not always disappear after the temperature has risen).

The relationship between changes in the rate of chemical and biochemical reactions and temperature is exponential, therefore, the most active components of fuels, contained in refinery products may react at a temperature of 40°C even several times as fast, compared with their reaction rates at –30°C. For that reason, the surface of fuel storage tanks must be protected from the direct impact of thermal and solar radiation.

Łosikow and Łukaszewicz reported [6] that the optimum storage temperature for petroleum products is in the range (20...30)°C, in which the rate of self-oxidation of hydrocarbons is rather low. Thermal decomposition of hydrocarbons may take place as parallel processes at very high temperatures, producing, *inter alia,* carbon dioxide and water.

A similar relationship which accelerates oxidation was observed for increased pressures. Pressure buildup is accompanied by an increase in the rate of oxidation reactions. The external factors discussed in herein lead to breaking the unsaturated bonds, as shown in (42), which generates long-chain compounds, leading to the formation of carbon deposits and settling of resinous compounds.

$$\text{nCH}\_{2}\text{=CH}\_{2} \xrightarrow{\text{p,T}} \text{[-CH}\_{2}\text{-CH}\_{2}\text{-]}\_{\text{n}}\tag{42}$$

Moisture is another weather condition which affects oxidation of fuels. Water takes part in certain reactions which involve radicals. The following forms of co-existence of fuel and water are of the highest importance both in the fuel storage and distribution:


These forms may convert between one another depending on ambient temperature and pressure conditions.

Solubility of water in the fuel is higher for higher temperatures and lower for higher molecular weights of hydrocarbons. When fuels are stored at low temperatures, their water content will crystallize forming ice crystals which may plug filters or tubing in the fuel system.

Information describing the mechanism of interaction between fuels and water is scarce in the available literature. There is a theory which says that the respective components of the fuel on storage undergo hydrolysis (this includes amines, polyesters, organometallic compounds, salts, esters of phosphoric acid, and sulfonic acids).

The presence of water in the fuel storage tanks also contributes to the growth of microorgan‐ isms, which are part of the cause of its oxidation. Along with the increasing count of micro‐ organisms, the concentration of surface active agents also increases. The water phase, both dispersed (emulsion) and its separated layer at the bottom of the storage tank, has its pH reduced by the activity of microorganisms, which is a potential cause of corrosion.

The effect of weather conditions may be moderated by using hermetic designs of storage tanks and by the use of optimum technological solutions (for instance, by the use of improvers). In addition, a fuel is expected to satisfy the requirements of quality as set in applicable standards and its long-term storage time should not go beyond safe limits. The effect of weather conditions may be moderated by using hermetic designs of storage tanks and by the use of optimum technological solutions (for instance, by the use of improvers). In addition, a fuel is expected to satisfy the requirements of quality as set in applicable standards and its long-term storage time should not go beyond safe limits.

#### *3.1.3. Oxidation catalysts*

following reactions (43…46):

**•** physical effects: crystallization of certain components of fuels, formation of ice crystals, change in solubility of oxygen and water, leading to the formation of new phases,

**•** chemical effects: change in the rate of oxidation, polymerization or corrosion, formation of

The rate and frequency of temperature fluctuations favors accumulation of higher amounts of oxygen and water in storage tanks. A significant drop or temporary fluctuations in the fuel's temperature cause changes which are not always reversible (any generated deposits or highly

The relationship between changes in the rate of chemical and biochemical reactions and temperature is exponential, therefore, the most active components of fuels, contained in refinery products may react at a temperature of 40°C even several times as fast, compared with their reaction rates at –30°C. For that reason, the surface of fuel storage tanks must be protected

Łosikow and Łukaszewicz reported [6] that the optimum storage temperature for petroleum products is in the range (20...30)°C, in which the rate of self-oxidation of hydrocarbons is rather low. Thermal decomposition of hydrocarbons may take place as parallel processes at very high

A similar relationship which accelerates oxidation was observed for increased pressures. Pressure buildup is accompanied by an increase in the rate of oxidation reactions. The external factors discussed in herein lead to breaking the unsaturated bonds, as shown in (42), which generates long-chain compounds, leading to the formation of carbon deposits and settling of

Moisture is another weather condition which affects oxidation of fuels. Water takes part in certain reactions which involve radicals. The following forms of co-existence of fuel and water

These forms may convert between one another depending on ambient temperature and

me 2 r 2 n nCH =CH -CH -CH - ¾¾®[ ] (42)

p, T monomer poly

**•** water as a separate phase at the bottom or on the walls of the tank or pipeline,

**•** ice as a separate phase at the bottom or on the walls of the tank or pipeline.

dispersed phases (such as emulsions, micelles), solids, deposits, etc.

dispersed phases do not always disappear after the temperature has risen).

from the direct impact of thermal and solar radiation.

resinous compounds.

116 Storage Stability of Fuels

**•** water dissolved in fuel,

**•** water emulsified in fuel,

pressure conditions.

**•** ice crystals dispersed in fuel,

temperatures, producing, *inter alia,* carbon dioxide and water.

2 2

are of the highest importance both in the fuel storage and distribution:

The rate of oxidation of petroleum products is higher in the presence of oxidation catalysts, which include metals (copper, lead, iron) and organic acid salts. Fig. 3 shows active compounds which are most frequently found in fuels and have a significant effect on the rate and course of degradation processes during long-term storage. **2.1.3. Oxidation catalysts** The rate of oxidation of petroleum products is higher in the presence of oxidation catalysts, which include metals (copper, lead, iron) and organic acid salts. Fig. 3 shows active compounds which are most frequently found in fuels and have a significant effect on the rate and course of degradation processes during long-term storage.

**Fig. 3. The chemical compounds which most frequently affect degradation processes Figure 3.** The chemical compounds which most frequently affect degradation processes during long-term storage

**during long-term storage**  Particularly active metals include copper, cobalt, iron, lead, manganese, zinc. Oxidation of hydrocarbons is based on decomposition of hydroperoxides which is accompanied by the Particularly active metals include copper, cobalt, iron, lead, manganese, zinc. Oxidation of hydrocarbons is based on decomposition of hydroperoxides which is accompanied by the formation of active peroxide and hydroxyl hydrocarbon free radicals, as shown in the following reactions (43...46):

formation of active peroxide and hydroxyl hydrocarbon free radicals, as shown in the

**Cu2+ + RH →Cu<sup>+</sup> + H+ + R∙ (43) Cu<sup>+</sup> + O2 →Cu2+ (44) Cu2+ + ROOH →Cu<sup>+</sup> + H+ + ROO∙ (45)** 

The presence of catalysts expedites branching of the chain of oxidative changes, and the resulting products accelerates corrosion of metal parts (engine, tank, tubing etc.). The

The effect of total sulfur content on the tendency of resinous deposits to be formed in fuels is not a linear relationship. Sulfur compounds are effective over a specific range of concentrations; as soon as that range is exceeded, their activity will cease (the phenomenon is also observed for alkaline nitrogen compounds). Sulfur compounds which are typically present in fuels and show such activity include derivatives of thiophenols, such as thiopenol, tetrahydrothiophenol, alkyl-substituted thiophenols, and condensed dibenzothiophenols. Studies reported by Mushrush [19] confirmed the generally known view that fuels which have a high sulfur content and are intended for storage are regarded as having poor stability and their long-term storage should rather be avoided. Moreover, a kind of synergistic effect of sulfur and nitrogen compounds was observed, resulting in that stability of fuels was

Addition of compounds which tend to inhibit oxidation will block the activity of oxidation catalysts in fuels. Such compounds are derivatives of aromatic amines, phenols, thioethers, dithiocarbamates, sulfur compounds, which have an ability to bind metals forming complex

**+ RO∙ (46)** 

**Cu<sup>+</sup> + ROOH →Cu2+ + OH-**

catalysts have different activities, depending on external conditions.

reduced by excessive reactivity and by formation of resinous agglomerates.

$$\text{Cu}^{2+} + \text{RH} \rightarrow \text{Cu}^{+} + \text{H}^{+} + \text{R}\cdot\text{}\tag{43}$$

$$\text{Cu}^+ + \text{O}\_2 \rightarrow \text{Cu}^{2+} \tag{44}$$

$$\text{Cu}^{2+} + \text{ROOH} \rightarrow \text{Cu}^{+} + \text{H}^{+} + \text{ROO} \cdot \text{} \tag{45}$$

$$\text{Cu}^+ + \text{ROOH} \rightarrow \text{Cu}^{2+} + \text{OH}^\cdot + \text{RO} \bullet \tag{46}$$

The presence of catalysts expedites branching of the chain of oxidative changes, and the resulting products accelerates corrosion of metal parts (engine, tank, tubing etc.). The catalysts have different activities, depending on external conditions.

The effect of total sulfur content on the tendency of resinous deposits to be formed in fuels is not a linear relationship. Sulfur compounds are effective over a specific range of concentra‐ tions; as soon as that range is exceeded, their activity will cease (the phenomenon is also observed for alkaline nitrogen compounds). Sulfur compounds which are typically present in fuels and show such activity include derivatives of thiophenols, such as thiopenol, tetrahy‐ drothiophenol, alkyl-substituted thiophenols, and condensed dibenzothiophenols. Studies reported by Mushrush [19] confirmed the generally known view that fuels which have a high sulfur content and are intended for storage are regarded as having poor stability and their long-term storage should rather be avoided. Moreover, a kind of synergistic effect of sulfur and nitrogen compounds was observed, resulting in that stability of fuels was reduced by excessive reactivity and by formation of resinous agglomerates.

Addition of compounds which tend to inhibit oxidation will block the activity of oxidation catalysts in fuels. Such compounds are derivatives of aromatic amines, phenols, thioethers, dithiocarbamates, sulfur compounds, which have an ability to bind metals forming complex bonds. What inhibitors do is they break oxidation process chains or decompose hydroperox‐ ides, peroxide radicals etc., to form non-active compounds. The presence of oxidation inhibi‐ tors in effect extends the possible time of storage and use of petroleum products.

#### *3.1.4. Products of oxidation*

As the process of oxidation continues, sparingly soluble deposits are formed in the fuels. In gasolines, agglomeration of deposits results from the formation of resins. The resins are organic compounds with a complex chemical structure, composed mainly of long-chained com‐ pounds, originating from polymerization. They are highly viscous, dark-brown or black compounds.

With regard to method of determination, resins in gasolines are classified as:

**•** actual resins (dissolved in fuel) – defined as the dry residue of fuel, they do not dissolve in n-heptane,


Owing to the very strong impact of resins on the performance properties of fuels, it is consid‐ ered that resin content is an essential measure of the quality of fuel and of its usefulness during long-term storage. After exceeding a certain concentration, actual resins tend to separate, forming deposits which then tend to accumulate on various components of the fuel system. In high-temperature conditions, the resins may convert to substances with a very high-density (lakes, carbon deposits etc.).

The potential resin content enables determination of the content of present resins, which may be formed in the fuel on storage. The potential resin content has no significant effect on the performance properties of fuels.

The following factors have an effect on the formation of carbon and other deposits, in the case of spontaneous ignition engines:


2+ + + Cu + RH Cu + H + R ® g (43)

2+ + + Cu + ROOH Cu + H + ROO ® g (45)

<sup>+</sup> 2+ - Cu + ROOH Cu + OH + RO ® g (46)

The presence of catalysts expedites branching of the chain of oxidative changes, and the resulting products accelerates corrosion of metal parts (engine, tank, tubing etc.). The catalysts

The effect of total sulfur content on the tendency of resinous deposits to be formed in fuels is not a linear relationship. Sulfur compounds are effective over a specific range of concentra‐ tions; as soon as that range is exceeded, their activity will cease (the phenomenon is also observed for alkaline nitrogen compounds). Sulfur compounds which are typically present in fuels and show such activity include derivatives of thiophenols, such as thiopenol, tetrahy‐ drothiophenol, alkyl-substituted thiophenols, and condensed dibenzothiophenols. Studies reported by Mushrush [19] confirmed the generally known view that fuels which have a high sulfur content and are intended for storage are regarded as having poor stability and their long-term storage should rather be avoided. Moreover, a kind of synergistic effect of sulfur and nitrogen compounds was observed, resulting in that stability of fuels was reduced by

Addition of compounds which tend to inhibit oxidation will block the activity of oxidation catalysts in fuels. Such compounds are derivatives of aromatic amines, phenols, thioethers, dithiocarbamates, sulfur compounds, which have an ability to bind metals forming complex bonds. What inhibitors do is they break oxidation process chains or decompose hydroperox‐ ides, peroxide radicals etc., to form non-active compounds. The presence of oxidation inhibi‐

As the process of oxidation continues, sparingly soluble deposits are formed in the fuels. In gasolines, agglomeration of deposits results from the formation of resins. The resins are organic compounds with a complex chemical structure, composed mainly of long-chained com‐ pounds, originating from polymerization. They are highly viscous, dark-brown or black

**•** actual resins (dissolved in fuel) – defined as the dry residue of fuel, they do not dissolve in

tors in effect extends the possible time of storage and use of petroleum products.

With regard to method of determination, resins in gasolines are classified as:

have different activities, depending on external conditions.

excessive reactivity and by formation of resinous agglomerates.

*3.1.4. Products of oxidation*

compounds.

118 Storage Stability of Fuels

n-heptane,

<sup>+</sup> 2+ Cu + O Cu <sup>2</sup> ® (44)


The following parameters characterize the tendency of diesel fuels to form deposits:


Care should also be taken to control the content of impurities, which lead to excessive wear and tear of the fuel supply system components.

#### *3.1.5. Accelerated ageing and stability tests*

Fuel stability tests are typically based on the method of determination of the induction period for gasolines. This applies to testing the oxidation stability of gasolines and aircraft fuels. The **ASTM D 873** test enables evaluation of the tendency of resinous deposits to be formed in fuels under accelerated ageing conditions. The test is carried out in a pressurized steel bomb. After the test is completed, the generated amounts of potential, adsorbed, washed, and unwashed resins are determined in accordance with the **EN-5** method, and the amount of deposits in the filtration apparatus is found in accordance with **ASTM D 6217**.

The accelerated ageing test according to **ASTM D 873** is useful in measuring the stability of gasolines (determination of potential resins) in the oxidation reaction conditions. The behavior of the gasoline sample in the test conditions is analyzed by determining the amount of generated deposits which do not dissolve in the hydrocarbon solvent, and the amount of soluble resins which were formed during the test. Although used for testing aircraft fuels, the test has been adapted to unleaded gasolines and is used worldwide. The induction period may also be used by the method according to **ISO 7536**.

Accelerated ageing of averaged distillates is typically determined by the method described in **ASTM D 4625**. The fuel is subjected to oxidation in mild conditions in the air at a temperature of 43.3°C. Samples are analyzed for an increased content of resinous compounds, structural changes in the resins (IR spectrum), and change in the color of the test sample. The test takes a long time and is costly, therefore, alternative faster methods are searched for to evaluate stability.

In the case of aircraft fuel stability tests, additionally, the thermo-oxidation stability test is performed to control any changes taking place. The test enables the assessment of the fuel's tendency to decompose and the susceptibility of deposits to be formed in the fuel system.

The **ASTM D 2274** or **ISO 12205** takes 40 hours. Based on a study which took several years to complete, a good correlation was shown between the oxidation stability test and the results obtained in field conditions. The method is intended for testing the oxidation stability of average petroleum distillates. The total quantity of filterable and adherent deposits which are formed in the process of oxidation is the final result of determination.

**ASTM 5304** describes another test, designed for the accelerated ageing of diesel fuels in the presence of pure oxygen under pressure. The method comprises the assessment of potential stability during the storage of average distillates. This applies to fresh distillates and those after storage, with or without stabilizers. The result is a gravimetric assessment of the total content of resins formed as the result of ageing.

The RANCIMAT test according to **EN 14112** is the method of assessment of oxidation stability both for diesel fuel and for biodiesel. Oxidation proceeds according to the radical mechanism. Oxidation products are transferred in a stream of air to the measuring cell filled with deionized water of which the conductivity is measured in a continuous manner. The conductivity vs. time diagram is the oxidation curve. Its inflexion point indicates the what is called the "induction period". The RANCIMAT test is also used for assessing the efficiency of oxidation inhibitors.

In the fuel stability tests according to **ASTM D 525** "Oxidation Stability of Gasoline (Induction Period Method)", oxidation stability is tested by observation of oxygen pressure drop in the test chamber, filled with the test fuel. The test is deemed completed after the oxygen pressure is 10% lower than the maximum value recorded in the test. The test results are similar to those obtained in the RANCIMAT test. The oxidation stability assessment criterion using the modified method described in ASTM D 525 and ASTM D 5304 is similar, although the test conditions, such as sample quantity, oxygen pressure and temperature are different.

The induction period tests may also be carried out according to the modified method described in **ASTM D 525** (the PetroOxy test). The induction period in the PetroOxy test is the duration of the test from the moment the test chamber was filled with oxygen to the time of recording a 10% pressure drop, compared with the highest value recorded in the system during the test.

The choice of the most suitable accelerated ageing test method ought to take into consideration the fact that the more similar are the testing conditions to natural storage conditions (that is, relatively low sample exposure temperature and long duration of test), the more reliable test results are obtained (e.g., ASTM D 4625). To obtain more details on the process of oxidation of the respective fuel components, it is necessary to carry out ageing tests using a model sample.

#### **3.2. Low-temperature stability of fuels**

The accelerated ageing test according to **ASTM D 873** is useful in measuring the stability of gasolines (determination of potential resins) in the oxidation reaction conditions. The behavior of the gasoline sample in the test conditions is analyzed by determining the amount of generated deposits which do not dissolve in the hydrocarbon solvent, and the amount of soluble resins which were formed during the test. Although used for testing aircraft fuels, the test has been adapted to unleaded gasolines and is used worldwide. The induction period may

Accelerated ageing of averaged distillates is typically determined by the method described in **ASTM D 4625**. The fuel is subjected to oxidation in mild conditions in the air at a temperature of 43.3°C. Samples are analyzed for an increased content of resinous compounds, structural changes in the resins (IR spectrum), and change in the color of the test sample. The test takes a long time and is costly, therefore, alternative faster methods are searched for to evaluate

In the case of aircraft fuel stability tests, additionally, the thermo-oxidation stability test is performed to control any changes taking place. The test enables the assessment of the fuel's tendency to decompose and the susceptibility of deposits to be formed in the fuel system.

The **ASTM D 2274** or **ISO 12205** takes 40 hours. Based on a study which took several years to complete, a good correlation was shown between the oxidation stability test and the results obtained in field conditions. The method is intended for testing the oxidation stability of average petroleum distillates. The total quantity of filterable and adherent deposits which are

**ASTM 5304** describes another test, designed for the accelerated ageing of diesel fuels in the presence of pure oxygen under pressure. The method comprises the assessment of potential stability during the storage of average distillates. This applies to fresh distillates and those after storage, with or without stabilizers. The result is a gravimetric assessment of the total content

The RANCIMAT test according to **EN 14112** is the method of assessment of oxidation stability both for diesel fuel and for biodiesel. Oxidation proceeds according to the radical mechanism. Oxidation products are transferred in a stream of air to the measuring cell filled with deionized water of which the conductivity is measured in a continuous manner. The conductivity vs. time diagram is the oxidation curve. Its inflexion point indicates the what is called the "induction period". The RANCIMAT test is also used for assessing the efficiency of oxidation

In the fuel stability tests according to **ASTM D 525** "Oxidation Stability of Gasoline (Induction Period Method)", oxidation stability is tested by observation of oxygen pressure drop in the test chamber, filled with the test fuel. The test is deemed completed after the oxygen pressure is 10% lower than the maximum value recorded in the test. The test results are similar to those obtained in the RANCIMAT test. The oxidation stability assessment criterion using the modified method described in ASTM D 525 and ASTM D 5304 is similar, although the test

conditions, such as sample quantity, oxygen pressure and temperature are different.

formed in the process of oxidation is the final result of determination.

of resins formed as the result of ageing.

also be used by the method according to **ISO 7536**.

stability.

120 Storage Stability of Fuels

inhibitors.

Low-temperature parameters of fuels are: cloud point, cold filter plugging filter, and flow temperature. These properties affect the possibility to start the engine at low ambient temper‐ atures and the fuel's behavior during its transmission, pumping and refuelling. While cloud point and cold filter plugging point are important for vehicle users, flow temperature makes a difference while the fuel is transported or pumped from storage tanks (especially at ambient temperatures below 0˚C).

The low-temperature characteristics of a fuel largely depend on which hydrocarbon fractions it contains. In diesel fuel, hydrocarbons chains are likely to precipitate at low temperatures. Therefore, a petroleum-based fuel may contain up to 20% paraffin hydrocarbons, which precipitate from the system as crystals because of their poor solubility in the process of cooling the fuel.

Cloud point (CP), according to ISO 3015, is a temperature at which paraffin crystals first start to precipitate as the fuel is cooled. On further cooling the fuel sample, the crystals will agglomerate and the fuel's density will increase throughout its volume until freezing point.

The highest temperature at which, after being cooled at normal conditions, a specific volume will not flow through a standard filter system within specified time limits is called the Cold Filter Plugging Point (CFPP) according to EN 116.

According to ISO 3016, the flow temperature is the lowest temperature at which a petroleum fuel sample is still able to flow as the temperature is lowered, in specified conditions.

A fuel which contains long hydrocarbon chains has a tendency to stratify and settle at the tank bottom, therefore, it is necessary to make sure the blend remains homogeneous, especially during long-term storage, and especially at low temperatures. Paraffin crystals at the cloud point or below the cloud point will travel to the bottom of the tank, gradually forming there a gelous layer, which is likely to cause an irreversible fluctuation of paraffins, eventually plugging the tubes and fuel pumps. To achieve the low-temperature parameter values which are specified in EN 590 (for transitional period, CFPP from -10˚C, for winter conditions-below -20˚C), it is indispensable to use appropriate additives for low-temperatures. Their function is to postpone the precipitation of paraffins from the fuel and/or prevent their agglomeration to sizes which cause filter plugging. The values of such parameters are largely affected by the fuel's contamination, its water content, and time of storage in the storage tanks. Mechanical impurities settle on filters and accelerate their plugging In addition, microparticles of dust and dirt tend to attract particles of paraffin hydrocarbons and enhance crystallization. Water content considerably affects low-temperature properties of diesel fuels. Resinous compounds (products of ageing) which may be formed in diesel fuels after prolonged storage interfere with their flow through the filter, accelerating crystallization.

#### **3.3. Corrosion resistance of fuels**

As the oxidation process continues, a variety of compounds are formed in the fuel, generally such as the following:


If present in excess in the fuel, the acidic compounds may cause corrosion of metal parts both during storage and in a working fuel system. Organic acids may accelerate ageing processes, form insoluble deposits and soaps with selected metals and contribute to the formation of permanent emulsions with water. Therefore, it is necessary to control the fuel's quality, monitoring such parameters as acid value, water content, and emulsification properties.

The copper corrosion test indicates potential problems in the case of prolonged contact of the fuel (which contains sulfur) with the structural components of fuel systems which are made of copper or bronze. For a majority of ready-to-use fuels, the parameter satisfies normative requirements.

Another test, based on ASTM D 665 ("Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water"), is performed in more stringent conditions. The test is used mainly for evaluation of the rust-preventing properties of turbine and hydraulic mineral oils. The method is based on visual assessment of corrosion damage on the surface of a steel roll after it was immersed in a mixture of the product with distilled water or a solution of inorganic salts at an elevated temperature, in conditions specified in the standard. After examination, the steel roll is washed with acetone and the corroded surface is evaluated with reference to the scale. Acidic products of oxidation may degrade structural materials, leading eventually to the presence of cracks, corrosion pits, damaged spots in metal surfaces, the construction material may swell or be dissolved. Such phenomena lead to changes in the material's microstructure which were present on its surface and throughout its volume, thus reducing its mechanical strength, that is, performance properties of the material. Changes which are caused by the fuel on the surface of metal parts may occur according to various mechanisms, for instance, due to galvanic processes. The driving force of galvanic corrosion is the difference in the electro‐ chemical potentials of different metals in the presence of the conducting fluid. The fuel's conductivity is increased by its ability to absorb water and by the presence of soluble ions of contaminants. Higher concentrations of inorganic components, salts, acids may promote corrosion by attacking the oxide film and/or increase conductivity, which more favors galvanization.

fuel's contamination, its water content, and time of storage in the storage tanks. Mechanical impurities settle on filters and accelerate their plugging In addition, microparticles of dust and dirt tend to attract particles of paraffin hydrocarbons and enhance crystallization. Water content considerably affects low-temperature properties of diesel fuels. Resinous compounds (products of ageing) which may be formed in diesel fuels after prolonged storage interfere

As the oxidation process continues, a variety of compounds are formed in the fuel, generally

**•** acidic compounds (organic acid, keto-and oxo acids, phenols) – they are aggressive to metals

**•** neutral compounds – those with no effect on the physico-chemical properties of fuels (alcohols, esters) or those which form deposits of all sorts due to condensation processes

If present in excess in the fuel, the acidic compounds may cause corrosion of metal parts both during storage and in a working fuel system. Organic acids may accelerate ageing processes, form insoluble deposits and soaps with selected metals and contribute to the formation of permanent emulsions with water. Therefore, it is necessary to control the fuel's quality, monitoring such parameters as acid value, water content, and emulsification properties.

The copper corrosion test indicates potential problems in the case of prolonged contact of the fuel (which contains sulfur) with the structural components of fuel systems which are made of copper or bronze. For a majority of ready-to-use fuels, the parameter satisfies normative

Another test, based on ASTM D 665 ("Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water"), is performed in more stringent conditions. The test is used mainly for evaluation of the rust-preventing properties of turbine and hydraulic mineral oils. The method is based on visual assessment of corrosion damage on the surface of a steel roll after it was immersed in a mixture of the product with distilled water or a solution of inorganic salts at an elevated temperature, in conditions specified in the standard. After examination, the steel roll is washed with acetone and the corroded surface is evaluated with reference to the scale. Acidic products of oxidation may degrade structural materials, leading eventually to the presence of cracks, corrosion pits, damaged spots in metal surfaces, the construction material may swell or be dissolved. Such phenomena lead to changes in the material's microstructure which were present on its surface and throughout its volume, thus reducing its mechanical strength, that is, performance properties of the material. Changes which are caused by the fuel on the surface of metal parts may occur according to various mechanisms, for instance, due to galvanic processes. The driving force of galvanic corrosion is the difference in the electro‐ chemical potentials of different metals in the presence of the conducting fluid. The fuel's conductivity is increased by its ability to absorb water and by the presence of soluble ions of contaminants. Higher concentrations of inorganic components, salts, acids may promote

with their flow through the filter, accelerating crystallization.

**3.3. Corrosion resistance of fuels**

(aldehydes, ketones, resins, asphaltenes).

such as the following:

(corrosion),

122 Storage Stability of Fuels

requirements.

Corrosion may also be caused by biochemical processes connected with microbial growth in the fuel, especially in the presence of water. Microbial growth in fuels is manifested by the presence of their metabolites, i.e., all sorts of sludge, slimy residue, emulsions or biofilms. On the one hand, such foul material will change the fuel's appearance and physico-chemical properties (decomposition) and, on the other, it will lead to changes in the structure of the shells the fuel tanks are made of (surface corrosion). The presence of water is the essential condition of the presence of microorganisms in fuel tanks. Water may migrate into the tank as moisture from the air, as precipitation water or ground water, or as the result of condensation on the tank walls. Biological activity of microorganisms in fuels also depends on the concen‐ tration of dissolved oxygen, optimum temperature, pH of the water environment, and the presence of hydrocarbons (especially saturated ones) and improvers (especially those which contain nitrogen). Apart from water, which is the source of building material for microbial cells, the fuel is the source of nutrients for the microbial growth. The most intense microbial growth is observed at the water-fuel interface. Energy, which is indispensable for the essential biological functions of microorganisms, is supplied as the result of biological oxidation of fuel components by the enzymes being produced, which work in the environment as long as there is an amount of matter left to be decomposed into such substances as are bio-available to the microbial cells. Decomposition of fuel into bio-available nutrients is continued regardless of microbial demand until the nutrients are entirely depleted or the volume of their metabolites is too large.

The kinetics of the biodegradation process is the fastest for n-alkanes, second fastest for branched alkanes, low-molecular aromatic compounds, and the slowest for polycyclic aromatic compounds [20]. Biodegradation of hydrocarbons may occur both in aerobic conditions (aerobic conversions), where oxygen is the electron acceptor, and in anaerobic conditions (anaerobic conversions, caused by anaerobic microorganisms), where a different electron acceptor is used, for instance, CO2, NO2, SO4 etc.

Changes which occur in fuels under the influence of microorganisms at stable conditions typically depend on the fractional composition of a given motor blend. LPG, kerosene or gasoline have rather low susceptibility to biodegradation. Biological instability is usually observed for diesel fuel with addition of a biocomponent in the form of FAME. Heavier petroleum products, obtained in high-temperature processes, are sterile and free of any content of microorganisms, even in their dormant forms. Contamination may occur only after temperature drop, i.e., in storage tanks, in the presence of water, air, non-petroleum based contaminants.

The pathway of decomposition of fuels induced by microorganisms is governed by the same laws of thermodynamics, kinetics, and catalysis as chemical reactions. Hydrocarbons decom‐ pose into fatty acids and alcohols, which are then used by microorganisms in metabolic processes, whereby water and carbon dioxide are obtained as final products. Microbial growth deteriorates the quality of fuels, as shown by the following parameters [21]:


Such changes will not usually disqualify any fuel, except for the phase separation index, of which the value may be so affected due to the microorganisms as to exclude the fuel from any further use. Changes in the fuel's composition are not the same for the entire tank volume. Any decomposition process taking place is local by nature, usually having the form of corrosion on the tank walls, and causes filter plugging in the fuel system.

Standard IP 385/94, developed by the Institute of Petroleum in London, provides a method to determine the count of microorganisms per unit volume of fuel at the fuel/water interface and in water.

In available literature reports there is not information on the permissible limits of contamina‐ tion of fuels with microorganisms. Although the contemporary petroleum products as used in aviation, navigation, and in road traffic do have a minor content of microorganisms (usually less than 50 cfu/dm3 ), the quality of such fuel is not affected by such contamination [22]. Since it is not possible to entirely prevent petroleum products from becoming contaminated with water, care should be taken to establish more stringent requirements and regularly monitor the permissible limits of the count of microorganisms in those fuels which are intended for long-term storage (protection by the use of biocidal products).

#### **4. Long-term storage**

A number of research projects have been carried out by the Automotive Industry Institute (PIMOT), Poland, to determine the effect of long-term storage of fuels on the their quality. The fuels tested included lead-free gasoline, aircraft fuel, diesel fuel, and fuel oil, before and after subjecting them to long-term storage.

The composition of the fuels tested was as follows:


The fuels were stored in steel drums, in variable weather conditions. The fuel was sampled at certain time intervals during storage to test their physico-chemical properties and chemical stability. The duration of such tests was 4 years and 2 years for gasoline and for diesel fuel, respectively.

#### **4.1. Gasolines**

**•** water separation index,

**•** condition of the water-fuel interface,

**•** content of mechanical impurities.

**•** chemical stability and thermal stability of fuel,

Such changes will not usually disqualify any fuel, except for the phase separation index, of which the value may be so affected due to the microorganisms as to exclude the fuel from any further use. Changes in the fuel's composition are not the same for the entire tank volume. Any decomposition process taking place is local by nature, usually having the form of

Standard IP 385/94, developed by the Institute of Petroleum in London, provides a method to determine the count of microorganisms per unit volume of fuel at the fuel/water interface and

In available literature reports there is not information on the permissible limits of contamina‐ tion of fuels with microorganisms. Although the contemporary petroleum products as used in aviation, navigation, and in road traffic do have a minor content of microorganisms (usually

it is not possible to entirely prevent petroleum products from becoming contaminated with water, care should be taken to establish more stringent requirements and regularly monitor the permissible limits of the count of microorganisms in those fuels which are intended for

A number of research projects have been carried out by the Automotive Industry Institute (PIMOT), Poland, to determine the effect of long-term storage of fuels on the their quality. The fuels tested included lead-free gasoline, aircraft fuel, diesel fuel, and fuel oil, before and after

**•** gasoline – containing various amounts of cracked gasoline (14...34) and various amounts of

The fuels were stored in steel drums, in variable weather conditions. The fuel was sampled at certain time intervals during storage to test their physico-chemical properties and chemical stability. The duration of such tests was 4 years and 2 years for gasoline and for diesel fuel,

), the quality of such fuel is not affected by such contamination [22]. Since

corrosion on the tank walls, and causes filter plugging in the fuel system.

long-term storage (protection by the use of biocidal products).

**•** surface tension, **•** resin content,

124 Storage Stability of Fuels

in water.

less than 50 cfu/dm3

**4. Long-term storage**

**•** diesel fuel.

respectively.

subjecting them to long-term storage.

The composition of the fuels tested was as follows:

MTBE (methyl-*tert*-butyl ether) (1,4...4,2),

From tests and analyses carried out after 4 years of storage it was found that the physicochemical properties of gasoline were not affected as regards the following parameters:


However, the following parameters did change after storage for 4 years:


Only slight changes after storing the fuel for 4 years were observed in: density, fractional composition, vapor pressure, content of water, and of MTBE.

With the lapse of storage time, all the gasoline samples showed reduced values of RON and MON. It was found that such changes may have been caused by chemical reactions taking place in the fuel ageing process, leading to the generation of resins with a low octane number in the fuel. After 4 years of storage, only the gasoline with the lowest cracked gasoline fraction was found to satisfy applicable requirements for MON. On the other hand, all samples of engine gasoline were compliant with such requirements in respect of RON.

The analysis confirmed that the present and total resin content tests of the gasoline samples were reliable in determining resistance of the fuel to external factors during long-term storage. Analysis of the generated amounts of actual/total resins confirmed the conclusions made after an analysis of changes in the octane number values. The studies indicate that a drop in the total resin content at the end of the first year of storage was connected with the fresh fuel stabilization in new conditions. Such stabilization processes were followed by the proper reactions of fuel oxidation during long-term storage.

For low percentages of the cracked gasoline fraction, only a slight increase was detected for actual resins, although as the cracked fraction was gradually increased (in the same period of time) the amount of generated resins was growing exponentially, going beyond the permis‐ sible values for the parameter, as specified in applicable standards. Analyses were performed to determine the fractional and chemical compositions of the gasoline samples by FIA and GCMS, which indicate that the extent of fuel degradation depended on the presence of unstable (reactive) hydrocarbon compounds, oxygen compounds, and improvers. The gasoline samples were found to contain a higher content of paraffin-naphthene compounds while less aromatic and olefin compounds were present. This indicates that the oxidation, polymerization and sedimentation processes were going on, leading to degradation of the fuel. IR spectroscopy studies indicate that the fuels' condition could be monitored provided that the compounds of interests (i.e., the resins) have been separated.

After the separation of acetone resins from the gasoline using column chromatography followed by an IR analysis, it was found that band intensities for the selected functional groups were becoming higher with the lapse of time, while variations in the spectrum course de‐ pended on the unstable, cracked fraction content. A quantitative analysis of the separated acetone resins was performed for reference, using the gravimetric method and it was found that the resin content had decreased after the first year of storage and increased in the years which followed. The amount of acetone resins in the test fuel changed at a faster rate, accom‐ panying an increase in the concentration of the cracked fraction.

Determination of oxidation stability by the induction-period method enabled identification of products with respect to their content of cracked-gasoline. Such determination appeared to be of little use in examining changes in the stability of gasolines on storage. The studies indicate that it possible to determine a forecast storage time for gasoline, using the accelerated ageing test (ASTM D 873) both for the fresh fuels and for those after long-term storage. An attempt was made to estimate the actual storage time of the gasolines. From the accelerated ageing tests for gasoline and from the Stavinoha equation [8], it was estimated that gasolines con‐ taining a package of improvers could safely be stored for 6 to 9 years, depending on how much of the cracked fraction gasoline they contain: specifically, the lower their cracked fraction content, the longer the storage time (for the percentages tested).

In the case of gasolines without improvers, storage time was up to 6 years while the depend‐ ence of time on the cracked fraction percentage was similar.

#### **4.2. Diesel fuel**

After storing diesel fuels and fuel oils for 2 years, the physico-chemical parameters which had changed were:


The studies indicate that the acid value depends on the content of improvers added to the fuels in their production process.

As in the case of gasolines, the content of acetone resins tended to decrease initially during storage, which was associated with the fuel's adaptation to the storage conditions. After the lapse of 18 months, as more acetone resins were formed in the fuel, their formation was not slowed down even by oxidation inhibitors. An IR analysis of the spectra obtained for the oil samples in the respective series of tests showed the presence of similar bands of characteristic vibrations of which the intensities depended on how much resins had been formed in the oil sample during storage. After storing the diesel oil for 2 years the samples were found to comply with EN 590 requirements for the parameters tested.

#### **5. Conclusion**

and olefin compounds were present. This indicates that the oxidation, polymerization and sedimentation processes were going on, leading to degradation of the fuel. IR spectroscopy studies indicate that the fuels' condition could be monitored provided that the compounds of

After the separation of acetone resins from the gasoline using column chromatography followed by an IR analysis, it was found that band intensities for the selected functional groups were becoming higher with the lapse of time, while variations in the spectrum course de‐ pended on the unstable, cracked fraction content. A quantitative analysis of the separated acetone resins was performed for reference, using the gravimetric method and it was found that the resin content had decreased after the first year of storage and increased in the years which followed. The amount of acetone resins in the test fuel changed at a faster rate, accom‐

Determination of oxidation stability by the induction-period method enabled identification of products with respect to their content of cracked-gasoline. Such determination appeared to be of little use in examining changes in the stability of gasolines on storage. The studies indicate that it possible to determine a forecast storage time for gasoline, using the accelerated ageing test (ASTM D 873) both for the fresh fuels and for those after long-term storage. An attempt was made to estimate the actual storage time of the gasolines. From the accelerated ageing tests for gasoline and from the Stavinoha equation [8], it was estimated that gasolines con‐ taining a package of improvers could safely be stored for 6 to 9 years, depending on how much of the cracked fraction gasoline they contain: specifically, the lower their cracked fraction

In the case of gasolines without improvers, storage time was up to 6 years while the depend‐

After storing diesel fuels and fuel oils for 2 years, the physico-chemical parameters which had

The studies indicate that the acid value depends on the content of improvers added to the fuels

As in the case of gasolines, the content of acetone resins tended to decrease initially during storage, which was associated with the fuel's adaptation to the storage conditions. After the lapse of 18 months, as more acetone resins were formed in the fuel, their formation was not slowed down even by oxidation inhibitors. An IR analysis of the spectra obtained for the oil samples in the respective series of tests showed the presence of similar bands of characteristic vibrations of which the intensities depended on how much resins had been formed in the oil

interests (i.e., the resins) have been separated.

panying an increase in the concentration of the cracked fraction.

content, the longer the storage time (for the percentages tested).

ence of time on the cracked fraction percentage was similar.

**•** oxidation stability at a temperature of 95°C,

**4.2. Diesel fuel**

126 Storage Stability of Fuels

changed were: **•** acid value,

**•** acetone resins content.

in their production process.

The analyses indicate that external factors involved in the storage of fuels lead to their degradation (oxidation), deteriorating their quality. Oxidation leads to the formation of highmolecular resins, gums, insoluble deposits and acidic compounds which are aggressive in contact with metals. The rate of oxidation depends on the fuel's physico-chemical properties, weather conditions and on the presence of compounds which either inhibit or activate oxidation.

Fuel stability, understood as its resistance to changes in quality, should be specified at the stages of transport, storage and use, however, there exist no legal documents establishing appropriate storage conditions for fuels and methods to monitor heir quality during long-term storage.

To better protect a fuel from factors which lead to its ageing, appropriate construction materials ought to be selected for the fuel-wetted parts, storage systems should be hermetic, and a selected package of improvers should be added to the fuel, to protect it from the dominant oxidative factor.

The fuel storage facility is a complex system and it is not possible to create a single algorithm describing all model processes of their ageing during storage. Therefore, it is necessary to continue research works on the subject, to search for and identify precursors of fuel oxidation processes and develop efficient methods to eliminate fuel oxidation and prevent its ageing.

#### **Author details**

Marlena Owczuk\* and Krzysztof Kołodziejczyk

\*Address all correspondence to: m.owczuk@pimot.eu

Department of Fuels, Biofuels and Lubricants, Automotive Industry Institute, Poland

#### **References**

[1] Directive 2003/17/EC of the European Parliament and of the Council of 3 March 2003 amending Directive 98/70/EC concerning the quality of gasoline and diesel fuel.


of FAME in respect of physical, chemical and performance properties; 3rd Scientific Conferemce ECOENERGY 2007], University of Agriculture in Lublin, Institute of Agrophysics, PAN – Lublin – Krasnobród.

[18] Jakóbiec J; Baranik M; Duda A. Wysoka jakość estrów metylowych kwasów tłuszczo‐ wych oleju rzepakowego to promocja transportu samochodowego [High quality of rapeseed oil fatty acid methyl esters promotes vehicle transport]; Archiwum Motory‐ zacji No. 1/2008..

[2] Wachal A., Materiały pędne i oleje silnikowe do współczesnych silników tłokowych, odrzutowych i rakietowych [Propellants and motor oils for contemporary piston-, jet-and rocket engines], Wydawnictwo Ministerstwa Obrony Narodowej, 1959.. [3] Beaver B.D.; Demunshi R., Sharief V., Tian D., and Teng Y., Development of oxygen scavenger additives for jet fuels, 5th International Conference on Stability and Han‐

[4] Černožukov N. I., Krejn S. E., okisl'ayemost' mineral'nych masel, Gostoptechizdat

[5] Iwanow K. I., Promieżutocznyje produkty i promieżutocznyje reakcji awtookislienija uglewodorodow [Intermediate products and reactions in autoxidation of hydrocar‐

[6] Łosikow B.W., Łukaszewicz P., Towaroznawstwo naftowe [Petroleum Products],

[8] Stavinoha L.L.; Westbrook S.R.; and McInnis L.A. Mechanism of deposit formation on fuel-wetted metal surfaces, 5th International Conference on Stability and Han‐

[10] Jones, E. G.; Balster, W. J.; Post, M. E. International Gas Turbine and Aeroengine Congress and Exposition: Cincinnati, OH, May 1993; Paper No. 92-GT-334. (Accept‐

[12] Heneghan, S. P.; Williams, T. F.; Martel, C. R.; Ballal, D. R. Trans. ASME, Jour. Eng.

[13] Pande S. G.; D.R. Hardy; The effect of copper, MDA and accelerated aging jet fuel thermal stability as measured by the gravimetric JFTOT, 5th International Confer‐ ence on Stability and Handling of Liquid Fuels Rotterdam, the Netherlands October

[14] Clark, R.H. In Proceedings of the 3rd International Conference on Stability and Han‐

[15] Hardy, D.R.; Beal, E.J.; Burnett, J.C. In Proceedings of the 4th International Confer‐ ence on Stability and Handling of Liquid Fuels, Orlando, Florida, 19-22 November,

[16] Černožukov N. I., Rafinacja produktów naftowych [Refining of petroleum products],

[17] Jakóbiec J; Ambrozik A. Badania FAME w zakresie oceny właściwości fizykoche‐ micznych i użytkowych; III Konferencja Naukowa EKOENERGIA 2007 [Examination

dling of Liquid Fuels Rotterdam, the Netherlands October 3-7, 1994.

ed for publication in Trans ASME, Jour. Eng. Gas Turb. Power.)

dling of Liquid Fuels, London, UK, 13-16 Sept. 1988, p. 283.

Wydawnictwo Naukowo – Techniczne, Warsaw 1968.

[11] Walling, C. Free Radicals in Solution, John Wiley and Sons, NY, 1957, p 427.

dling of Liquid Fuels Rotterdam, the Netherlands October 3-7, 1994.

[Oxidation of mineral oils], Gostoptechizdat, Moscow 1955

Państwowe Wydawnictwo Techniczne, Warsaw 1953

[7] Bone W., Haffner A., Proc. Roy. Soc., A 143, 16, 1933.

[9] Heneghan, S. P.; Zabarnick, S. Fuel 1994, 73,35.

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3-7, 1994.

bons], Moscow 1949.

128 Storage Stability of Fuels


## **Corrosiveness of Fuels During Storage Processes**

Monika Ziółkowska and Dorota Wardzińska

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59803

#### **1. Introduction**

Corrosion, or destruction of materials in consequence of chemical or electrochemical interac‐ tions between the material and its environment, has been a major problem in industry for years. The petrochemical industry being no exception, corrosiveness has been blamed, as the major perpetrator, for costly failures of equipment. Knowing well and understanding the process of corrosion enables its effective control and prevention of the problems caused by corrosion. Bear in mind that corrosion attacks a variety of materials, not just metals alone: the destructive effect of the environment on ceramics, plastics, as well as composites is also observed.

In a more complex environment, it is more difficult to investigate and find a solution to the corrosive effect. Paradoxically, corrosion is a common problem wherever petroleum products are in contact with metal parts and alloys, whether during production, distribution, operation, or storage – even though non-polar hydrocarbons contained in the fuel do not cause corrosion. Corrosion changes are caused by certain constituents of the fuel: sulfur compounds, organic acids, and water-soluble inorganic acids and bases. Among the aforementioned compounds, the most aggressive ones, having the highest corrosive effect are active sulfur compounds (e.g., free sulfur, hydrogen sulfide), especially in the presence of water. The content of water in the refinery fuels is usually very low (30-80 ppm) and its effect on corrosion rate is insignificant. If increased (for instance, due to the penetration of steam into storage tanks), it does have an effect on corrosion processes. In gasoline with a water content of 80 ppm or less, the rate of corrosion on carbon steel is 0.001 mm/year; in those with a water content of 200 ppm – corrosion rate is as high as 0.4 mm/year [1]. Part of the steel or metal surface is wetted with water, which forms thin (from 3-10 μm), interrupted water films between the metal and the organic phase. Since oxygen has different solubilities (higher for the organic and lower for the aqueous phase), anode or cathode regions are formed on the steel or metal surface. This creates favorable conditions for electrochemical corrosion in a given environment. Also microbial growth in fuels causes corrosion damage during the storage and transport of fuels. Above a certain limit,

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

which usually is low, the presence of each of the aforementioned factors of corrosion is unacceptable. Bear in mind that fuel ageing and corrosion are coexistent processes. According to certain reports, the presence in fuels of metal ions originating from corrosion processes accelerates the formation of deposits and gums [2] which, in turn, lead to problems in the fuel distribution system, such as filter plugging or damage to engine parts. Therefore, it is very important to determine the corrosive effect of fuels on the construction materials, widely used in automobile engines or storage tanks.

#### **2. Corrosion in storage tanks**

Corrosion is also a problem during fuel storage in storage tanks. Corrosion leads to the formation of products which are likely to cause changes in the quality of fuels (affecting their physical and chemical properties), may accelerate oxidation of hydrocarbon ingredients of the fuel – this leads to higher acid values and to the development of gum and sludge in the fuel. Carbon-steel storage tanks, intended for long-term (5 years a minimum) storage of fuels, may affect the chemical stability of fuels (increased content of oxidation products) [1].

Any sludge developed in the fuel will promote microbiological corrosion. Steam and atmos‐ pheric oxygen may dissolve in the fuel during storage. Water and atmospheric oxygen may penetrate into the storage tanks during their evacuation and during what is called "tank breathing". As temperature fluctuates between day and night, water with its contaminants may be separated from a fuel being stored in aboveground tanks, forming a thin aqueous electrolyte film. Typically, the film is formed on the tank walls and travels downwards, to reach the bottom. This creates conditions in which electrochemical corrosion processes are likely to occur. In storage tanks, there are several corrosion zones with different susceptibilities to corrosion:


The mechanisms of corrosion taking place on the inner parts of the tank are more complex, compared with those describing the corrosion processes that may occur on its outer surface. The rate and type of corrosion taking place in aboveground tanks depends on what petroleum

**Figure 1.** Corrosion zones in storage tanks

which usually is low, the presence of each of the aforementioned factors of corrosion is unacceptable. Bear in mind that fuel ageing and corrosion are coexistent processes. According to certain reports, the presence in fuels of metal ions originating from corrosion processes accelerates the formation of deposits and gums [2] which, in turn, lead to problems in the fuel distribution system, such as filter plugging or damage to engine parts. Therefore, it is very important to determine the corrosive effect of fuels on the construction materials, widely used

Corrosion is also a problem during fuel storage in storage tanks. Corrosion leads to the formation of products which are likely to cause changes in the quality of fuels (affecting their physical and chemical properties), may accelerate oxidation of hydrocarbon ingredients of the fuel – this leads to higher acid values and to the development of gum and sludge in the fuel. Carbon-steel storage tanks, intended for long-term (5 years a minimum) storage of fuels, may

Any sludge developed in the fuel will promote microbiological corrosion. Steam and atmos‐ pheric oxygen may dissolve in the fuel during storage. Water and atmospheric oxygen may penetrate into the storage tanks during their evacuation and during what is called "tank breathing". As temperature fluctuates between day and night, water with its contaminants may be separated from a fuel being stored in aboveground tanks, forming a thin aqueous electrolyte film. Typically, the film is formed on the tank walls and travels downwards, to reach the bottom. This creates conditions in which electrochemical corrosion processes are likely to occur. In storage tanks, there are several corrosion zones with different susceptibilities

**•** Zone 1 – Top of the tank (inner surfaces of the roof and side walls), which contacts the fuel

**•** Zone 2 – Splashing zone: a border between the gas phase and liquid phase. The size of it is variable and depends on how much fuel is in the tank and how the tank is filled/evacuated. **•** Zone 3 – Liquid zone: that part of the storage tank which is permanently in contact with the

**•** Zone 4 – Bottom of the tank and, optionally, its immediately adjacent region. That part of the aboveground tanks is most exposed to contact with water, any salts that may be dissolved in it and its organic and inorganic deposits. The organic deposits are composed of hydrocarbons and bio-films, whereas the inorganic deposits may comprise corrosion

The mechanisms of corrosion taking place on the inner parts of the tank are more complex, compared with those describing the corrosion processes that may occur on its outer surface. The rate and type of corrosion taking place in aboveground tanks depends on what petroleum

liquid fuel; same as above, its size depends on how much fuel is in the tank.

affect the chemical stability of fuels (increased content of oxidation products) [1].

in automobile engines or storage tanks.

132 Storage Stability of Fuels

**2. Corrosion in storage tanks**

to corrosion:

vapors.

products, salts, and sand [1].

product is stored, on solubility of water and oxygen in that kind of fuel, tank capacity, filling/ evacuation frequency, and on temperature. Also important are the design features of the tank itself, such as roof type (floating or fixed), the presence of pontoons, tank breathing solutions, as well as its location. Typical tanks for storing gasoline or petroleum are provided with floating roofs and pontoons which limit fuel evaporation and losses through safety valves [1].

Depending on the type of fuel, corrosion processes may take place in different tank zones. In aboveground storage tanks for gasoline, corrosion is observed usually on the south-facing part of the tank. This is probably due to the fact that the day and night temperature fluctuations are higher on the south-facing surface. The south part of the storage tank is more exposed to sunlight, this causes a local temperature increase in daytime and improves the solubility of water in the fuel. In the night, the temperature goes down, again, causing separation of water and its adsorption on the steel surface of the tank walls. Such circumstances, combined with the presence of oxygen, lead to electrochemical corrosion. During the storage of gasoline, the bottom surfaces of storage tanks are less exposed to corrosion.

#### **2.1. The mechanism of corrosion taking place in fuels**

With the exception of noble metals, all metals are thermodynamically unstable under normal conditions and it is natural for the system (comprising metals and their alloys) to achieve a form with a better thermodynamic stability (oxidized form). Although a number of types and mechanisms of corrosion are known, it is believed that in the case of fuels there occur electro‐ chemical corrosion (in the presence of electrolytes) and chemical corrosion (in the absence of electrolytes). Compounds which are formed as the result of corrosion, also called corrosion products, may accelerate or inhibit corrosion, or may have no effect on the further course of the metal destruction process. In the case of certain metals, weakly soluble metal compounds (usually oxides) are formed, creating a protective film on its surface, it reduces the rate of corrosion to negligible values – the metal then remains passive. The presence of such naturally formed surface films has an effect on the corrosion resistance of such construction materials as alloy steels, titanium alloys, or aluminum. The oxide layer significantly inhibits corrosion of the metal on which it is formed. This enables designing of components and storage tank fittings without any extra safety devices against corrosions. For instance, in alloy steels, the formation of a thin passive film is connected with a suitable content of chromium in the steel. Under certain conditions, a chromium-rich oxide film on the surface will practically prevent general corrosion. A problem appears as soon as the passive film is destroyed locally and its continuity is interrupted, leading to the formation of active-passive galvanic cells and to the development of corrosion pits. In such galvanic cells the area which is devoid of the passive film becomes the anode where the metal is oxidized while the passivated metal area functions as the cathode. This is very dangerous because the anode/cathode surface area ratio is often unfavorable, very much accelerating metal digestion. For certain conditions, this may have serious consequences, for instance, perforation of the tank walls. The Figure below illustrates the idea of how the galvanic cell works, on the example of iron.

**Figure 2.** Galvanic cell – the principle of operation

The rate of electrochemical corrosion depends on the nature of the protective film being formed on the metal or alloy surface, the presence of polar solvents (especially water) in the environ‐ ment of the metal (fuel), the presence of electrolytes (salts, acids, bases), as well as temperature. For a given material, corrosion resistance depends on its structure, composition and the various forces acting upon it. In the case of metal, its electrochemical potential is important as well. Metals having a positive electrochemical potential in relation to hydrogen will undergo oxidation more readily and the process is the more intensive the higher the potential.

To explain the mechanisms of corrosion is a great challenge because fuels are very complex mixtures and corrosion processes are complicated. Corrosion interactions remain to be the subject of studies globally. Studies are carried out to investigate the details of the mechanism of corrosion – both in order to fill some gaps in our knowledge of the problem, and to find better ways to prevent corrosion and minimize corrosion damage. Understanding corrosion processes is important both for economic and safety reasons. One must not forget that losses caused by corrosion involve not only damaged or destroyed tanks, pumps, engines, but also loss of energy, utilities, or the human effort involved in the manufacturing of materials or products. In the case of fuel storage, on the one hand, the ongoing corrosion processes may destroy tanks or fittings, generating high costs of maintenance or even creating a necessity to replace the damaged parts, posing realistic environmental hazard (leakage of fuel); on the other hand, the products of corrosion may affect the quality of fuel during storage, intensifying the process of fuel ageing as well as vehicle engine problems.

formed surface films has an effect on the corrosion resistance of such construction materials as alloy steels, titanium alloys, or aluminum. The oxide layer significantly inhibits corrosion of the metal on which it is formed. This enables designing of components and storage tank fittings without any extra safety devices against corrosions. For instance, in alloy steels, the formation of a thin passive film is connected with a suitable content of chromium in the steel. Under certain conditions, a chromium-rich oxide film on the surface will practically prevent general corrosion. A problem appears as soon as the passive film is destroyed locally and its continuity is interrupted, leading to the formation of active-passive galvanic cells and to the development of corrosion pits. In such galvanic cells the area which is devoid of the passive film becomes the anode where the metal is oxidized while the passivated metal area functions as the cathode. This is very dangerous because the anode/cathode surface area ratio is often unfavorable, very much accelerating metal digestion. For certain conditions, this may have serious consequences, for instance, perforation of the tank walls. The Figure below illustrates

The rate of electrochemical corrosion depends on the nature of the protective film being formed on the metal or alloy surface, the presence of polar solvents (especially water) in the environ‐ ment of the metal (fuel), the presence of electrolytes (salts, acids, bases), as well as temperature. For a given material, corrosion resistance depends on its structure, composition and the various forces acting upon it. In the case of metal, its electrochemical potential is important as well. Metals having a positive electrochemical potential in relation to hydrogen will undergo

To explain the mechanisms of corrosion is a great challenge because fuels are very complex mixtures and corrosion processes are complicated. Corrosion interactions remain to be the subject of studies globally. Studies are carried out to investigate the details of the mechanism of corrosion – both in order to fill some gaps in our knowledge of the problem, and to find better ways to prevent corrosion and minimize corrosion damage. Understanding corrosion processes is important both for economic and safety reasons. One must not forget that losses caused by corrosion involve not only damaged or destroyed tanks, pumps, engines, but also loss of energy, utilities, or the human effort involved in the manufacturing of materials or products. In the case of fuel storage, on the one hand, the ongoing corrosion processes may

oxidation more readily and the process is the more intensive the higher the potential.

the idea of how the galvanic cell works, on the example of iron.

**Figure 2.** Galvanic cell – the principle of operation

134 Storage Stability of Fuels

Corrosion of metals while in contact with the fuels occurs under the influence of products which are formed by oxidation of gasoline, diesel fuel or biofuels. Such factors cause morpho‐ logical changes taking place on metal parts, which is manifested by changes in coloration of the fuel, among other things. In the case of biofuels and alternative fuels, the problem of their corrosive effect is not very well known yet; this is because of the variability of raw materials and progress in the production technology of such fuels.

#### **2.2. Corrosive effect of biodiesel on copper, aluminum, carbon steel and stainless steel**

Biodiesel is perceived as a novelty fuel, based on renewable sources, having a potential to replace the conventional diesel fuel. From the point of view of chemistry, biodiesel is a combination of fatty acid methyl or ethyl esters. In spite of its numerous advantages, it does have certain drawbacks. Test reports indicate that biodiesel causes more corrosion, compared with the petroleum-based diesel fuel, especially in the presence of water [3].Corrosive effect on metals is one of the major parameters of biofuels, determining their usefulness as engine fuels.

Studies on the corrosive effect of Jatropha biodiesel were carried out before, using such materials as copper, zinc, lead, tin, bronze. It was found that the rate of oil degradation was increased by oxidation which, in turn, was catalyzed by the presence of the metal (the most pronounced catalytic effect was recorded for copper) [4, 5].

Reported in [6] are findings of studies concerning the effect of biodiesel and bioethanol on corrosion for selected materials. Immersion tests were carried out at room temperatures which demonstrated that copper and aluminum are susceptible to corrosion both in biodiesel and in bioethanol. Corrosion pits were also observed on carbon-steel and cast-iron surfaces. A research team of Fazal reported, in [7], their findings on the effect of palm-oil biodiesel on the structural materials of engine components, and compared their findings with similar tests on the conventional diesel fuel. Their findings indicate that the extent of corrosion was higher for biodiesel, compared with diesel fuel. Copper and aluminum appeared to be quite readily corroded in biodiesel and stainless steel was found to resist corrosion.

The corrosive effect of biodiesel and bioethanol on copper, aluminum, carbon steel and stainless steel was also investigated in [8]. Immersion corrosion tests were carried out in accordance with ASTM G32-72 at 43ºC for two months. For comparison, similar tests were carried out for a petroleum-based diesel fuel. The results indicate that corrosion rate of copper in biodiesel was roughly 6 times as high as that in diesel fuel. A similar phenomenon occurred when carbon steel plates were immersed in biodiesel: corrosion rate was 12 times as high as that in diesel fuel. On the other hand, corrosion rate of stainless steel and aluminum in biodiesel was only slight, in the same range as that found for diesel fuel. In the case of aluminum, its surface was covered with a thin, strongly sticking layer of oxides (passive film). The passive film forms a barrier between the metal surface and the fuel, preventing also any access of oxygen of which the presence is essential in corrosion processes; this results in lower corrosion rates. Materials such as copper or carbon steel are readily oxidized. When exposed to biodiesel, metal oxides are formed, including: CuO, Cu2O, Fe2O3 etc., which adhere less strongly to metal surfaces, compared with aluminum [8]. Later in the tests, surface analyses were carried out using SEM, EDS, XPS, and the concentrations of the respective ions in biodiesel before and after the corrosion tests were measured using AAS. Based on their findings, the authors proposed a possible mechanism of the reaction for copper and iron, which could have been working during the corrosion processes taking place. Initially, at room temperatures, copper slowly reacts with oxygen, forming the red copper(I) oxide according to the following reaction:

$$4\text{ Cu} + \text{O}\_2 \rightarrow 2\text{ Cu}\_2\text{O}\tag{1}$$

At elevated temperatures, the black copper(II) oxide may be formed directly (reaction 2).

$$2\,\text{Cu} + \text{O}\_2 \rightarrow 2\,\text{CuO} \tag{2}$$

If water is present in the biodiesel, processes may take place which result in its degradation. The presence of water leads to hydrolysis of esters and, eventually, to the formation of free fatty acids and glycerol. Each of the oxides formed in reactions 1 and 2 may react with fatty acids, forming organic salts (reactions 3 and 4) and water, which accelerates corrosion.

$$2\text{CuO} + 4\text{R'COOH} \rightarrow 2\text{Cu} \text{(R'COO)}\_2 + 2\text{H}\_2\text{O} \tag{3}$$

$$\text{Cu}\_2\text{O} + 2\text{R'COOH} \rightarrow 2\text{Cu(R'COO)} + \text{H}\_2\text{O} \tag{4}$$

Similar reactions take place for iron, exposed to biodiesel. Initially, metallic iron is oxidized to form iron oxide (reaction 5).

$$\text{\textbullet 4Fe} + \text{\textbullet O}\_2 \rightarrow 2\text{Fe}\_2\text{O}\_3 \tag{5}$$

In the next step, the iron oxide may react with the fatty acids being formed in the biodiesel (reaction 6).

$$\text{Fe}\_2\text{O}\_{3\text{\textquotedblleft}R'}\text{\textquotedblright}\text{COOH}\rightarrow\text{2Fe}(\text{R'COO})\_3\text{\textquotedblleft}\text{H}\_2\text{O}\tag{6}$$

Another possibility is that iron reacts directly with fatty acids, forming organic salts which are adsorbed on the metal surface, in addition to hydrogen (reaction 7).

$$2\text{Fe} + 6\text{R'COOH} \rightarrow 2\text{Fe} \text{(R'COO)}\_3 + 3\text{H}\_2\uparrow\tag{7}$$

The corrosion processes taking place were also indicated by changes in the fuel's coloration after exposure of the test metal samples. Its color depended on what kind of ions were released from the test samples. Specifically, a greenish shade of biodiesel after exposure of copper plates was attributed to the presence of Cu+2 ions in the fuel. A brownish shade after testing the carbon steel plates may indicate the presence in the fuel of Fe+2 or Fe+3 ions. Discoloration of biodiesel after immersion tests with aluminum samples is attributed to Al+3 ions, whereas a slightly yellowish shade after exposure of the stainless steel samples – to Fe+3 or Cr+3 ions. In addition to discoloration, sufficient evidence confirming the passage of the metal ions into the solution was that the weight of the test samples was reduced after the corrosion tests.

film forms a barrier between the metal surface and the fuel, preventing also any access of oxygen of which the presence is essential in corrosion processes; this results in lower corrosion rates. Materials such as copper or carbon steel are readily oxidized. When exposed to biodiesel, metal oxides are formed, including: CuO, Cu2O, Fe2O3 etc., which adhere less strongly to metal surfaces, compared with aluminum [8]. Later in the tests, surface analyses were carried out using SEM, EDS, XPS, and the concentrations of the respective ions in biodiesel before and after the corrosion tests were measured using AAS. Based on their findings, the authors proposed a possible mechanism of the reaction for copper and iron, which could have been working during the corrosion processes taking place. Initially, at room temperatures, copper slowly reacts with oxygen, forming the red copper(I) oxide according to the following reaction:

At elevated temperatures, the black copper(II) oxide may be formed directly (reaction 2).

If water is present in the biodiesel, processes may take place which result in its degradation. The presence of water leads to hydrolysis of esters and, eventually, to the formation of free fatty acids and glycerol. Each of the oxides formed in reactions 1 and 2 may react with fatty acids, forming organic salts (reactions 3 and 4) and water, which accelerates corrosion.

Similar reactions take place for iron, exposed to biodiesel. Initially, metallic iron is oxidized to

In the next step, the iron oxide may react with the fatty acids being formed in the biodiesel

Another possibility is that iron reacts directly with fatty acids, forming organic salts which are

adsorbed on the metal surface, in addition to hydrogen (reaction 7).

form iron oxide (reaction 5).

(reaction 6).

136 Storage Stability of Fuels

4 Cu + O 2 Cu O 2 2 ® (1)

2 Cu + O 2 CuO <sup>2</sup> ® (2)

® ( ) <sup>2</sup> <sup>2</sup> 2CuO + 4R'COOH 2Cu R'COO + 2H O (3)

® ( ) Cu O + 2R'COOH 2Cu R'COO + H O 2 2 (4)

® ( ) 2 3 + <sup>2</sup> <sup>3</sup> Fe O 6R'COOH 2Fe R'COO + 3H O (6)

4Fe + 3O 2Fe O 2 23 ® (5)

With the exception of stainless steel, metal surfaces tend to change coloration when reacting with biodiesel, for instance, the copper plate was coated with some black material. Examination indicates [8] that the black deposit may be composed of oxides or organic salts. On the other hand, the presence of organic salts and no iron oxide was observed on the carbon steel plate. Organic salts may also be present on aluminum surfaces. Compared with copper surfaces, aluminum showed only slight discoloration though the entire surface grew a little darker. Certain products of oxidation of the surface of metal or metal alloys, such as ions or deposits, may penetrate into the biodiesel fuel, causing changes in its physico-chemical properties, other products may react with free fatty acids leading, eventually, to the formation of fatty acid organic salts on metal surfaces. This is potentially the main cause of diversification in the appearance of biodiesel and metal surfaces. Examinations were carried out using atomic absorption spectroscopy (AAS), which confirmed that the content of copper and iron ions in biodiesel was high after corrosion.

The carbon steel surface in biodiesel was found to have a higher content of carbon and oxide after the corrosion test. This was caused by the reaction between metal oxides and fatty acids to form salts, which strongly stick to the metal surfaces.

Also traces of carbon, oxygen, nitrogen and sulfur were detected on non-corroded metal surfaces. This was caused by exposure of the metal to the influence of air, and its reaction with atmospheric oxygen whereby metal oxides are generated and certain organic contaminants are absorbed on the surface. Literature reports indicate that, after corrosion, the content of carbon was higher on copper, carbon steel and aluminum and lower on stainless steel. This indicates the formation of organic deposits, such as fatty acid salts, and oxides. The deposits strongly adhere to metal surfaces and are hard to remove with a solvent such as acetone. No significant changes were noticed on stainless steel, indicating the absence of organic deposits on the metal surfaces. Spectral analysis of stainless steel suggested the presence of carbon complexes, including organic deposits and amorphous carbon. The formation of carbon complexes reduces the carbon content on steel, compared with other metal surfaces, where such complexes are not formed.

Certain organic deposits tend to develop on metal surfaces only after exposure to atmospheric air; this was observed for such metals as copper, carbon steel, or aluminum. In such tests, the metal surfaces had a higher carbon content and the level of –COOH carboxylic groups also was higher after corrosion in biodiesel. The increased content of carboxylic groups indicates oxidation of the biodiesel and the formation of carboxylic acids. Reactions between carboxylic acids and metal oxides produce organic salts which deposit upon the metal surfaces [8].

Such conclusions are confirmed by IR spectral analysis. After the corrosion tests, the intensity of the peaks corresponding to iron oxides (Fe3O4 and Fe2O3) was higher, indicating the presence of such oxides; at the same time the iron peak intensity was lower. This explains why organic salts are present on the steel surface, while the content of iron is lower. The same phenomenon was observed for the aluminum surface. A higher intensity of the Al2O3 peak and lower intensity of the aluminum peak indicates the formation of organic salts. Aluminum reacts quite readily with atmospheric oxygen, generating Al2O3, and tends to adsorb organic compounds. It is safe to say that the formation of an aluminum oxide film inhibits corrosion reactions with the metal surface. Oxygen, dissolved in biodiesel induces further formation of the aluminum oxide film, preventing access to the surface.

From among copper, aluminum, carbon steel and stainless steel, the rate of corrosion in biodiesel was the highest for copper (0.02334 mm/year), second highest for carbon steel (0.01819 mm/year), 0.00324 mm/year for aluminum and 0.00087 mm/year for stainless steel. The corrosion products included mainly fatty acid salts, and metal oxides. It was also dem‐ onstrated that decomposition of biodiesel is catalyzed by elements such as copper and iron, creating an environment for corrosion of engine components [8].

#### **2.3. Corrosive effect of biodiesel on copper and brass**

Copper and copper alloys undergo corrosion in fuels probably according to the same mecha‐ nism as iron and iron alloys. However, the corrosion processes taking place on copper are less destructive and slower – corrosion pits, if present all, are limited to extreme conditions; corrosion is usually manifested by discoloration of the metal or alloy surface. It was demon‐ strated that even the lowest concentrations of sulfur present in the fuel will take part in corrosion reactions taking place on copper, forming sulfides on its surface. Destructive oxidation reactions are catalyzed by Cu2+ ions which pass into the fuel due to corrosion processes. According to literature, the effect is observed when the concentration of copper ions in the fuel is between 100 and 3000 ppm [2].

Oxidation of biodiesel leads to the formation of various products such as peroxides and hydroperoxides. During degradation, these products transform into shorter-chain compounds such as low-molecular weight acids, aldehydes, ketones, and alcohols [9]. Moreover, oxidative polymerization may lead to the formation of macromolecular chemical compounds [10]. Thus, oxidation of biodiesel intensifies its corrosive effect, affecting the physico-chemical properties of the fuel [7]. The corrosiveness of biodiesel is also connected with the presence in it of contaminants, such as: water, alcohol, free fatty acids.

Studies on the effect of light, temperatures, and selected metals on corrosiveness and degra‐ dation of biodiesel are reported in [11]. The corrosiveness studies were carried out for copper and brass using the immersion method according to the standards ASTM G31 and ASTM G1. The experiment was continued for 5 days at room temperatures (ASTM G1) in the presence or absence of light. The studies were also carried out at 55ºC for comparison with the findings of tests according to ASTM G31 (with air bubbling). In the tests referred to in ASTM G31, the test samples were placed at 3 different levels (totally immersed in fuel, partly immersed in fuel, exposed to fuel vapors only). The copper and brass plates had more corrosion-induced changes after being immersed in biodiesel, compared with the test plates exposed to fuel vapors only. The corrosion processes were accelerated by bubbling with air, leading to larger weight losses in the copper and brass plates. After 5 days of exposure, a slightly higher corrosion rate was recorded for the plates exposed to light at room temperatures, compared with those tested in the absence of light. Interestingly enough, in the tests carried out in the dark, corrosion rate was much slower for copper and for brass after increasing the temperature to 55ºC; presum‐ ably, this was caused by the fact that oxygen is less soluble at higher temperatures. Moreover, biodiesel samples were found to have higher viscosities and higher water contents after the corrosion tests according to ASTM G31.

#### **2.4. Corrosive effect of bioethanol on aluminum**

metal surfaces had a higher carbon content and the level of –COOH carboxylic groups also was higher after corrosion in biodiesel. The increased content of carboxylic groups indicates oxidation of the biodiesel and the formation of carboxylic acids. Reactions between carboxylic acids and metal oxides produce organic salts which deposit upon the metal surfaces [8].

Such conclusions are confirmed by IR spectral analysis. After the corrosion tests, the intensity of the peaks corresponding to iron oxides (Fe3O4 and Fe2O3) was higher, indicating the presence of such oxides; at the same time the iron peak intensity was lower. This explains why organic salts are present on the steel surface, while the content of iron is lower. The same phenomenon was observed for the aluminum surface. A higher intensity of the Al2O3 peak and lower intensity of the aluminum peak indicates the formation of organic salts. Aluminum reacts quite readily with atmospheric oxygen, generating Al2O3, and tends to adsorb organic compounds. It is safe to say that the formation of an aluminum oxide film inhibits corrosion reactions with the metal surface. Oxygen, dissolved in biodiesel induces further formation of the aluminum

From among copper, aluminum, carbon steel and stainless steel, the rate of corrosion in biodiesel was the highest for copper (0.02334 mm/year), second highest for carbon steel (0.01819 mm/year), 0.00324 mm/year for aluminum and 0.00087 mm/year for stainless steel. The corrosion products included mainly fatty acid salts, and metal oxides. It was also dem‐ onstrated that decomposition of biodiesel is catalyzed by elements such as copper and iron,

Copper and copper alloys undergo corrosion in fuels probably according to the same mecha‐ nism as iron and iron alloys. However, the corrosion processes taking place on copper are less destructive and slower – corrosion pits, if present all, are limited to extreme conditions; corrosion is usually manifested by discoloration of the metal or alloy surface. It was demon‐ strated that even the lowest concentrations of sulfur present in the fuel will take part in corrosion reactions taking place on copper, forming sulfides on its surface. Destructive oxidation reactions are catalyzed by Cu2+ ions which pass into the fuel due to corrosion processes. According to literature, the effect is observed when the concentration of copper ions

Oxidation of biodiesel leads to the formation of various products such as peroxides and hydroperoxides. During degradation, these products transform into shorter-chain compounds such as low-molecular weight acids, aldehydes, ketones, and alcohols [9]. Moreover, oxidative polymerization may lead to the formation of macromolecular chemical compounds [10]. Thus, oxidation of biodiesel intensifies its corrosive effect, affecting the physico-chemical properties of the fuel [7]. The corrosiveness of biodiesel is also connected with the presence in it of

Studies on the effect of light, temperatures, and selected metals on corrosiveness and degra‐ dation of biodiesel are reported in [11]. The corrosiveness studies were carried out for copper and brass using the immersion method according to the standards ASTM G31 and ASTM G1.

oxide film, preventing access to the surface.

138 Storage Stability of Fuels

creating an environment for corrosion of engine components [8].

**2.3. Corrosive effect of biodiesel on copper and brass**

in the fuel is between 100 and 3000 ppm [2].

contaminants, such as: water, alcohol, free fatty acids.

The presence of alcohols in the fuel has a significant effect on its corrosion behavior. The alcohol itself is a corrosive medium, additionally, it is able to dissolve in and be mixed with water at any ratio. Studies on the effect of bioethanol on corrosion for an aluminum alloy (A348) were described in [12]. Corrosive properties were tested by its immersion in fuel blend samples, made of unleaded gasoline with various concentrations of bioethanol (10, 15 and 20% (v/v)). The tests were carried out at temperatures 60, 80 and 100o C, by immersing the test metal samples in the prepared blends for 24 hours. The test results indicate the presence of local corrosion pits on the surface of the aluminum alloy plates after continuing the tests for 24 hours at a temperature of 100o C. The extent of corrosion was higher for higher concentrations of ethanol in the blends, as indicated by the higher number of corrosion pits on the surface of the test alloy samples. No corrosion damage was observed at lower temperatures (60 and 80o C).

Presumably, a protective film of hydrated aluminum oxide was formed on the aluminum alloy surface during the tests at temperatures below and including 80o C. Only after increasing the temperature to 100o C was the reaction triggered between aluminum and bioethanol (reaction 8).

$$\text{6C}\_2\text{H}\_5\text{OH} + 2\text{Al} \rightarrow 3\text{H}\_2 + 2\text{Al} \text{(C}\_2\text{H}\_5\text{O})\_3 \tag{8}$$

The reaction involves substitution of the aluminum atom originating from the metal or from the oxide film with a hydrogen atom originating from bioethanol; hydrogen and aluminum acetate are formed.

Reported in [13] are results of studies, intended to determine the effect of oxygen present in the fuel on the corrosive properties of aluminum alloy. The test fuel blend was composed of 80% unleaded gasoline and 20% of 99.9% pure ethanol. The fuel blends were heated up to 100o C after being subjected to bubbling with nitrogen or gaseous oxygen for 2 hours. This provided fuels with different concentrations of oxygen. The effect of oxygen, dissolved in the fuel, on a high-temperature corrosion of aluminum alloy was investigated in electrochemical tests, using impedance spectroscopy and by evaluating the condition of the surface of the aluminum alloy samples before and after the tests. It was shown that corrosion resistance was increased with the increasing concentration of oxygen dissolved in the fuel, indicating the passive film formation on the test sample surface. On the other hand, it was demonstrated that bioethanol may be oxidized by the oxygen dissolved in the fuel, generating acetic acid and water. Each of the products is able to have an effect on corrosion of aluminum alloys. In further tests, the authors attempted to determine the effect of those factors on aluminum alloy destruction processes: to achieve this, specified amounts of acetic acid and/or water (0, 0.1, 0.5, 1.0 % v/v) were added to a fuel blend comprising 80% unleaded gasoline and 20% bioethanol after passing nitrogen through the blend. The test alloy plates were immersed for 6 hours in the prepared fuel blends after heating the blends to 100o C. It was demonstrated that water had a beneficial effect on the corrosion resistance of the aluminum alloy, having formed a hydrated aluminum oxide film on the metal surface, which acted as a protective film. Changes on the surface of the alloy sample were not observed for blends with a water content of more than 0.5 % v/v. Addition of acetic acid worked quite differently: it had a harmful effect, destroying the protective film and causing corrosion pits even at as low concentrations as 0.1 % v/v, and the process was intensified by addition of more acetic acid.

#### **3. Corrosion processes in fuels during storage**

Storage of fuels is accompanied by various processes, including oxidation, condensation, and polymerization. Contaminants which were detected during long-term storage of fuels included products of corrosion, organic particulate matter, and deposits. Part of such impur‐ ities tended to settle at the tank bottom and side walls, although most of them remained in the fuel and were present in the engine chamber during its operation. Such impurities were not flammable, they tended to escalate soot formation during combustion, affecting the engine's performance. Certain components and impurities (iron oxides, sulfides) also had an effect on the fuels tribology, causing increased wear and tear of the surfaces of the engine's major components. That is why investigating the influence of the storage process on the durability of the selected materials and on the quality of the fuel itself seemed so important. That fuels have a corrosive effect on materials they contact in at every step of their life cycle, is an established fact. What remains to be provided is the findings of studies describing the influence of the degree of fuel ageing on its corrosiveness – both to the materials applied in design solutions for storage tanks, and to the materials applied in vehicle engines. It is the objective of this paper to find out whether and how, if at all, the corrosive effect of various fuels varies in time during storage.

#### **3.1. Corrosiveness tests of gasoline and diesel fuels**

The gasoline and diesel fuels which are the national strategic reserve have a zero content of bio-components. The content of bio-components in fuels is known to affect the kinetics of corrosion processes. In order to determine the actual effect of bio-components on the rate of corrosion, and on fuel ageing processes, tests were carried out using fuels with and without a content of bio-components.

The following fuels were used in the tests:

provided fuels with different concentrations of oxygen. The effect of oxygen, dissolved in the fuel, on a high-temperature corrosion of aluminum alloy was investigated in electrochemical tests, using impedance spectroscopy and by evaluating the condition of the surface of the aluminum alloy samples before and after the tests. It was shown that corrosion resistance was increased with the increasing concentration of oxygen dissolved in the fuel, indicating the passive film formation on the test sample surface. On the other hand, it was demonstrated that bioethanol may be oxidized by the oxygen dissolved in the fuel, generating acetic acid and water. Each of the products is able to have an effect on corrosion of aluminum alloys. In further tests, the authors attempted to determine the effect of those factors on aluminum alloy destruction processes: to achieve this, specified amounts of acetic acid and/or water (0, 0.1, 0.5, 1.0 % v/v) were added to a fuel blend comprising 80% unleaded gasoline and 20% bioethanol after passing nitrogen through the blend. The test alloy plates were immersed for 6 hours in

a beneficial effect on the corrosion resistance of the aluminum alloy, having formed a hydrated aluminum oxide film on the metal surface, which acted as a protective film. Changes on the surface of the alloy sample were not observed for blends with a water content of more than 0.5 % v/v. Addition of acetic acid worked quite differently: it had a harmful effect, destroying the protective film and causing corrosion pits even at as low concentrations as 0.1 % v/v, and

Storage of fuels is accompanied by various processes, including oxidation, condensation, and polymerization. Contaminants which were detected during long-term storage of fuels included products of corrosion, organic particulate matter, and deposits. Part of such impur‐ ities tended to settle at the tank bottom and side walls, although most of them remained in the fuel and were present in the engine chamber during its operation. Such impurities were not flammable, they tended to escalate soot formation during combustion, affecting the engine's performance. Certain components and impurities (iron oxides, sulfides) also had an effect on the fuels tribology, causing increased wear and tear of the surfaces of the engine's major components. That is why investigating the influence of the storage process on the durability of the selected materials and on the quality of the fuel itself seemed so important. That fuels have a corrosive effect on materials they contact in at every step of their life cycle, is an established fact. What remains to be provided is the findings of studies describing the influence of the degree of fuel ageing on its corrosiveness – both to the materials applied in design solutions for storage tanks, and to the materials applied in vehicle engines. It is the objective of this paper to find out whether and how, if at all, the corrosive effect of various fuels varies

The gasoline and diesel fuels which are the national strategic reserve have a zero content of bio-components. The content of bio-components in fuels is known to affect the kinetics of

C. It was demonstrated that water had

the prepared fuel blends after heating the blends to 100o

140 Storage Stability of Fuels

the process was intensified by addition of more acetic acid.

**3. Corrosion processes in fuels during storage**

**3.1. Corrosiveness tests of gasoline and diesel fuels**

in time during storage.


The above-mentioned fuels were stored in underground storage tanks to cause their ageing in field conditions. Samples were collected from the storage tanks at certain intervals in order to find out how the physico-chemical properties of the stored fuels were changing and to test the fuels for their corrosive effect. The fuel samples were collected after 6 and 12 months of storage. Tests of the corrosive effect of fuels during storage were carried out using test samples of the following metals or alloys:


The above-mentioned materials are commonly used as structural materials for vehicle engines. For instance, aluminum alloys are used for making engine blocks, cylinder heads, oil sumps, drive shafts, and rocker arms. Owing to its good conductive properties, copper is used for making windings for alternators, starters, or ignition coils. Brass is used for tubes in fuel distribution systems, tubing for coolers, electrical connections. Steel is used for making major components, including connecting rods, timing gear, and piston pins. Moreover, each of the selected metals and alloys has a different corrosion resistance (different electrochemical potential), which adds to the comprehensiveness of evaluation of the corrosive behavior of fuels. If a fuel contains factors of corrosiveness, such as sulfur compounds, organic acids, and water soluble inorganic acids and bases, in the presence of water and oxygen, each of the abovementioned metal/alloy plates will be affected by electrochemical corrosion, although the process will be running at different rates. A copper plate with an electrochemical potential of 0.337 V in relation to the hydrogen electrode is classified as a semi-noble metal. In theory, its electrochemical corrosion resistance is high although, if water soluble acids are present (pH below 6.5), it may be corroded in oxygen depolarization conditions. The corrosion resistance of steel depends on its chemical composition. Higher carbon concentrations have an adverse effect on corrosion resistance (C45 grade steel is expected to be corroded sooner than S235JR grade).

Test samples of the selected materials in the form of 15 x 50 x 2 mm plates were sanded with abrasive paper with different grain-sizes, so as to obtain smooth and scratch-free test surfaces. The resulting plates were then defatted with acetone and dried.

#### **3.2. Corrosiveness tests of fuels by the immersion method**

The tests were carried out by immersion, using a dedicated, custom-made glass connected vessel which is illustrated in Figure 3. One part of the glass vessel was filled with 100 ml of the test fuel and the other remained empty. The vessel was heated for about 2 minutes to remove any air therefrom, so that the volume just above the fuel was filled with fuel vapors. Both parts of the vessel were then plugged with stoppers having plates of a same kind attached to them. The plates were weighed previously to an accuracy of 0.0001 g, using an analytical balance. The length of the threads was selected so as to enable one plate to be fully immersed in the fuel in one part of the vessel. The other plate, attached in the other part of the vessel, was suspended at the same height. The resulting measuring vessel was placed in a Binder KBF/ KBF-ICH chamber in an oxygen free environment.

**Figure 3.** Measuring vessel diagram

The tests were carried out in mixed cycles of ambient temperature and elevated temperature, as follows, respectively:


The duration of a single cycle was 24 hours, including 8 hours at an elevated temperature and the other 16 hours at an ambient temperature. The tests were completed after 16.5 cycles. The vessel remained at elevated temperatures for a total of 100 hours. The conditions simulated temperature fluctuations between day and night, prevailing in storage tanks.

After the tests were completed, the plates were taken out from the vessels, cleaned with acetone, dried, and weighed to the same accuracy as before the measurements.

Corrosion was measured in terms of the sample weight changes before and after exposure in fuels with different degrees of ageing. In addition, the condition of the plate surfaces was visually examined and evaluated with regard to the size, distribution, and type of changes on the plate surfaces.

#### **4. Corrosive effect on metals**

The corrosive effect of petroleum products was tested by two groups of methods: visual evaluation of samples of materials after exposure in fuels – this only enabled finding out whether corrosion did or not take place. If corrosion changes have taken place, the method provides no information on the rate of the process, therefore, it is important to add to the information a specific quantitative description of the changes taking place.

Changes in the weight of the test material before and after exposure in the given fuel were adopted as the criterion of the evaluation of the corrosive effect of the test fuels. To enable a comparison of the changes taking place in the test metals and steels, it was necessary to use a parameter, taking into account the different densities of the test materials. This purpose is served by linear corrosion rate, defined as the cross-sectional loss per year [mm/year]. The linear corrosion rate was found from the following formula:

$$K\_L = \frac{(m\_1 - m\_2) \times 24 \times 365}{\rho T \bar{s} \times 1000} = 8.76 \times \frac{A m}{\rho T \bar{s}}$$

wherein:

Test samples of the selected materials in the form of 15 x 50 x 2 mm plates were sanded with abrasive paper with different grain-sizes, so as to obtain smooth and scratch-free test surfaces.

The tests were carried out by immersion, using a dedicated, custom-made glass connected vessel which is illustrated in Figure 3. One part of the glass vessel was filled with 100 ml of the test fuel and the other remained empty. The vessel was heated for about 2 minutes to remove any air therefrom, so that the volume just above the fuel was filled with fuel vapors. Both parts of the vessel were then plugged with stoppers having plates of a same kind attached to them. The plates were weighed previously to an accuracy of 0.0001 g, using an analytical balance. The length of the threads was selected so as to enable one plate to be fully immersed in the fuel in one part of the vessel. The other plate, attached in the other part of the vessel, was suspended at the same height. The resulting measuring vessel was placed in a Binder KBF/

The tests were carried out in mixed cycles of ambient temperature and elevated temperature,

C for diesel fuel containing up to 7% (V/V) FAME (B7) and with no FAME (B0),

The duration of a single cycle was 24 hours, including 8 hours at an elevated temperature and the other 16 hours at an ambient temperature. The tests were completed after 16.5 cycles. The vessel remained at elevated temperatures for a total of 100 hours. The conditions simulated

After the tests were completed, the plates were taken out from the vessels, cleaned with

Corrosion was measured in terms of the sample weight changes before and after exposure in fuels with different degrees of ageing. In addition, the condition of the plate surfaces was

octane gasoline containing a maximum of 15% ETBE and no bioethanol (E0).

temperature fluctuations between day and night, prevailing in storage tanks.

acetone, dried, and weighed to the same accuracy as before the measurements.

C for 95 octane gasoline containing a maximum of 5% (V/V) bioethanol (E5) and for 98

The resulting plates were then defatted with acetone and dried.

**3.2. Corrosiveness tests of fuels by the immersion method**

KBF-ICH chamber in an oxygen free environment.

**Figure 3.** Measuring vessel diagram

as follows, respectively:

142 Storage Stability of Fuels

**•** 40o

**•** 25o

m1 – sample weight before corrosion [g],

m2 – sample weight after corrosion [g],

∆m – change in sample weight [g],

ρ – density of material [g/cm<sup>3</sup> ],

T – time of exposure at elevated temperatures [hr],

S – surface area of sample [m2 ].

Findings for each of the test fuels after 6 and 12 months of storage are given below.

#### **4.1. Corrosive effect of 95 octane gasoline containing ethanol**

In the corrosive-effect tests for commercial 95 octane gasoline (containing a maximum of 5 % V/V of ethanol) after 6 months of storage, weight changes were observed for all of the exposed metal and steel plates. Such weight changes were recorded after exposure to liquid gasoline and to gasoline vapors. The linear rate of the process of corrosion was found from weight changes and density of the metal (steel), taking into account the duration of exposure of the test samples at elevated temperature (Figure 4).

Based on the test results, it was found that such corrosion rates are rather insignificant, and fall in the range from 0.001 to 0.004 mm/year. The highest corrosion rate was recorded in the high corrosion resistance) – Table 1.

case of lead samples, both for the plate immersed in the liquid gasoline, and for the one exposed to its vapors.The lowest corrosion rates in the test gasoline were recorded for the plates made of S235JR (constructional steel), which indicates its optimum corrosion resistance. For the other materials, such as copper, brass, aluminum and C45 carbon steel, corrosion rates were similar. None of the materials was affected by corrosion to a significant degree, which was assessed as corrosion resistance grade 2 – materials with high corrosion resistance – Table 1. carbon steel, corrosion rates were similar.None of the materials was affected by corrosion to a significant degree, which was assessed as corrosion resistance grade 2 – materials with

**Figure 4.** Linear corrosion rate for the test metals and steel, obtained for 95 octane gasoline after 6 months of storage

Figure 4. Linear corrosion rate for the test **metals and steel, obtained for 95 octane** 


resistance **Table 1.** Corrosion resistance groups

Table 1. Corrosion resistance groups

With low resistance V 8 1.0 - 5.0 9 5.0 - 10.0 Non-resistant VI 10 > 10.0 Figure 5 shows the appearance of the plates after the tests. Only the copper plate was slightly discolored after being immersed in gasoline. Its discoloration was uniform and present all over the surface.

7 0.5 -1.0

case of lead samples, both for the plate immersed in the liquid gasoline, and for the one exposed to its vapors.The lowest corrosion rates in the test gasoline were recorded for the plates made of S235JR (constructional steel), which indicates its optimum corrosion resistance. For the other materials, such as copper, brass, aluminum and C45 carbon steel, corrosion rates were similar. None of the materials was affected by corrosion to a significant degree, which was assessed as

carbon steel, corrosion rates were similar.None of the materials was affected by corrosion to a significant degree, which was assessed as corrosion resistance grade 2 – materials with

Figure 4. Linear corrosion rate for the test **metals and steel, obtained for 95 octane** 

**Figure 4.** Linear corrosion rate for the test metals and steel, obtained for 95 octane gasoline after 6 months of storage

**Corrosion resistance group Symbol Corrosion resistance grade Vp [mm/year]** Entirely resistant I 1 > 0.0001

Entirely resistant I 1 > 0.0001

Non-resistant VI 10 > 10.0

Figure 5 shows the appearance of the plates after the tests. Only the copper plate was slightly discolored after being immersed in gasoline. Its discoloration was uniform and present all over

**grade Vp [mm/year]**

2 0.001- 0.005 3 0.005 - 0.01

4 0.01 - 0.05 5 0.05 - 0.1

6 0.1 - 0.5 7 0.5 -1.0

8 1.0 - 5.0 9 5.0 - 10.0

2 0.001- 0.005

3 0.005 - 0.01

4 0.01 - 0.05

5 0.05 - 0.1

6 0.1 - 0.5

7 0.5 -1.0

8 1.0 - 5.0

9 5.0 - 10.0

**resistance group Symbol Corrosion resistance** 

IV

Non-resistant VI 10 > 10.0

corrosion resistance grade 2 – materials with high corrosion resistance – Table 1.

high corrosion resistance) – Table 1.

144 Storage Stability of Fuels

**gasoline after 6 months of storage** 

Highly resistant II

Resistant III

With low resistance V

Table 1. Corrosion resistance groups

**Corrosion**

Highly resistant II

Resistant III

With reduced resistance IV

With low resistance V

**Table 1.** Corrosion resistance groups

With reduced resistance

the surface.

**Figure 5.** Appearance of samples after corrosion tests for 95 octane gasoline after 6 months of storage a) copper, b) brass c) lead, d) aluminum, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors) Figure 5. Appearance of samples after corrosion tests for 95 octane gasoline after 6 months of storage a) copper, b) brass c) lead, d) aluminum, e) C45 steel and f) S235JR steel (left:

A similar phenomenon was observed for the same gasoline after 12 months of storage: exposure did not cause any significant changes in the test materials. Here again, the highest corrosion rate was observed for lead, although it was slightly lower, compared with that for gasoline after 6 months of storage (Figure 6). sample exposed to liquid, right: sample exposed to vapors) A similar phenomenon was observed for the same gasoline after 12 months of storage: exposure did not cause any significant changes in the test materials. Here again, the highest corrosion rate was observed for lead, although it was slightly lower, compared with that for gasoline after 6 months of storage (Figure 6).

Figure 6. Linear corrosion rate for the test metals and steel, obtained for 95 octane gasoline **Figure 6.** Linear corrosion rate for the test metals and steel, obtained for 95 octane gasoline after 12 months of storage

appeared to be much lower than that for gasoline after 6 months of storage. This is probably due to the formation, on the surface of aluminum, of a passive film (the metal is capable of

Interestingly enough, the calculated corrosion rate in the case of aluminum

after 12 months of storage

Interestingly enough, the calculated corrosion rate in the case of aluminum appeared to be much lower than that for gasoline after 6 months of storage. This is probably due to the formation, on the surface of aluminum, of a passive film (the metal is capable of self-passiva‐ tion). This is connected with the presence of a thin film of oxygen or other compounds on the metal surface. The film is a specific barrier to the environment. Hence, passivation is accom‐ panied by a decrease in the rate of degradation of the substrate. In the case of copper, brass, and the S235JR steel, corrosion rates after 12 months were roughly twice as high, compared with the tests for gasoline after 6 months of storage. On the other hand, one must bear in mind that the results are burdened with a rather serious error, which may be connected both with the fact that the plates were not uniform, and with the complexity of the processes leading to corrosiveness. self-passivation). This is connected with the presence of a thin film of oxygen or other compounds on the metal surface. The film is a specific barrier to the environment. Hence, passivation is accompanied by a decrease in the rate of degradation of the substrate. In the

The above results were confirmed also by observation of the plates after exposure in the test fuel and its vapors. No changes were visible to the naked eye either on the plates exposed to the liquid gasoline or to its vapors. All of the test materials after exposure in a gasoline containing bioethanol after 12 months of storage were rated as materials with corrosion resistance grade 2. case of copper, brass, and the S235JR steel, corrosion rates after 12 months were roughly twice as high, compared with the tests for gasoline after 6 months of storage. On the other hand, one must bear in mind that the results are burdened with a rather serious error, which may be connected both with the fact that the plates were not uniform, and with the complexity of the processes leading to corrosiveness. The above results were confirmed also by observation of the plates after exposure in the test fuel and its vapors. No changes were visible to the naked eye either on the plates

#### **4.2. Corrosive effect of 98 octane gasoline containing ETBE** exposed to the liquid gasoline or to its vapors. All of the test materials after exposure in a gasoline containing bio-ethanol after 12 months of storage were rated as materials with

given environment (corrosion resistance grades 1-2).

corrosion resistance grade 2.

after 6 months of storage

For 98 octane gasoline containing ETBE after 6 months of storage, the highest corrosion rate was recorded for lead (Figure 7). Interestingly enough, in this case, a higher corrosion rate was noted for the plate exposed to the gasoline vapors. The lowest corrosion rate after exposure in gasoline was recorded for brass. For the other metals and steels, the rates were comparable. One should bear in mind that the ranges of linear corrosion rate for all of the test materials were rather not large, indicating their good corrosion resistance in a given environment (corrosion resistance grades 1-2). **4.2. Corrosive effect of 98 octane gasoline containing ETBE**  For 98 octane gasoline containing ETBE after 6 months of storage, the highest corrosion rate was recorded for lead (Figure 7). Interestingly enough, in this case, a higher corrosion rate was noted for the plate exposed to the gasoline vapors. The lowest corrosion rate after exposure in gasoline was recorded for brass. For the other metals and steels, the rates were comparable. One should bear in mind that the ranges of linear corrosion rate for all of the test materials were rather not large, indicating their good corrosion resistance in a

**Figure 7.** Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline after 6 months of storage

Figure 7. Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline

condition after the test was similar to that before the test. The appearance of the plates after

the tests for gasoline after 6 months of storage is shown in Figure 8.

No significant changes were noted by observation of the plate surfaces, their

No significant changes were noted by observation of the plate surfaces, their condition after the test was similar to that before the test. The appearance of the plates after the tests for gasoline after 6 months of storage is shown in Figure 8.

Interestingly enough, the calculated corrosion rate in the case of aluminum appeared to be much lower than that for gasoline after 6 months of storage. This is probably due to the formation, on the surface of aluminum, of a passive film (the metal is capable of self-passiva‐ tion). This is connected with the presence of a thin film of oxygen or other compounds on the metal surface. The film is a specific barrier to the environment. Hence, passivation is accom‐ panied by a decrease in the rate of degradation of the substrate. In the case of copper, brass, and the S235JR steel, corrosion rates after 12 months were roughly twice as high, compared with the tests for gasoline after 6 months of storage. On the other hand, one must bear in mind that the results are burdened with a rather serious error, which may be connected both with the fact that the plates were not uniform, and with the complexity of the processes leading to

The above results were confirmed also by observation of the plates after exposure in the test fuel and its vapors. No changes were visible to the naked eye either on the plates exposed to the liquid gasoline or to its vapors. All of the test materials after exposure in a gasoline containing bioethanol after 12 months of storage were rated as materials with corrosion

The above results were confirmed also by observation of the plates after exposure

For 98 octane gasoline containing ETBE after 6 months of storage, the highest

self-passivation). This is connected with the presence of a thin film of oxygen or other compounds on the metal surface. The film is a specific barrier to the environment. Hence, passivation is accompanied by a decrease in the rate of degradation of the substrate. In the case of copper, brass, and the S235JR steel, corrosion rates after 12 months were roughly twice as high, compared with the tests for gasoline after 6 months of storage. On the other hand, one must bear in mind that the results are burdened with a rather serious error, which may be connected both with the fact that the plates were not uniform, and with the

For 98 octane gasoline containing ETBE after 6 months of storage, the highest corrosion rate was recorded for lead (Figure 7). Interestingly enough, in this case, a higher corrosion rate was noted for the plate exposed to the gasoline vapors. The lowest corrosion rate after exposure in gasoline was recorded for brass. For the other metals and steels, the rates were comparable. One should bear in mind that the ranges of linear corrosion rate for all of the test materials were rather not large, indicating their good corrosion resistance in a given environment

corrosion rate was recorded for lead (Figure 7). Interestingly enough, in this case, a higher corrosion rate was noted for the plate exposed to the gasoline vapors. The lowest corrosion rate after exposure in gasoline was recorded for brass. For the other metals and steels, the rates were comparable. One should bear in mind that the ranges of linear corrosion rate for all of the test materials were rather not large, indicating their good corrosion resistance in a

Figure 7. Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline

**Figure 7.** Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline after 6 months of storage

condition after the test was similar to that before the test. The appearance of the plates after

the tests for gasoline after 6 months of storage is shown in Figure 8.

No significant changes were noted by observation of the plate surfaces, their

in the test fuel and its vapors. No changes were visible to the naked eye either on the plates exposed to the liquid gasoline or to its vapors. All of the test materials after exposure in a gasoline containing bio-ethanol after 12 months of storage were rated as materials with

**4.2. Corrosive effect of 98 octane gasoline containing ETBE**

**4.2. Corrosive effect of 98 octane gasoline containing ETBE** 

complexity of the processes leading to corrosiveness.

given environment (corrosion resistance grades 1-2).

corrosiveness.

146 Storage Stability of Fuels

resistance grade 2.

(corrosion resistance grades 1-2).

after 6 months of storage

corrosion resistance grade 2.

**Figure 8.** Appearance of samples after corrosion tests for 98 octane gasoline after 6 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

Tests for 98 gasoline containing ETBE indicate that, after 12 months of storage, corrosion rates were lower (for aluminum, lead, C45 steel and S235JR steel) or remained at a similar level (copper, C45 steel and S235JR steel), which is a desirable phenomenon (Figure 9).

In the case of brass plates immersed in gasoline, corrosion rates after 12 months and after 6 months of storage were similar. In the case of brass, a difference was noted for exposure to vapors – corrosion rate was 3 times as high after 12 months of storage. The calculated corrosion rates for all the metals and alloys were very low, indicating that the test materials demonstrate good resistance. A decrease in corrosion rates after 12 months may be connected with changes in the fuel taking place during storage, which promote passivation of metal surfaces (formation of thin film of metal oxides or organic salts).

No changes were observed on the test plate surfaces for gasoline after 12 months of storage (Figure 10).

after 12 months of storage

(Figure 9).

Figure 8. Appearance of samples after corrosion tests for 98 octane gasoline after 6 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left:

a similar level (copper, C45 steel and S235JR steel), which is a desirable phenomenon

sample exposed to liquid, right: sample exposed to vapors)

Figure 9. Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline **Figure 9.** Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline after 12 months of storage

**Figure 10.** Appearance of samples after corrosion tests for 98 octane gasoline after 12 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

#### **4.3. Corrosive effect of diesel fuel containing up to 7% FAME**

For lead samples after exposure in the diesel fuel containing up to 7% FAME, corrosion rate was very high after just 6 months of storage (Figure 11).

Figure 10. Appearance of samples after corrosion tests for 98 octane gasoline after 12 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR

steel (left: sample exposed to liquid, right: sample exposed to vapors)

corrosion rate was very high after just 6 months of storage (Figure 11).

No changes were observed on the test plate surfaces for gasoline after 12 months

of storage (Figure 10).

Figure 11. Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage **Figure 11.** Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage

Corrosion rate for lead was more than 33 times as high as that for brass. According to the data shown in Table 1, lead may be classified as a material with corrosion resistance grade 7 (out of 10 grades). The other test metals and steels showed better corrosion resistance: copper and brass were classified as materials with corrosion resistance grade 4, aluminum, C45 steel and S235JR steel – as materials with corrosion resistance grade 2. Corrosion rate for lead was more than 33 times as high as that for brass. According to the data shown in Table 1, lead may be classified as a material with corrosion resistance grade 7 (out of 10 grades). The other test metals and steels showed better corrosion resistance: copper and brass were classified as materials with corrosion resistance grade 4, aluminum, C45 steel and S235JR steel – as materials with corrosion resistance grade 2.

For a better illustration of their corrosion rates, Figure 12 shows the test results obtained for the test materials with the exception of lead. For a better illustration of their corrosion rates, Figure 12 shows the test results

obtained for the test materials with the exception of lead.

after exposure in the test fuel.

F igure 12. Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage **Figure 12.** Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage

**Figure 10.** Appearance of samples after corrosion tests for 98 octane gasoline after 12 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to

Figure 8. Appearance of samples after corrosion tests for 98 octane gasoline after 6 months of storage a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left:

corrosion rates were lower (for aluminum, lead, C45 steel and S235JR steel) or remained at a similar level (copper, C45 steel and S235JR steel), which is a desirable phenomenon

Figure 9. Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline

**Figure 9.** Linear corrosion rate for the test metals and steel, obtained for 98 octane gasoline after 12 months of storage

In the case of brass plates immersed in gasoline, corrosion rates after 12 months and after 6 months of storage were similar. In the case of brass, a difference was noted for exposure to vapors – corrosion rate was 3 times as high after 12 months of storage. The calculated corrosion rates for all the metals and alloys were very low, indicating that the test materials demonstrate good resistance. A decrease in corrosion rates after 12 months may be connected with changes in the fuel taking place during storage, which promote passivation

of metal surfaces (formation of thin film of metal oxides or organic salts).

Tests for 98 gasoline containing ETBE indicate that, after 12 months of storage,

sample exposed to liquid, right: sample exposed to vapors)

For lead samples after exposure in the diesel fuel containing up to 7% FAME, corrosion rate

**4.3. Corrosive effect of diesel fuel containing up to 7% FAME**

was very high after just 6 months of storage (Figure 11).

vapors)

(Figure 9).

148 Storage Stability of Fuels

after 12 months of storage

Among these test materials, the highest corrosion rate was observed for brass, the lowest was observed for the S235JR steel. The appearance of the samples after subjecting them to the corrosive effect tests is Among these test materials, the highest corrosion rate was observed for brass, the lowest was observed for the S235JR steel.

shown in Figure 13. Significant changes were observed in the case of lead, following its direct exposure to diesel fuel (Figure 13 d)), as confirmed by the calculated corrosion rates. The lead plate is coated with corrosion products. No changes were detected in the lead plate after exposure to the fuel vapors. The copper and brass plates showed discoloration

Figure 13. Appearance of samples after corrosion tests for diesel fuel containing up to 7% FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and

f) S235JR steel (left: sample exposed to liquid, right: – after exposure in vapors)

The appearance of the samples after subjecting them to the corrosive effect tests is shown in Figure 13. Significant changes were observed in the case of lead, following its direct exposure to diesel fuel (Figure 13 d)), as confirmed by the calculated corrosion rates. The lead plate is coated with corrosion products. No changes were detected in the lead plate after exposure to the fuel vapors. The copper and brass plates showed discoloration after exposure in the test fuel.

**Figure 13.** Appearance of samples after corrosion tests for diesel fuel containing up to 7% FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: – after exposure in vapors)

In corrosive effect tests after storing diesel fuel for 12 months, lead appeared to be the material with the highest susceptibility to corrosion. The calculated linear corrosion rate for lead was approx. 0.450 mm/year, which is slightly lower than that obtained after exposure in diesel fuel after 6 months of storage. In the case of copper and brass, corrosion rate was much reduced (Figure 14).

Corrosion rates for aluminum, C45 steel and S235JR steel were comparable and very low, indicating their good resistance to the effect of diesel fuel as early as in the initial period of its storage.

Figure 15 shows the appearance of copper, brass, and aluminum plates after exposure in commercial diesel fuel after 12 months of storage. Macroscopic examination of the plates indicates no corrosion changes on their surface.

In corrosive effect tests after storing diesel fuel for 12 months, lead appeared to be

the material with the highest susceptibility to corrosion. The calculated linear corrosion rate

rate was much reduced (Figure 14).

The appearance of the samples after subjecting them to the corrosive effect tests is shown in Figure 13. Significant changes were observed in the case of lead, following its direct exposure to diesel fuel (Figure 13 d)), as confirmed by the calculated corrosion rates. The lead plate is coated with corrosion products. No changes were detected in the lead plate after exposure to the fuel vapors. The copper and brass plates showed discoloration after exposure in the test

**Figure 13.** Appearance of samples after corrosion tests for diesel fuel containing up to 7% FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: –

In corrosive effect tests after storing diesel fuel for 12 months, lead appeared to be the material with the highest susceptibility to corrosion. The calculated linear corrosion rate for lead was approx. 0.450 mm/year, which is slightly lower than that obtained after exposure in diesel fuel after 6 months of storage. In the case of copper and brass, corrosion rate was much reduced

Corrosion rates for aluminum, C45 steel and S235JR steel were comparable and very low, indicating their good resistance to the effect of diesel fuel as early as in the initial period of its

Figure 15 shows the appearance of copper, brass, and aluminum plates after exposure in commercial diesel fuel after 12 months of storage. Macroscopic examination of the plates

fuel.

150 Storage Stability of Fuels

after exposure in vapors)

(Figure 14).

storage.

indicates no corrosion changes on their surface.

Figure 14. Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage **Figure 14.** Linear corrosion rate for test samples of metals and steel, obtained for a diesel fuel containing up to 7% FAME after 6 months of storage

**Figure 15.** Appearance of samples after corrosion tests for diesel fuel containing up to 7% FAME after 6 months of storage: a) copper, b) brass c) aluminum d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors) FAME after 6 months of storage: a) copper, b) brass c) aluminum d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

Figure 15. Appearance of samples after corrosion tests for diesel fuel containing up to 7%

#### **4.4. Corrosive effect of diesel fuel containing no FAME**

The same kind of tests were carried out for a diesel fuel containing no FAME. Also in this case, the highest susceptibility to corrosion in a fuel stored for 6 months was found for lead (linear corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was recorded for brass; moreover, corrosion rate was higher for the brass samples immersed in the liquid fuel, compared with those obtained for the plates exposed to vapors. The same kind of tests were carried out for a diesel fuel containing no FAME. Also in this case, the highest susceptibility to corrosion in a fuel stored for 6 months was found for lead (linear corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was **Corrosive effect of diesel fuel containing no FAME**  The same kind of tests were carried out for a diesel fuel containing no FAME. Also in this case, the highest susceptibility to corrosion in a fuel stored for 6 months was found for

recorded for brass; moreover, corrosion rate was higher for the brass samples immersed in

lead (linear corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was

the liquid fuel, compared with those obtained for the plates exposed to vapors.

**Corrosive effect of diesel fuel containing no FAME** 

the aluminum and the steel samples (Figure 17).

Figure 16. Linear corrosion rate for the test metals and steels, obtained for diesel fuel containing ETBE after 6 months of storage **Figure 16.** Linear corrosion rate for the test metals and steels, obtained for diesel fuel containing ETBE after 6 months of storage Figure 16. Linear corrosion rate for the test metals and steels, obtained for diesel fuel

Same as in the case of the previous diesel fuel, high corrosion resistance was observed for the aluminum and the steel samples (Figure 17). Same as in the case of the previous diesel fuel, high corrosion resistance was observed for the aluminum and the steel samples (Figure 17). containing ETBE after 6 months of storage Same as in the case of the previous diesel fuel, high corrosion resistance was observed for

Figure 17. Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel with no FAME after 6 months of storage Figure 17. Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel with no FAME after 6 months of storage **Figure 17.** Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel with no FAME after 6 months of storage

Figure 18 shows the appearance of the plates after immersion tests for diesel fuel, after 6 months of storage.

corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was recorded for brass; moreover, corrosion rate was higher for the brass samples immersed in the liquid fuel,

in this case, the highest susceptibility to corrosion in a fuel stored for 6 months was found for lead (linear corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was recorded for brass; moreover, corrosion rate was higher for the brass samples immersed in

in this case, the highest susceptibility to corrosion in a fuel stored for 6 months was found for lead (linear corrosion rate of about 0.170 mm/year) (Figure 16). Such rate was much higher, compared with the other test metals and steels. The second highest corrosion rate was recorded for brass; moreover, corrosion rate was higher for the brass samples immersed in

Figure 16. Linear corrosion rate for the test metals and steels, obtained for diesel fuel

**Figure 16.** Linear corrosion rate for the test metals and steels, obtained for diesel fuel containing ETBE after 6 months

Figure 16. Linear corrosion rate for the test metals and steels, obtained for diesel fuel

Same as in the case of the previous diesel fuel, high corrosion resistance was observed for

Same as in the case of the previous diesel fuel, high corrosion resistance was observed for the

Same as in the case of the previous diesel fuel, high corrosion resistance was observed for

Figure 17. Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel

Figure 17. Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel

Figure 18 shows the appearance of the plates after immersion tests for diesel fuel, after 6 months

**Figure 17.** Linear corrosion rate for copper, aluminum and brass after exposure in diesel fuel with no FAME after 6

the liquid fuel, compared with those obtained for the plates exposed to vapors.

the liquid fuel, compared with those obtained for the plates exposed to vapors.

The same kind of tests were carried out for a diesel fuel containing no FAME. Also

The same kind of tests were carried out for a diesel fuel containing no FAME. Also

compared with those obtained for the plates exposed to vapors.

**Corrosive effect of diesel fuel containing no FAME** 

**Corrosive effect of diesel fuel containing no FAME** 

containing ETBE after 6 months of storage

containing ETBE after 6 months of storage

aluminum and the steel samples (Figure 17).

of storage

152 Storage Stability of Fuels

with no FAME after 6 months of storage

with no FAME after 6 months of storage

months of storage

of storage.

the aluminum and the steel samples (Figure 17).

the aluminum and the steel samples (Figure 17).

**Figure 18.** Appearance of samples after corrosion tests for a diesel fuel containing no FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f) S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

Changes were detected in the appearance of the plate surfaces for copper, brass, and lead which were exposed to the test fuel. Such changes were not observed in the case of plates after their exposure to the effect of vapors.

After storingthedieselfuelwithnoFAMEfor12months,thehighestcorrosionratewasrecorded for lead (0.165 mm/year). The other test metals and steels showed high corrosion resistance (grade 2) (Table 1), and linear corrosion rate did not exceed 0.001 mm/year (Figure 19).

The appearance of the test samples after corrosion tests for a diesel fuel collected after 12 months of storage is illustrated in Figure 20. The surface of the copper plate after direct exposure to the fuel had not changed, which was the case for diesel fuel after 6 months of storage. This is confirmed by the value of linear corrosion rate, which is 4 times as low for the tests in the fuel after 1 year of storage.

Changes in the appearance of the plate surfaces were detected only for lead, which was exposed to the diesel fuel.

mm/year (Figure 19).

after their exposure to the effect of vapors.

months of storage.

After storing the diesel fuel with no FAME for 12 months, the highest corrosion rate

Figure 18 shows the appearance of the plates after immersion tests for diesel fuel, after 6

Figure 18. Appearance of samples after corrosion tests for a diesel fuel containing no FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f)

Changes were detected in the appearance of the plate surfaces for copper, brass, and lead which were exposed to the test fuel. Such changes were not observed in the case of plates

S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

Figure 19. Linear corrosion rate for copper, aluminum, brass, C45 and S235JR steel after exposure in diesel fuel with no FAME after 12 months of storage **Figure 19.** Linear corrosion rate for copper, aluminum, brass, C45 and S235JR steel after exposure in diesel fuel with no FAME after 12 months of storage

**Figure 20.** Appearance of samples after corrosion tests for a diesel fuel containing no FAME after 12 months of storage: a) copper, b) brass c) aluminum d) lead e) C45 steel f) S235JR steel (left: sample exposed to liquid, right: sample ex‐ posed to vapors)

#### **5. Conclusion**

The present studies indicate that the test samples of gasolines showed a lower corrosive effect on the materials, selected for the corrosion tests, compared with diesel fuels. Regardless of the type of fuel, the highest rate of corrosion among the test samples of steel and other materials was observed for lead, both after 6 and after 12 months of storage. In the case of diesel fuels after 6 months of storage, a more pronounced effect on degradation of the test lead sample was observed for diesel fuel with FAME. Corrosion rate for lead in the diesel fuel with FAME was 3.5 times as high as that for the diesel fuel without FAME. For the other metals and alloys, tested in diesel fuel with FAME, corrosion rate was also higher, compared with the oil without the bio-component; this confirms literature reports relating to the adverse effect of the biocomponent on the durability of structural materials. A macroscopic examination of the condition of the plate surfaces, tested in diesel fuels, indicated changes in the surface of copper, brass, and lead samples. For gasolines, the highest weight losses were recorded, again, for lead; in the case of gasoline with ETBE, higher losses were observed for the plates exposed to the gasoline vapors, compared with those directly immersed in the fuel. This may be attributed to the different composition and higher vapor pressure of the gasoline with ETBE. Compati‐ bility with the test fuel samples was the highest in the case of steel and aluminum. Considering the duration of fuel storage time, corrosion rate was reduced for both test samples of diesel fuel after 12 months of storage, which is a beneficial phenomenon. On the other hand, the plates were exposed to the fuel for a rather short duration (approx. 17 days). In the aspect of the fuel blends' complexity and their storage for a relatively short period of time, the aforementioned tests ought to have been be carried out in fuels, exposed to storage for even longer durations. It seems that it would be beneficial to test the corrosiveness of the test fuel samples also with the use of other methods, such as impedance spectroscopy, techniques enabling a microscopic examination of the surface condition and composition, as well as measuring the rate of the corrosion, initiated in the early phase of storage.

#### **Author details**

Monika Ziółkowska1,2 and Dorota Wardzińska1\*

\*Address all correspondence to: d.wardzinska@pimot.eu

1 Department of Fuels, Biofuels and Lubricants, Automotive Industry Institute, Poland

2 Quality Centre, Automotive Industry Institute, Poland

#### **References**

**5. Conclusion**

posed to vapors)

The present studies indicate that the test samples of gasolines showed a lower corrosive effect on the materials, selected for the corrosion tests, compared with diesel fuels. Regardless of the

**Figure 20.** Appearance of samples after corrosion tests for a diesel fuel containing no FAME after 12 months of storage: a) copper, b) brass c) aluminum d) lead e) C45 steel f) S235JR steel (left: sample exposed to liquid, right: sample ex‐

Figure 18 shows the appearance of the plates after immersion tests for diesel fuel, after 6

Figure 18. Appearance of samples after corrosion tests for a diesel fuel containing no FAME after 6 months of storage: a) copper, b) brass c) aluminum, d) lead, e) C45 steel and f)

Changes were detected in the appearance of the plate surfaces for copper, brass, and lead which were exposed to the test fuel. Such changes were not observed in the case of plates

was recorded for lead (0.165 mm/year). The other test metals and steels showed high corrosion resistance (grade 2) (Table 1), and linear corrosion rate did not exceed 0.001

Figure 19. Linear corrosion rate for copper, aluminum, brass, C45 and S235JR steel after

**Figure 19.** Linear corrosion rate for copper, aluminum, brass, C45 and S235JR steel after exposure in diesel fuel with no

exposure in diesel fuel with no FAME after 12 months of storage

After storing the diesel fuel with no FAME for 12 months, the highest corrosion rate

S235JR steel (left: sample exposed to liquid, right: sample exposed to vapors)

after their exposure to the effect of vapors.

mm/year (Figure 19).

154 Storage Stability of Fuels

FAME after 12 months of storage

months of storage.


## **Autoxidation of Fuels During Storage**

Joanna Czarnocka, Anna Matuszewska and Małgorzata Odziemkowska

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59807

#### **1. Introduction**

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156 Storage Stability of Fuels

p.369-74

Petroleum fuels are expected not to show any chemical changes during storage under certain conditions. Yet, a slow process of uncontrollable oxidation, also called autoxidation or selfoxidation, of which the mechanism is not thoroughly investigated, may occur even in a stable storage environment. The problem of autoxidation of fuels has gained particular importance after the introduction of product streams originating from the deep-processing of petroleum, e.g., cracking, as components of fuels.

The chemical changes, involved in the degradation of fuels, are not very well known yet, therefore, it is hard to predict the duration of storage for such fuel or to control the rate of its ageing. Stability of fuels during storage depends on their chemical composition, especially, the presence in them of compounds containing heteroatoms of oxygen, sulfur, nitrogen, traces of metal ions which catalyze oxidation processes, as well as on their storage conditions, such as temperature, access of light, possibility to absorb oxygen.

Hydrocarbons, the essential component of petroleum-based fuels, are likely during storage to react with atmospheric oxygen and with one another. Products which originate from oxygen‐ ation will undergo further changes, resulting in change of coloration, the presence of nonvolatile, macromolecular substances (gum), as well as development of particulate matter followed by sediment/deposit [1]. There are many theories on the autoxidation of liquid hydrocarbons. One of them is based on the chain mechanism of radical reactions, formulated by Backstrom [2].

The theory is based on chain radical reactions with participation of peroxy and hydrocarbon free radicals, leading to the precipitation of substances which contaminate storage tanks, promote corrosion of transfer pipelines, cause filter plugging and similar problems in the fuel distribution system.

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Descriptions of the specific steps of radical reactions are available from numerous reports. The changes, at all times, involve the presence of peroxides, as shown in the diagrams below [3]:

**a.** Initiation:

$$\text{R-H} + \text{Initiator (e.g., light, } \begin{array}{c} \text{temperature, } \text{catalyst} \end{array} \rightarrow \text{R-} \bullet \text{(H} \bullet \text{)}$$

**b.** Propagation:

$$\begin{array}{c} \text{R}\bullet \text{+O}\_{2} \rightarrow \text{R-O-O}\bullet\\ \text{R-O-O}\bullet \text{+R-H} \rightarrow \text{R}\bullet \text{+R-O-H} \end{array}$$

**c.** Termination:

R• + R• →R-R RO2• + RO2• → ROOR + O2, inactive products (alcohols, ketones) R• + R-O-O• → ROOR RO• (or RO• 2 ) + ROOH → different products ROOH → non-radical products

**d.** Chain branching:

R-O-O-H→ RO•+•OH RH + RO•→ R• + ROH RH +•OH <sup>→</sup> R• +H2O

wherein: R−H – denotes hydrocarbon,

R• – hydrocarbon radical, R-O-O• – peroxide radical, R-O-O-H – hydroperoxide.

In the initiation step, the reactions generating hydrocarbon free radicals R● run at a very slow rate, especially at ambient temperatures. The process rate may be increased by temperature and the presence of transition metals. The position where the hydrogen atom is detached from the hydrocarbon molecule is determined by the force of the C-H bond and the resonanse energy of the radicals being formed; the higher the resonance energy, the weaker the C-H bond force [4].

The chain growth or propagation step consists of two reactions. In the first reaction, which is irreversible, the hydrocarbon radical being formed in the initiation step reacts instantaneously with molecular oxygen, whereby a peroxide radical is formed. In the second reaction, which determines the rate of chain propagation, the peroxide radical will attach a hydrogen atom after detaching it from another hydrocarbon molecule. This produces a hydroperoxide and a hydrocarbon radical which is able to react with another oxygen molecule in accordance with the first reaction of the propagation step. The rate of C-H bond cleavage (initiation – the first reaction of the propagation step), depends on the type of substitutes: those hydrocarbons which contain a tertiary hydrogen or a hydrogen in the alpha position relative to the double bond or to an aromatic ring, are the most susceptible to oxidation.

Descriptions of the specific steps of radical reactions are available from numerous reports. The changes, at all times, involve the presence of peroxides, as shown in the diagrams below [3]:

R-H + Initiator (e.g., *light*, *temperature*, *catalyst*) → R• +(H•)

R-O-O• + R-H → R•+ R-O-O-H

RO2• + RO2• → ROOR + O2, inactive products (alcohols, ketones)

R-O-O-H→ RO•+•OH RH + RO•→ R• + ROH RH +•OH <sup>→</sup> R• +H2O

R• – hydrocarbon radical, R-O-O• – peroxide radical, R-O-O-H – hydroperoxide.

In the initiation step, the reactions generating hydrocarbon free radicals R● run at a very slow rate, especially at ambient temperatures. The process rate may be increased by temperature and the presence of transition metals. The position where the hydrogen atom is detached from the hydrocarbon molecule is determined by the force of the C-H bond and the resonanse energy of the radicals being formed; the higher the resonance energy,

The chain growth or propagation step consists of two reactions. In the first reaction, which is irreversible, the hydrocarbon radical being formed in the initiation step reacts instantaneously

) + ROOH → different products

R• +O2→ R-O-O•

**a.** Initiation:

158 Storage Stability of Fuels

**b.** Propagation:

**c.** Termination:

R• + R• →R-R

(or RO• 2

wherein: R−H – denotes hydrocarbon,

the weaker the C-H bond force [4].

RO•

**d.** Chain branching:

R• + R-O-O• → ROOR

ROOH → non-radical products

The chain termination step includes recombination between the radicals and decomposition of hydroperoxides to non-radical products, such as alcohols, ketones, acids; they may react further, leading to macromolecular substances.

Branching of the reaction chain accelerates the formation of free radicals in the system – every single radical gives three more, which may participate in the process of propagation. That step is initiated by homolytic decomposition, at a slow rate, of the hydroperoxide R-O-O-H to the radicals RO• and •OH, which then very fast recombine with hydrocarbons, detaching the hydrogen atom therefrom. Cleavage of the O-O bond is characterized by a very high activation energy and it matters when temperature in the system is higher than 150˚C [5]. The process is also accelerated by transition metal ions. The chain branching reactions are very important in the aspect of product stability. At high temperature conditions, aldehydes and ketones are able to react, producing acids and macromolecular substances.

The necessary conditions for autoxidation to take place in a system are: the suitable pressure of oxygen (50 Tr) [6], temperature in the range 30-120˚C, the presence of a certain amount of hydroxides which initiate the aforementioned processes, or other factors which favor the formation of free radicals, for instance, active metal ions, water, microorganisms, light. Organic peroxides are merely intermediate products which may decompose to free, reactive radicals which initiate chain branching. In the initial stage of autoxidation, when the amount of free radicals in the system is small, the changes occur slowly. After the induction period, oxidation is rapidly accelerated due to the autocatalytic effect of intermediate products which accumu‐ late in the fuel, and due to the reaction chain branching. The rate of oxidation reaches its maximum value after which it slowly decreases [7].

How the autoxidation chain is formed in the absence of oxidation promoters such as sunlight, sources of free radicals and metals, remains to be unknown. For gasolines which contain cracked components, it is safe to assume that active radicals as well as traces of metal ions originating from catalysts are present in the system. Moreover, as shown by Nagpal J.M. et al. [8], a tendency to develop gum depends on the amount and type of unsaturated hydrocarbons, present in the fuel. Studies reported by Pereira et al. confirmed [9] that not all of the olefins which are present in fuels will be transformed into resins to the same extent. Among those studied, the olefins forming secondary allyl radicals (cyclohexene and 2,4-hexadiene) had the highest contribution to the formation of macromolecular substances. The stability of allyl and alkyl radicals and the autoxidation reaction mechanism, confirming the occurrence of chain radical reactions, were also discussed.

#### **2. Tests of RON 95 and RON 98 gasolines**

Gasolines and compounds generated in degradation process have a complex chemical structure, therefore, it is hard to develop a reliable test to determine oxidation stability of fuels during storage. Although there do exist standard laboratory procedures for evaluation of oxidation stability of gasolines at certain intervals during storage, such procedures do not enable prediction of the potential duration of their further storage. Such methods are based on accelerated ageing processes which require elevated temperatures, high concentration of oxygen in the test sample and the presence of free metal ions which catalyze the reaction. The chemistry of the reactions taking place in accelerated ageing conditions may be far from changes associated with autoxidation of fuels during ambient storage. Therefore, chemical stability tests for fuels (such as induction period, potential resin content) are characterized by different correlations to changes occurring in real conditions in the storage tank. As mentioned before, some of the compounds taking part in oxidative chain reactions in gasolines are organic peroxides. The process is also affected by the content of unsaturated compounds, especially the precursors of secondary allyl radicals (e.g., cyclohexene, 2,4-hexadiene). In reactions involving peroxide radicals, which may occur at low (ambient) temperatures, the peroxide radicals are attached to the double bond.

As part of their experimental work on autoxidation of gasolines, including the effect of organic peroxides and cyclic olefins on the process, the present authors have studied the correlation between changes in the chemical composition of fuel and its chemical stability after the lapse of a certain time of storage. Two types of winter-time gasoline with the research octane numbers (RON) of 95 and 98, respectively, compliant with the quality requirements of EN 228 were tested [10]. Oxygen compounds were present in both types, with the total oxygen being lower than 2.7%(m/m). The RON 95 gasoline contained ethanol and ethyl *tert*-butyl ether (EETB), while the RON 98 gasoline contained only ethyl *tert*-butyl ether. Moreover, gasoline samples doped with 5.3 M solution of *tert-*butyl-hydroperoxide (TBHP) in decane were prepared. TBHP, a stable organic substance and a convenient source of radicals, is used for simulating oxidized liquid conditions [11]. The concentration of TBHP in the fuel was 50 millimols per liter.

Moreover, gasolines were included in the test which were doped with cyclohexene, a repre‐ sentative of cyclic olefins contributing to the intensification of autoxidation processes. Owing to environmental requirements, the total content of olefins in the fuels had to be a maximum of 18% (v/v), therefore, the maximum concentration of cyclohexene was 3% (v/v).

The gasoline samples were stored for 6 months in laboratory conditions at 15˚C. Rapid ageing tests were carried out at certain intervals and chemical stability was monitored using selected methods.

The laboratory samples and methods of testing are shown in Table 1.


**Table 1.** Gasoline test samples and methods of test

**2. Tests of RON 95 and RON 98 gasolines**

160 Storage Stability of Fuels

radicals are attached to the double bond.

millimols per liter.

methods.

Gasolines and compounds generated in degradation process have a complex chemical structure, therefore, it is hard to develop a reliable test to determine oxidation stability of fuels during storage. Although there do exist standard laboratory procedures for evaluation of oxidation stability of gasolines at certain intervals during storage, such procedures do not enable prediction of the potential duration of their further storage. Such methods are based on accelerated ageing processes which require elevated temperatures, high concentration of oxygen in the test sample and the presence of free metal ions which catalyze the reaction. The chemistry of the reactions taking place in accelerated ageing conditions may be far from changes associated with autoxidation of fuels during ambient storage. Therefore, chemical stability tests for fuels (such as induction period, potential resin content) are characterized by different correlations to changes occurring in real conditions in the storage tank. As mentioned before, some of the compounds taking part in oxidative chain reactions in gasolines are organic peroxides. The process is also affected by the content of unsaturated compounds, especially the precursors of secondary allyl radicals (e.g., cyclohexene, 2,4-hexadiene). In reactions involving peroxide radicals, which may occur at low (ambient) temperatures, the peroxide

As part of their experimental work on autoxidation of gasolines, including the effect of organic peroxides and cyclic olefins on the process, the present authors have studied the correlation between changes in the chemical composition of fuel and its chemical stability after the lapse of a certain time of storage. Two types of winter-time gasoline with the research octane numbers (RON) of 95 and 98, respectively, compliant with the quality requirements of EN 228 were tested [10]. Oxygen compounds were present in both types, with the total oxygen being lower than 2.7%(m/m). The RON 95 gasoline contained ethanol and ethyl *tert*-butyl ether (EETB), while the RON 98 gasoline contained only ethyl *tert*-butyl ether. Moreover, gasoline samples doped with 5.3 M solution of *tert-*butyl-hydroperoxide (TBHP) in decane were prepared. TBHP, a stable organic substance and a convenient source of radicals, is used for simulating oxidized liquid conditions [11]. The concentration of TBHP in the fuel was 50

Moreover, gasolines were included in the test which were doped with cyclohexene, a repre‐ sentative of cyclic olefins contributing to the intensification of autoxidation processes. Owing to environmental requirements, the total content of olefins in the fuels had to be a maximum

The gasoline samples were stored for 6 months in laboratory conditions at 15˚C. Rapid ageing tests were carried out at certain intervals and chemical stability was monitored using selected

of 18% (v/v), therefore, the maximum concentration of cyclohexene was 3% (v/v).

The laboratory samples and methods of testing are shown in Table 1.

#### **2.1. Usefulness of infra-red spectroscopy for studies on autoxidation of gasolines**

Infra-red spectroscopy enables examination of the structure of molecules. It helps determine what functional groups the analyzed compound has in it. Functional groups are a group of several atoms connected with one another by means of chemical bonds (for instance, carbonyl group –C=O, hydroxyl group –OH). When present in the molecule of a given chemical compound, functional groups have characteristic vibration bands which are visible in the spectrum. A given functional group which is present in various compounds has similar vibration frequencies.

Autoxidation of hydrocarbons in gasolines leads, in the first place, to alcohols. They, in turn, are further oxidized to form aldehydes and ketones, leading to organic acids. The compounds have the following characteristic groups: hydroxyl group (for alcohols and acids) and carbonyl group (for acids, aldehydes and ketones). Although gasoline has a very rich IR spectrum, the characteristic bands for the aforementioned fuel oxidation products are manifested in easily identified ranges. For the hydroxyl group, it is a wide range with its maximum around 3300 cm-1, while the carbonyl group gives a signal in the spectrum in the range from 1750 to 1650 cm-1, depending on the type of compound (acids, aldehydes, esters, ketones). The location of the respective bands may be shifted, depending on the structure of a given compound (vicinity of other groups). IR spectroscopy is a fast and rather simple method, therefore, it was selected by the authors for establishing the correlation between changes in the chemical composition of fuels and their current chemical stability.

Oxidation stability of the gasolines during storage was tested at regular intervals. The results were compared with the IR spectra of the collected samples. The spectra were recorded using the Magna 750 apparatus from Nicolet in the wave number range from 4000 to 400 cm-1, in a 0.065 mm thick KBr cell.

Comparison of the IR spectra for the RON 95 gasoline, recorded after various times of storage, enabled detection of changes in the area which is typical of the O-H and C=O groups (Figure 1).

**Figure 1.** IR spectra, recorded for RON 95 gasoline samples: 1) fresh, 2) after 3 months of storage, and 3) after 6 months of storage.

The RON 95 gasoline had a content of alcohol (ethanol added during its production), therefore, the fresh fuel sample had a spectrum with the wide, strong band with its maximum at ab. 3330 cm-1, typical of the O-H groups. The intensity of that band was increasing during the initial months of storage and decreasing in the later months. The changes are clearly seen after comparing the surface areas below that particular peak in the respective spectra – Table 2. Lowering of the spectrum for gasoline after 6 months of storage, clearly seen in Figure 1, is probably due to the fact that the sample was darker, com‐ pared with its initial color. Changes in the coloration of fuel during its ageing process are due to the formation of chromophore groups in it.


**Table 2.** Surface area below the peak, typical of the OH group for RON\_95 gasoline samples

The largest number of hydroxyl groups were detected in the RON\_95 (4) gasoline sample, as shown by the largest surface area below the peak in question (Table 2). The surface area was growing larger during the initial months of storage, which is attributed to the formation of oxidation products (alcohols and/or acids). During the next months, the surface area was getting smaller which may have been caused by two phenomena: firstly, oxidation of alcohols to ketones and aldehydes, and the resulting disappearance of the O-H group; secondly, development in gasoline of some compounds in the form of gum. The emergence of carbonyl compounds is indicated by changes observed near the band at ab. 1743 cm-1 – Figure 2. Moreover, it should also be borne in mind that alcohol, an oxygen compound added to the gasoline, may be the precursor of the ageing processes.

**Figure 1.** IR spectra, recorded for RON 95 gasoline samples: 1) fresh, 2) after 3 months of storage, and 3) after 6 months

The RON 95 gasoline had a content of alcohol (ethanol added during its production), therefore, the fresh fuel sample had a spectrum with the wide, strong band with its maximum at ab. 3330 cm-1, typical of the O-H groups. The intensity of that band was increasing during the initial months of storage and decreasing in the later months. The changes are clearly seen after comparing the surface areas below that particular peak in the respective spectra – Table 2. Lowering of the spectrum for gasoline after 6 months of storage, clearly seen in Figure 1, is probably due to the fact that the sample was darker, com‐ pared with its initial color. Changes in the coloration of fuel during its ageing process are

> **Sample number 3330 cm-1 band** RON\_95 (1) 8426 RON\_95 (2) 9120 RON\_95 (3) 9746 RON\_95 (4) 11021 RON\_95 (5) 9198 RON\_95 (6) 8677 RON\_95 (K) 7009

**Table 2.** Surface area below the peak, typical of the OH group for RON\_95 gasoline samples

due to the formation of chromophore groups in it.

of storage.

162 Storage Stability of Fuels

**Figure 2.** IR spectra in the range 2700-1300 cm-1, recorded for RON 95 gasoline samples: 1) fresh, 2) after 3 months of storage, and 3) after 6 months of storage

There was a small, narrow peak in the spectrum of the fresh gasoline sample, with its maximum at ab. 1743 cm-1 (Figure 2). Minor changes were detected around that band in the spectra of gasoline samples after various periods of storage two more peaks were detected for lower wave numbers (ab. 1730 and 1710 cm-1), tending to combine and form a wider, single band. The peaks may be attributed to various compounds containing a carbonyl group, which are formed in the fuel as the result of chemical changes during its storage.

In their own research, the present authors have demonstrated that changes observed in the RON 98 gasoline spectra after various periods of storage were different, compared with those observed for RON 95. The only similarity was that the gasoline spectrum was lowered after long-term storage. Figure 3 shows IR spectra, recorded for the RON 98 gasoline samples: fresh and those being stored for 3 and 6 months. Changes in the typical areas for the hydroxyl and carbonyl groups were less intense than those for RON 95. Surface areas below those bands were observed to change irregularly with time, although they were larger in every case than those in the fresh sample. The phenomenon made it impossible to predict the tendency of such changes – Table 3. Nonetheless, it was found that changes (increase or decrease) in the surface area below the characteristic band for the hydroxyl group involved a similar change below the peak for the carbonyl group. Such behavior was attributed by the authors to transformation processes taking place in oxidation products as well as to the precipitation of certain com‐ pounds in the fuel in the form of gum.

**Figure 3.** IR spectra, recorded for RON 98 gasoline samples: 1) fresh, 2) after 3 months of storage, and 3) after 6 months of storage


**Table 3.** Surface area below the characteristic peak for the OH group in RON\_98 gasoline

The authors' own research indicates that introduction to the fresh RON 95 gasoline of perox‐ ides (precursors of oxidation) has no effect, during the short observation period, on changes in the chemical composition of the fuel due to autoxidation. The spectra of the fuels with TBHP, recorded during storage, were not different from those of the fuels without TBHP. Changes within the characteristic band for the carbonyl group, occurring in the gasoline during storage, were similar to those observed in the fuel without peroxides – the emergence of low intensity peaks below 1743 cm-1, combined of low intensity peaks, combined to form a wider band. Similar changes as for the fuel without the compound were obsrved also in the surface area below the characteristic peak for hydroxyl groups: an initial increase in surface area during the first two months of storage was followed by its reduction. Comparison of the spectra for the RON 95 gasoline samples (fresh, stored for 6 months, and the one doped with peroxides and stored for 6 months) indicates that the presence of peroxides has caused a minor increase in the characteristic peaks for the carbonyl and hydroxyl groups, compared with the fuel which was stored without addition of TBHP.

long-term storage. Figure 3 shows IR spectra, recorded for the RON 98 gasoline samples: fresh and those being stored for 3 and 6 months. Changes in the typical areas for the hydroxyl and carbonyl groups were less intense than those for RON 95. Surface areas below those bands were observed to change irregularly with time, although they were larger in every case than those in the fresh sample. The phenomenon made it impossible to predict the tendency of such changes – Table 3. Nonetheless, it was found that changes (increase or decrease) in the surface area below the characteristic band for the hydroxyl group involved a similar change below the peak for the carbonyl group. Such behavior was attributed by the authors to transformation processes taking place in oxidation products as well as to the precipitation of certain com‐

**Figure 3.** IR spectra, recorded for RON 98 gasoline samples: 1) fresh, 2) after 3 months of storage, and 3) after 6 months

**Sample Band 3635-3281 cm-1 Band 1746 cm-1** RON\_98 (1) 306 77 RON\_98 (3) 542 182 RON\_98 (4) 466 97 RON\_98 (5) - 132 RON\_98 (6) 569 119 RON\_98 (K) 388 110

**Table 3.** Surface area below the characteristic peak for the OH group in RON\_98 gasoline

pounds in the fuel in the form of gum.

164 Storage Stability of Fuels

of storage

Similar studies, carried out on the RON 98 gasoline, indicate that the presence of TBHP has not affected the nature of the fuel's autoxidation. The increase or reduction of the surface area below the characteristic vibration bands for the O-H and C=O groups, has not had any specific tendency. Comparison of the spectra for the RON 98 gasoline samples (fresh, stored with and stored without peroxides) has shown that addition of TBHP in this case has not had an observable effect on its oxidation rate.

The findings clearly indicate that autoxidation rate is affected by the type of gasoline while the oxidation precursor in the form of hydroperoxide, added to the gasoline, tends to enhance the differences.

Introduction to the gasolines to be stored of a substance which is susceptible to oxidation is expected to enhance their autoxidation. The hypothesis was validated by the findings reported by the present authors only for the RON 95 gasoline. The presence of cyclohexene in the fuel during storage has had a minor effect on the increase in the surface area below the characteristic peaks for the O-H and C=O groups. After the olefin was added to the RON 98 fuel, no significant changes in the nature of changes were observed in the bands under consideration for the hydroxyl and carbonyl groups. The results confirm the earlier conclusion made by the authors, concerning the effect of the gasoline type on the rate of processes which occur during its storage.

The tests, carried out by the authors, indicate that infra-red spectroscopy is useful for moni‐ toring the processes taking place in gasolines, even during short-time storage. The processes are chiefly oxidative, as confirmed by changes, for instance, in intensity, shape, or the emer‐ gence of new peaks in the IR spectrum in the wave number ranges which are typical of the carbonyl and hydroxyl groups. They depend, for instance, on the type of gasoline stored (for instance, RON 98 is more stable) and on the presence of precursors of oxidation. When testing the stored gasolines periodically, the authors did not find any changes in their physicochemical properties. The IR method enables the recording of even minor chemical changes resulting in deterioration of the fuel, which do not involved changes in physico-chemical properties, as determined by standardized methods.

#### **2.2. Application of rapid ageing tests for evaluation of gasolines**

Along with examination of their physico-chemical properties, the authors tested the gasolines periodically for their oxidation stability during storage and recorded the spectra of fuels after subjecting them to accelarated ageing.

Oxidation stability is commonly evaluated by determination of induction period according to ISO 7536 [12].

Induction period indicates the tendency of gasoline to develop gum during storage. The parameter is measured by the time that has lapsed from the beginning of the test to the occurrence of what is called "break point". According to the standard, the break point is defined as the point on the curve showing the pressure vs. time relationship before which the pressure drops by exactly 14 kPa within 15 minutes and after which the pressure increases by not less than 14 kPa within 15 minutes. It is assumed that a chemically stable gasoline will not, under conditions of test, be oxidized within less than 240-480 minutes [13].

In the stability tests according to the aforementioned standard, none of the gasoline samples actually had a break point after storage. Analyses of the pressure vs. time relationship during a 24-hour test indicate an incessant pressure drop, although the values were lower, compared with those required for the break point. The authors are of the opinion that pressure drop in the test cell indicated chemical reactions taking place in the system. IR spectra of fuel samples, prepared before and after the oxidation test clearly indicate the presence of oxidation products in the fuel.

Figure 4 shows a typical IR spectrum of RON 95 gasoline after 6 months of storage, subjected to the induction period test. Weak signals in the range that is characteristic for carbonyl groups, visible in the spectrum recorded after 6 months of storage (Figure 2 and Figure 3), are combined to form a strong, single band with its maximum at ab. 1710 cm-1.

The spectrum of the fuel subjected to the induction period tests showed a strong, sharp signal with its maximum at the wave number 2332 cm-1. The IR spectra recorded for samples collected after various times of storage and subjected to accelerated ageing indicate that the peak intensity at 2332 cm-1 increases slightly with longer storage times. The peak is characteristic, for instance, for the triple bond in nitriles C≡N. In that range, the signal may also originate from the C≡C group, although the characteristic peak for that group in pure compounds is in the lower wave number range 2100-2270 cm-1. On the other hand, it should be noted that in the case in question, the sample was a complex mixture of organic compounds which may have led to band shifts in the spectrum. In the case of alkynes, where the triple bond is at the terminal carbon atom, the spectrum also has signals generated by the ≡C-H group: a strong and sharp band of stretching vibrations at ab. 3300 cm-1 and deformation bands in the ranges 1220-1370 cm-1 and 610-700 cm-1. In the analyzed spectrum of RON 95 gasoline (sample after induction period), the presence of the aforementioned bands cannot be confirmed expressly because, if present at all, such bands were masked by other strong signals. Only a weak peak was visible at ab. 620 cm-1, which might indicate that alkynes are one of the products of autoxidation of the fuel. Although such compounds might be just a temporary product, it would take detailed tests to confirm the hypothesis.

**Figure 4.** IR spectrum of RON 95 gasoline after 6 months of storage, after induction period tests

**2.2. Application of rapid ageing tests for evaluation of gasolines**

under conditions of test, be oxidized within less than 240-480 minutes [13].

to form a strong, single band with its maximum at ab. 1710 cm-1.

would take detailed tests to confirm the hypothesis.

subjecting them to accelarated ageing.

ISO 7536 [12].

166 Storage Stability of Fuels

in the fuel.

Along with examination of their physico-chemical properties, the authors tested the gasolines periodically for their oxidation stability during storage and recorded the spectra of fuels after

Oxidation stability is commonly evaluated by determination of induction period according to

Induction period indicates the tendency of gasoline to develop gum during storage. The parameter is measured by the time that has lapsed from the beginning of the test to the occurrence of what is called "break point". According to the standard, the break point is defined as the point on the curve showing the pressure vs. time relationship before which the pressure drops by exactly 14 kPa within 15 minutes and after which the pressure increases by not less than 14 kPa within 15 minutes. It is assumed that a chemically stable gasoline will not,

In the stability tests according to the aforementioned standard, none of the gasoline samples actually had a break point after storage. Analyses of the pressure vs. time relationship during a 24-hour test indicate an incessant pressure drop, although the values were lower, compared with those required for the break point. The authors are of the opinion that pressure drop in the test cell indicated chemical reactions taking place in the system. IR spectra of fuel samples, prepared before and after the oxidation test clearly indicate the presence of oxidation products

Figure 4 shows a typical IR spectrum of RON 95 gasoline after 6 months of storage, subjected to the induction period test. Weak signals in the range that is characteristic for carbonyl groups, visible in the spectrum recorded after 6 months of storage (Figure 2 and Figure 3), are combined

The spectrum of the fuel subjected to the induction period tests showed a strong, sharp signal with its maximum at the wave number 2332 cm-1. The IR spectra recorded for samples collected after various times of storage and subjected to accelerated ageing indicate that the peak intensity at 2332 cm-1 increases slightly with longer storage times. The peak is characteristic, for instance, for the triple bond in nitriles C≡N. In that range, the signal may also originate from the C≡C group, although the characteristic peak for that group in pure compounds is in the lower wave number range 2100-2270 cm-1. On the other hand, it should be noted that in the case in question, the sample was a complex mixture of organic compounds which may have led to band shifts in the spectrum. In the case of alkynes, where the triple bond is at the terminal carbon atom, the spectrum also has signals generated by the ≡C-H group: a strong and sharp band of stretching vibrations at ab. 3300 cm-1 and deformation bands in the ranges 1220-1370 cm-1 and 610-700 cm-1. In the analyzed spectrum of RON 95 gasoline (sample after induction period), the presence of the aforementioned bands cannot be confirmed expressly because, if present at all, such bands were masked by other strong signals. Only a weak peak was visible at ab. 620 cm-1, which might indicate that alkynes are one of the products of autoxidation of the fuel. Although such compounds might be just a temporary product, it

The authors observed that the oxidation tendency of RON 95 gasoline was getting higher after every month of storage, as indicated by the intensity of the signal, generated by the C≡C group. Even though a slight decrease in the surface areas below the characteristic peaks for the O-H and C=O groups was observed, this may have been caused by the ever more intense precipi‐ tation from the fuel of some compounds in the form of gum or by other, unidentified changes.

Subjecting the RON 98 gasoline samples to accelerated oxidation clearly improved the intensity of the signals attributed to the carbonyl group at 1730 cm-1 and to the hydroxyl group at the wave number 3322 cm-1 – Figure 6. Interestingly enough, compared with the spectrum for the RON 95 gasoline, the carbonyl band has its maximum shifted towards higher wave numbers. This may have been caused by the formation of other compounds containing a carbonyl group.

As in the case of the RON 95 gasoline, a peak having its maximum at the wave number 2332 cm-1 and a weak signal at ab. 620 cm-1 were observed in the spectrum for the RON 98 gasoline which was recorded in the sample after the induction period. The bands were attributed by the authors to unsaturated compounds having a triple bond (Figure 5). The signal at 2332 cm-1 in the spectrum for the RON 98 gasoline had a lower intensity, com‐ pared with that for RON 95, which might be due to the lower quantity of groups having a multiple bond in the structures of compounds formed in the fuel during its oxidation. However, the surface area below the peaks at the wave numbers 3300 cm-1 and 1730 cm-1 for RON 98 was getting smaller with the lapse of every month of storage; the same phenomenon was observed in the lower-octane number gasoline.

Addition of peroxides to the gasolines and subjecting the latter to the accelerated oxidation thereafter, has confirmed the nature of changes taking place in their composition, which were observed by the authors in the samples without peroxides. In the spectra of fuels, there is an

**Figure 5.** IR spectrum for RON 98 gasoline after 6 months of storage, after induction period test

intense band at the wave number of ab. 1720-1710 cm-1, indicating the presence of compounds containing a carbonyl group. Moreover, the surface area below the peak (3300 cm-1) corre‐ sponding to the –OH group vibrations tended to increase. A band appeared which had its maximum at 2332 cm-1; its intensity varied with the fuel type and duration of storage. In the gasoline samples with peroxides added to them, part of the compounds containing –OH and C=O groups tended to precipitate or were further transformed during the subsequent months of storage. This was indicated by the slightly reduced surface area in the peaks of interest in the spectra of those fuels, subjected to accelerated oxidation. Similar changes were also observed in the fuel samples with cyclohexene added to them.

#### **2.3. Application of the induction period in the evaluation of gasolines**

The authors' own findings indicate that it is not possible to differentiate between the chemical stabilities of gasolines during short-term storage using the induction period method according to ISO 7536. Therefore, chemical stability was evaluated using the oxidation stability test based on rapid small-scale oxidation according to EN 16091 [14]. The test also uses the pressure drop criterion vs. duration of test. However, for the purpose of this test, the induction period is defined as the time that has lapsed from the commencement of test to the critical point, understood as a pressure drop by 10% relative to the maximum pressure, recorded during oxidation. The oxidation stability findings obtained by this method are referred to later in this paper as the "induction period by micro method".

The method is mainly used for determining oxidation stability of diesel oils, methyl esters and mixtures thereof, although it is also useful for determining the chemical stability of gasolines, in suitably selected conditions of test. The gasoline samples described above were tested by the authors in the following conditions: temperature 140˚C, initial pressure of oxygen - 500 kPa, sample volume - 5 cm3 . Findings for RON 95 are shown in Figure 6, those for RON 98 are shown in Figure 7.

**Figure 6.** Induction period by micro method for RON 95 gasoline samples vs. duration of storage

intense band at the wave number of ab. 1720-1710 cm-1, indicating the presence of compounds containing a carbonyl group. Moreover, the surface area below the peak (3300 cm-1) corre‐ sponding to the –OH group vibrations tended to increase. A band appeared which had its maximum at 2332 cm-1; its intensity varied with the fuel type and duration of storage. In the gasoline samples with peroxides added to them, part of the compounds containing –OH and C=O groups tended to precipitate or were further transformed during the subsequent months of storage. This was indicated by the slightly reduced surface area in the peaks of interest in the spectra of those fuels, subjected to accelerated oxidation. Similar changes were also

The authors' own findings indicate that it is not possible to differentiate between the chemical stabilities of gasolines during short-term storage using the induction period method according to ISO 7536. Therefore, chemical stability was evaluated using the oxidation stability test based on rapid small-scale oxidation according to EN 16091 [14]. The test also uses the pressure drop criterion vs. duration of test. However, for the purpose of this test, the induction period is defined as the time that has lapsed from the commencement of test to the critical point, understood as a pressure drop by 10% relative to the maximum pressure, recorded during oxidation. The oxidation stability findings obtained by this method are referred to later in this

The method is mainly used for determining oxidation stability of diesel oils, methyl esters and mixtures thereof, although it is also useful for determining the chemical stability of

observed in the fuel samples with cyclohexene added to them.

paper as the "induction period by micro method".

**2.3. Application of the induction period in the evaluation of gasolines**

**Figure 5.** IR spectrum for RON 98 gasoline after 6 months of storage, after induction period test

168 Storage Stability of Fuels

**Figure 7.** Induction period by micro method for RON 98 gasoline samples vs. duration of storage

During a 6-month period of storage under established conditions, induction period by micro method according to EN 16091 was shorter for every test sample. As expected, the gasolines without addition of olefin or peroxide were characterized by longer induction periods, compared with the doped gasoline samples, while the RON 95 gasoline showed lower stability, compared with RON 98. In both gasoline types, addition of TBHP reduced the induction period more than did addition of cyclohexene. The most markedly reduced induction period in the samples with the peroxide and olefin was observed during the initial two months of storage, and changes were only slight in the months which followed; for pure gasoline samples the tendency was observed after three months of storage.

The authors are of the opinion that the findings have confirmed the hypothesis that addition of an organic peroxide or cycloolefin to the fuel has had a deteriorating effect on the chemical stability of the fuels. Degradation of gasoline was accelerated by the substances. This proves the assumption that the substances take part in autoxidation processes. Regardless of the chemical composition of the initial fuel, the degrading effect of TBHP was more prominent than that of cyclohexene. Moreover, the findings indicate that the standard method to measure the induction period for gasoline during storage is not reliable in detecting minor changes in the chemical composition of hydrocarbons which occur due to autoxidation. On the other hand, measuring the induction period using the micro method may be useful in evaluating the chemical stability of fuels. Moreover, it is a rapid test, which is an advantage.

#### **2.4. The contents of gum, inherent and potential resins vs. chemical stability of fuels**

When studying the chemical stability of fuels during storage, the contents of inherent resins and gum was determined according to ISO 6246 [15]. The term "gum" is understood as the residue on evaporation of the test fuel, not subjected to further chemical treatment. It comprises an n-heptane-insoluble portion, non-volatile compounds such as contaminants, and additives. What remains after washing the resins with n-heptane and the solvent evaporation is a residue, referred to as "inherent resins"; its maximum level in the finished fuel is limited at 5 mg per 100 ml fuel.

The resistance of fuel to chemical changes is determined by its potential resin content, though only with reference to its current condition. The parameters may indicate oxidation processes taking place in the fuel, though it should be remembered that the value of the parameter will be increased by the improvers, added to the fuels. Even though the potential resins do not tend to cause any specific disturbances with the operation of fuels they may, in unfavorable conditions, settle in transfer lines and fuel filters, plugging them.

The content of gum vs. time of storage is shown in Figures 8 and 9; that of inherent resins is shown in Table 4.

In the case of RON 95, the lowest values of gum content were found in the gasoline samples with no admixtures. The parameter showed a growing tendency for samples with addition of cyclohexene, although no distinct tendency was found for the gasoline with a peroxide content.

For RON 98 – whether pure or with addition of cyclohexene – no tendency to change the gum content was detected. To the contrary, in the gasoline samples with a peroxide content, the

**Figure 8.** Gum content in the RON 95 gasoline vs. time of storage

During a 6-month period of storage under established conditions, induction period by micro method according to EN 16091 was shorter for every test sample. As expected, the gasolines without addition of olefin or peroxide were characterized by longer induction periods, compared with the doped gasoline samples, while the RON 95 gasoline showed lower stability, compared with RON 98. In both gasoline types, addition of TBHP reduced the induction period more than did addition of cyclohexene. The most markedly reduced induction period in the samples with the peroxide and olefin was observed during the initial two months of storage, and changes were only slight in the months which followed; for pure gasoline samples the

The authors are of the opinion that the findings have confirmed the hypothesis that addition of an organic peroxide or cycloolefin to the fuel has had a deteriorating effect on the chemical stability of the fuels. Degradation of gasoline was accelerated by the substances. This proves the assumption that the substances take part in autoxidation processes. Regardless of the chemical composition of the initial fuel, the degrading effect of TBHP was more prominent than that of cyclohexene. Moreover, the findings indicate that the standard method to measure the induction period for gasoline during storage is not reliable in detecting minor changes in the chemical composition of hydrocarbons which occur due to autoxidation. On the other hand, measuring the induction period using the micro method may be useful in evaluating

the chemical stability of fuels. Moreover, it is a rapid test, which is an advantage.

**2.4. The contents of gum, inherent and potential resins vs. chemical stability of fuels**

When studying the chemical stability of fuels during storage, the contents of inherent resins and gum was determined according to ISO 6246 [15]. The term "gum" is understood as the residue on evaporation of the test fuel, not subjected to further chemical treatment. It comprises an n-heptane-insoluble portion, non-volatile compounds such as contaminants, and additives. What remains after washing the resins with n-heptane and the solvent evaporation is a residue, referred to as "inherent resins"; its maximum level in the finished fuel is limited at 5 mg per

The resistance of fuel to chemical changes is determined by its potential resin content, though only with reference to its current condition. The parameters may indicate oxidation processes taking place in the fuel, though it should be remembered that the value of the parameter will be increased by the improvers, added to the fuels. Even though the potential resins do not tend to cause any specific disturbances with the operation of fuels they may, in unfavorable

The content of gum vs. time of storage is shown in Figures 8 and 9; that of inherent resins is

In the case of RON 95, the lowest values of gum content were found in the gasoline samples with no admixtures. The parameter showed a growing tendency for samples with addition of cyclohexene, although no distinct tendency was found for the gasoline with a peroxide content. For RON 98 – whether pure or with addition of cyclohexene – no tendency to change the gum content was detected. To the contrary, in the gasoline samples with a peroxide content, the

conditions, settle in transfer lines and fuel filters, plugging them.

tendency was observed after three months of storage.

100 ml fuel.

170 Storage Stability of Fuels

shown in Table 4.

**Figure 9.** Gum content in the RON 98 gasoline vs. time of storage

value of the parameter was found to increase with storage time. When comparing the gum content in pure RON 95 and RON 98 samples vs. time of storage, lower values of the parameter were detected for alcohol-ether gasoline.

Table 4 shows the inherent resin content.


**Table 4.** Inherent resin content in gasoline after storage

Based on the analysis of the findings shown above, it was not possible to find any explicit relationship between the inherent resin content in the gasoline samples and their time of storage. After 6 months storage, the inherent resin content was determined only in one sample out of a total of six samples tested.

The induction period data according to ISO 7536 for the stored samples did not make it possible to expressly evaluate the ageing tendency of fuels during long-term storage and they did not correlate with the resin content according to ISO 6246; therefore, stability tests were carried out according to ASTM D 873 [16]. Ageing was carried out for 4 hours under oxygen flow conditions: pressure 690-705 kPa, temperature 100˚C. The result of determination was the potential resin content, being the sum of solubles and insolubles. In the present test, insolubles are defined as a deposit which adheres to the glass wall of a test cell from which an aged fuel was removed along with precipitates and solubles; such insolubles are determined from an increase in the weight of the test cell after the test, as compared with the clean test cell, weighed before the test. Soluble resins are regarded as oxidation products which are dissolved in the aged fuel plus deposits which adhere to the cell walls, soluble in a toluene-acetone mixture. A non-volatile residue on evaporation of the aged fuel and the solvent which was used for washing the test cell after the test is the soluble resin content.

Determination of the potential resin content enables the evaluation of the fuel's ability to develop gum and deposits and is an additional indicator of chemical stability for gasolines.

Results for the solubles content of the gasoline samples vs. time of storage are shown in the graphs below. The insolubles content is shown in Table 5.

**Figure 10.** The soluble resins content according to ASTM D 873 in RON 95

**Storage time**

172 Storage Stability of Fuels

**RON 95**

**Table 4.** Inherent resin content in gasoline after storage

washing the test cell after the test is the soluble resin content.

graphs below. The insolubles content is shown in Table 5.

out of a total of six samples tested.

**RON 95 +OLEF**

**Inherent resins mg/100 ml**

**RON 98**

**RON 98 +OLEF**

**RON 98 +PEROX**

**RON 95 +PEROX**

Initial condition 0.0 0.4 0.5 0.0 0.4 0.5

1 month 0.0 0.0 0.0 0.2 0.0 0.0

2 months 0.0 0.0 0.6 0.0 0.6 0.0

3 months 0.0 0.4 0.0 0.0 0.0 0.4

4 months 0.4 0.0 0.8 0.0 0.3 0.0

5 months 0.3 0.0 0.0 0.0 0.2 0.6

6 months 0.0 0.0 1.4 0.0 0.0 0.0

Based on the analysis of the findings shown above, it was not possible to find any explicit relationship between the inherent resin content in the gasoline samples and their time of storage. After 6 months storage, the inherent resin content was determined only in one sample

The induction period data according to ISO 7536 for the stored samples did not make it possible to expressly evaluate the ageing tendency of fuels during long-term storage and they did not correlate with the resin content according to ISO 6246; therefore, stability tests were carried out according to ASTM D 873 [16]. Ageing was carried out for 4 hours under oxygen flow conditions: pressure 690-705 kPa, temperature 100˚C. The result of determination was the potential resin content, being the sum of solubles and insolubles. In the present test, insolubles are defined as a deposit which adheres to the glass wall of a test cell from which an aged fuel was removed along with precipitates and solubles; such insolubles are determined from an increase in the weight of the test cell after the test, as compared with the clean test cell, weighed before the test. Soluble resins are regarded as oxidation products which are dissolved in the aged fuel plus deposits which adhere to the cell walls, soluble in a toluene-acetone mixture. A non-volatile residue on evaporation of the aged fuel and the solvent which was used for

Determination of the potential resin content enables the evaluation of the fuel's ability to develop gum and deposits and is an additional indicator of chemical stability for gasolines.

Results for the solubles content of the gasoline samples vs. time of storage are shown in the

**Figure 11.** The soluble resin content according to ASTM D 873 in RON 98


**Table 5.** The insolubles content according to ASTM D 873 in gasoline during storage

The gasoline samples with a peroxide content have a very high soluble resins content, except that, for RON 95, the value is nearly twice as high as that for RON 98 (for samples stored for more than 3 months).

For alcohol-ether gasoline without admixtures and for that with an admixture of cyclohexene, the value of that parameter grows moderately with storage time while there is an observable tendency toward change in similar ether-based gasoline samples.

Every oxidized sample was tested to determine its content of insolubles, although no different storage times. No precipitated or suspended deposit was detected in the fuel in either sample after ageing.

#### **3. Tests on diesel oil**

The chemical instability of diesel oil is caused by the presence in the fuel of compounds which act as precursors of the formation macromolecular structures with limited solubility. Gener‐ ally, such compounds include components containing nitrogen and sulfur, reactive olefins, as well as organic acids.

A quite well known mechanism, generating insolubles in diesel oil, is the transformation of phenalenones and indoles to indolephenalene salt complexes. The reaction is favored by acid conditions. Phenalenones are formed by oxidation of active olefins whereas indoles are a natural component of fuels. Organic acids, the indispensable catalyst for the reaction, are usually present in components of fuels or are generated by oxidation of mercaptanes to form sulfonic acids. The mechanism of deposit formation may be interrupted by neutralization of acidic conditions or elimination of the precursors with the use of hydrogen or suitable additives having a stabilizing or antioxidative properties.

**Figure 12.** The mechanism of formation of deposits in diesel oil.

**Storage time**

174 Storage Stability of Fuels

more than 3 months).

**3. Tests on diesel oil**

well as organic acids.

having a stabilizing or antioxidative properties.

after ageing.

**RON 95**

**RON 95 +OLEF**

**Table 5.** The insolubles content according to ASTM D 873 in gasoline during storage

tendency toward change in similar ether-based gasoline samples.

**Insolubles mg/100 ml**

**RON 98**

**RON 98 +OLEF**

**RON 98 +PEROX**

**RON 95 +PEROX**

Initial condition 0.2 1.1 1.0 0.3 1.2 1.1 1 month 0.2 1.1 0.5 0.3 1.4 1.5 2 months 0.4 0.9 1.2 0.5 1.3 0.7 3 months 0.9 0.6 0.8 1.1 0.7 0.7 4 months 0.5 0.5 0.6 1.0 0.7 1.2 5 months 0.7 0.6 0.9 1.0 0.4 0.7 6 months 0.8 0.9 0.9 1.0 0.8 1.1

The gasoline samples with a peroxide content have a very high soluble resins content, except that, for RON 95, the value is nearly twice as high as that for RON 98 (for samples stored for

For alcohol-ether gasoline without admixtures and for that with an admixture of cyclohexene, the value of that parameter grows moderately with storage time while there is an observable

Every oxidized sample was tested to determine its content of insolubles, although no different storage times. No precipitated or suspended deposit was detected in the fuel in either sample

The chemical instability of diesel oil is caused by the presence in the fuel of compounds which act as precursors of the formation macromolecular structures with limited solubility. Gener‐ ally, such compounds include components containing nitrogen and sulfur, reactive olefins, as

A quite well known mechanism, generating insolubles in diesel oil, is the transformation of phenalenones and indoles to indolephenalene salt complexes. The reaction is favored by acid conditions. Phenalenones are formed by oxidation of active olefins whereas indoles are a natural component of fuels. Organic acids, the indispensable catalyst for the reaction, are usually present in components of fuels or are generated by oxidation of mercaptanes to form sulfonic acids. The mechanism of deposit formation may be interrupted by neutralization of acidic conditions or elimination of the precursors with the use of hydrogen or suitable additives

S.J. Marshaman and P. David [17] proposed a reaction mechanism comprising several steps and leading to the development of deposits by oxidation of phenalenes to phenalenones, followed by addition to the phenalenones of indoles, which later form indolephenalene salt complexes in an acidic environment. The reaction mechanism is shown in Figure 12.

S.J. Marshaman and P. David developed methods to monitor the level of phenalenes and phenalenones, compounds leading to the development of deposits in fuels. It was a twoway study: the fuels were subjected to long-term ambient storage while, at the same time, rapid ageing tests were carried out at elevated temperatures and/or oxygen at a positive pressure. The concentration of phenalenes and phenalenones was measured by chromato‐ graphic methods (HPLC). After being subjected to a standard ageing test according to ASTM D 2274, the fuel samples did not show any tendency to develop large amounts of insolu‐ bles. On the other hand, when subjected to the test method simulating long-term storage conditions in accordance with ASTM D 4625, the fuel samples showed a regular increase in the amount of gum/resin or deposits, corresponding to an increase in the time of storage. After 16 weeks of testing, the amount of filterable deposits was 4.0 mg/100 cm3 , and that of resins was 6.9 mg/100 cm3 , with the initial value being less than 0.1mg/100 cm3 . For the initial 2 months of storage, the fuel was not undergoing any intensive ageing and a significant increment in the amount of total deposits was recorded only after ab. 12 weeks.

In the case of the fuels which were stored in steel tanks in ambient conditions, the amount of insolubles was observed to grow around the 30th week of storage, and the total deposit was 16.8 mg/100 cm3 after 50 weeks.

The levels of phenalenes (and their alkyl homologs) and phenalenones were measured during the entire process of ambient storage of the fuel samples. The tests indicate that, for longer storage times, the content of phenalenes decreased from 860 mg/l to 135 mg/l in the 46th week of storage, while that of phenalenones increased from 15 mg/l to 188 mg/l. This confirms the proposed mechanism for oxidation of phenalenes to phenalenones. The present authors monitored the fuel for coloration and acidity while carrying on this study. They found that changes in the color of fuel was connected with increased concentrations of phenalenones: the longer the storage time, the darker the fuel. Its darkening was deemed to indicate the formation of precursors of deposits. Furthermore, the present authors have demonstrated that the presence in the fuel of strong acids, such as aromatic sulfonic acids, accelerates the formation of deposits.

Indole, along with its alkyl derivatives, is another important compound, participating in the development of deposits. L.A. Beranek *et al.* [18] have carried out studies on fuel blends comprising straight run distillate (SRD) and light cycle oil (LCO) fractions. The latter are classified as chemically unstable compounds. Compounds such as 2-methylindole, 3-methyl‐ indole and 1-phenalenone were introduced, additionally, to the model fuel samples. The samples were subjected to rapid ageing in pressurized bombs for a period of 64 hours at elevated oxygen pressure conditions, at a temperature of 95o C.

For the fuel blends which were composed of straight run distillate only, the amount of insolubles was not observed to increase, regardless of whether indoles and phenalenone were added thereto. In the case of fuel blends comprising, additionally, a fraction rich in cyclic compounds, the level of deposit after the test was definitely higher for the samples with a content of indoles and phenalenone. The findings also indicate a decrease in the concentration of indoles in the samples which were subjected to rapid ageing; on the contrary, the content of phenalenone was not reduced. More deposit is formed for the samples containing 2 methylindole, compared with those containing 3-methylindole.

One more mechanism explaining the course of the fuel ageing process is postulated, especially for low-sulfur fuels which contain biocomponents, where the formation of deposits and gum is a multi-step radical reaction, involving the participation of hydroperoxides. Dan Li *et al.* [19] have described studies on the thermooxidative stability of aviation fuel. They have observed that degradation of aviaton fuels may be caused by the short-term effect of high temperatures. To validate the hypothesis, they tested several samples of aviation fuel by subjecting them to oxidation at various temperatures (120–180o C) for a maximum of 20 hours. Small amounts of the fuel were sampled at intervals in the process of testing in order to determine the level of hydroperoxides and for spectral analyses.

The content of hydroperoxides in the test fuel samples varied, depending on time and oxidation temperature. The general relationship observed during the test, regardless of process temperature, was that the level of hydroperoxides will increase up to a certain maximum, followed by a decrease to a certain level. The higher the process temperature, the faster the increase in the hydroperoxide concentration. The hydroperoxide was formed at a higher rate than decomposition, which resulted in an increase in the resultant amount of the hydroper‐ oxide. The authors emphasize that such behavior is in agreement with the free radicals mechanism, as described by Zabarnick [20]; hydroperoxides may be regarded as intermediate compounds in the consecutive reactions of autoxidation of hydrocarbons. During the tests, it was also observed that the fuel with a higher content of polar components would be oxidized sooner. Generally, the presence of polar components is the principal factor of instability. An analysis of the FTIR spectra of fuel samples, collected at various intervals during the process of oxidation, indicated the development in the fuels of structures which are typical of oxidation products: carbonyl and hydroxyl groups were identified. The peak intensity of such groups was the higher, the longer the time of oxidation.

monitored the fuel for coloration and acidity while carrying on this study. They found that changes in the color of fuel was connected with increased concentrations of phenalenones: the longer the storage time, the darker the fuel. Its darkening was deemed to indicate the formation of precursors of deposits. Furthermore, the present authors have demonstrated that the presence in the fuel of strong acids, such as aromatic sulfonic acids, accelerates the formation

Indole, along with its alkyl derivatives, is another important compound, participating in the development of deposits. L.A. Beranek *et al.* [18] have carried out studies on fuel blends comprising straight run distillate (SRD) and light cycle oil (LCO) fractions. The latter are classified as chemically unstable compounds. Compounds such as 2-methylindole, 3-methyl‐ indole and 1-phenalenone were introduced, additionally, to the model fuel samples. The samples were subjected to rapid ageing in pressurized bombs for a period of 64 hours at

For the fuel blends which were composed of straight run distillate only, the amount of insolubles was not observed to increase, regardless of whether indoles and phenalenone were added thereto. In the case of fuel blends comprising, additionally, a fraction rich in cyclic compounds, the level of deposit after the test was definitely higher for the samples with a content of indoles and phenalenone. The findings also indicate a decrease in the concentration of indoles in the samples which were subjected to rapid ageing; on the contrary, the content of phenalenone was not reduced. More deposit is formed for the samples containing 2-

One more mechanism explaining the course of the fuel ageing process is postulated, especially for low-sulfur fuels which contain biocomponents, where the formation of deposits and gum is a multi-step radical reaction, involving the participation of hydroperoxides. Dan Li *et al.* [19] have described studies on the thermooxidative stability of aviation fuel. They have observed that degradation of aviaton fuels may be caused by the short-term effect of high temperatures. To validate the hypothesis, they tested several samples of aviation fuel by subjecting them to

the fuel were sampled at intervals in the process of testing in order to determine the level of

The content of hydroperoxides in the test fuel samples varied, depending on time and oxidation temperature. The general relationship observed during the test, regardless of process temperature, was that the level of hydroperoxides will increase up to a certain maximum, followed by a decrease to a certain level. The higher the process temperature, the faster the increase in the hydroperoxide concentration. The hydroperoxide was formed at a higher rate than decomposition, which resulted in an increase in the resultant amount of the hydroper‐ oxide. The authors emphasize that such behavior is in agreement with the free radicals mechanism, as described by Zabarnick [20]; hydroperoxides may be regarded as intermediate compounds in the consecutive reactions of autoxidation of hydrocarbons. During the tests, it was also observed that the fuel with a higher content of polar components would be oxidized sooner. Generally, the presence of polar components is the principal factor of instability. An analysis of the FTIR spectra of fuel samples, collected at various intervals during the process

C.

C) for a maximum of 20 hours. Small amounts of

elevated oxygen pressure conditions, at a temperature of 95o

methylindole, compared with those containing 3-methylindole.

oxidation at various temperatures (120–180o

hydroperoxides and for spectral analyses.

of deposits.

176 Storage Stability of Fuels

S. Gernigon *et al.* [21] investigated the probability of inhibiting the hydrocarbons radical oxidation reaction by using suitable anti-oxidants. They selected BHT (butylated hydroxyto‐ luene), 2,4-DTBP (2,4-di–tert-butylphenol), TBMP (2-tert-butyl-4-methylphenol). Their studies were carried out on four selected, pure hydrocarbons, representing aviation fuel components. Degradation of the test hydrocarbons was carried out by oxidation at a temperature of 185o C for 72 hours. For identification of the compounds being formed by oxidation, as well as to study the kinetics of decomposiiton of hydrocarbons and the additives used, the hydrocarbon samples were analyzed by GC/MS, GC and FTIR. The authors found that the hydrocarbons were decomposed in the process of oxidation forming new compounds, usually ketones, alcohols and carboxylic acids. The longer the time of degradation, the higher the amount of such compounds. Moreover, it was found that the amount of antioxidants was reduced in the course of oxidation and their efficiency depended on the concentration of a given additive, oxidation time, and composition of the fuel. BHT degradation products were identified as ketones, alcohols, carboxylic acids and BHT dimers. The antioxidants tested were found to be more effective toward alkanes, compared with cyclic compounds. Deposits were not formed in the degraded hydrocarbons, even though oxidation was taking place, as confirmed by the presence of ketones, carboxylic acids, and alcohols.

The process of degradation of stability of fuels relates also to products which contain biocom‐ ponents; in diesel oil, the biocomponent is fatty acid methyl esters (FAME). Esters are a nontoxic, sulfur-free, biodegradable biocomponent with low oxidation stability. In esters, stability largely depends on the profile of the fatty acids they are made from. Polyunsaturated fatty acids are more reactive, compared with saturated compounds. G. Karavalakis *et al.* [22] report that esters react with oxygen via an autoxidation mechanism involving a radical reaction through the steps of initiation, propagation, chain branching and termination. The essential products of oxidation include allyl hydroperoxides, unstable products which form secondary products of oxidation such as aldehydes and ketones, cyclic acids, polymeric compounds. The presence of macromolecular, polymerized compounds, may lead to the development of gum.

G. Karavalakis, S. Stournas and D. Karonis [23] have studied the oxidation stability of biodiesel (100% FAME) and blends of diesel oil with FAME, according to the methodology described in EN 14214 and EN 15751. The authors have demonstrated that the larger the ester content in diesel oil, the shorter its induction period, therefore, the product is more susceptible to oxidation. Susceptibility of such blends to oxidation depended also on the type of raw material the ester was made from, and on the composition of diesel oil.

The present authors have attempted to validate the hypothesis, proposed by S.J. Marshaman and P. David, concerning the mechanism of deposit formation due to the oxidation of phena‐ lenes in relation to the contemporarily used low-sulfur diesel oils with a content of fatty acid methyl esters. Methyl esters are known to be readily decomposed to acids; therefore, it was assumed in the present work that the acid being formed by fuel oxidation may favor the transformation of phenalenones and indoles to indolephenalene salt complexes, finally generating deposits and gum. An indole derivative (2-methylindole) was added to the test samples in order to intensify the process of oxidation of diesel oil. The tests were continued for 6 months while monitoring the fuel degradation rate with the use of normative and supplementary tests enabling the measurement of deposits and gum and of the induction period.

#### **3.1. Methods of tests**

Several accelerated ageing tests, which are most commonly used for determining the degree of fuel degradation, were selected for the study. All of the selected tests are dedicated to testing straight run distillates. The research problem assumed by the authors requires determination of the amount of deposits which may potentially be developed from degradation of a fuel doped with 2-methylindole; for that reason the ASTM D 5304 and EN ISO 12205 tests were used. Both these tests determine the amount of filterable and adherent insolubles (gum). All the same, in order to verify the theory of fuel degradation without deposit formation, two tests were selected EN 16091 (PetroOxy) and EN 15751 (Rancimat), of which the result is presented as the induction period. Elevated temperatures were used in each of the four proposed methods for determining oxidation stability of fuels, oxygen environment was used in three of them. In the method according to ASTM D 5304 and EN 16091, oxygen is used at a pressure in the range 700-800 kPa.

Definitions of terms:


The Rancimat test is carried out mainly for straight run distillates with a content of more than 2% (V/V) FAME as well as for FAME as a pure biofuel. The other tests may be used for testing a petroleum-based hydrocarbons fuel or one with biocomponents. The test conditions are described in the Table below.


**Table 6.** Conditions of rapid oxidation tests

generating deposits and gum. An indole derivative (2-methylindole) was added to the test samples in order to intensify the process of oxidation of diesel oil. The tests were continued for 6 months while monitoring the fuel degradation rate with the use of normative and supplementary tests enabling the measurement of deposits and gum and of the induction

Several accelerated ageing tests, which are most commonly used for determining the degree of fuel degradation, were selected for the study. All of the selected tests are dedicated to testing straight run distillates. The research problem assumed by the authors requires determination of the amount of deposits which may potentially be developed from degradation of a fuel doped with 2-methylindole; for that reason the ASTM D 5304 and EN ISO 12205 tests were used. Both these tests determine the amount of filterable and adherent insolubles (gum). All the same, in order to verify the theory of fuel degradation without deposit formation, two tests were selected EN 16091 (PetroOxy) and EN 15751 (Rancimat), of which the result is presented as the induction period. Elevated temperatures were used in each of the four proposed methods for determining oxidation stability of fuels, oxygen environment was used in three of them. In the method according to ASTM D 5304 and EN 16091, oxygen is used at a pressure

**1.** Filterable insolubles – deposit/sediment formed during the test, which may be removed from the fuel by filtration using a filter pore size of 0.8 μm. This type of deposits includes both the particulate matter and deposits washed out using a unary solvent (isooctane).

**2.** Adherent insolubles (gum) – deposit formed during the test, sticking to the glass parts of the fuel filtration system. Deposits from the walls are washed using a ternary solvent

**4.** Induction period in the Rancimat test – the time that lapses from the commencement of measurement to the time when the formation of oxidation products is severely intensified,

**5.** Induction period in the PetroOxy test – the time that lapses from the commencement of measurement to the time when oxygen pressure in the test chamber is 10% below its initial

The Rancimat test is carried out mainly for straight run distillates with a content of more than 2% (V/V) FAME as well as for FAME as a pure biofuel. The other tests may be used for testing a petroleum-based hydrocarbons fuel or one with biocomponents. The test conditions are

**3.** Total insolubles – a sum of filterable and adherent deposits.

as recorded by changes in electrolytic conductivity.

period.

178 Storage Stability of Fuels

**3.1. Methods of tests**

in the range 700-800 kPa.

before evaporating it.

described in the Table below.

Definitions of terms:

value.

A spectrophotometric analysis of the fuel samples was performed using the FT-IR spectro‐ photometer Magna 750 from Nicolet. The fuel samples were subjected to oxidation at a temperature of 140o C, under oxygen-flow conditions at a pressure of 700 kPa. After ageing, the fuel samples were subjected to a spectral analysis. The spectra were measured in a 0.065 mm thick KBr cell in the wave number range from 4000 to 400 cm-1. The same technique was used for measuring the samples not subjected to rapid ageing.

#### **3.2. The use of rapid ageing tests in the assessment of rate of change in fuels during storage**

The test material consisted of two samples of diesel oil with different contents of fatty acid methyl esters. Each sample was doped with 130 mg/kg of 2-methylindole. The value was assessed based on literature reports. The samples were stored at a room temperature for 6 months in a dark place and were collected for testing at 30-day intervals. Their compositions are shown in the Table below.


**Table 7.** Compositions of samples and methods of tests

When testing the diesel oil samples with a low content of FAME, no effect of an increased level of indole compounds on the rate of fuel degradation, expressed as the induction period, was observed. The induction period value was lower after 6 months of storage, compared with the beginning of tests. For sample A (without indole), the induction period was reduced from the initial 41.9 minutes to 32.8 minutes, for the sample AM (with indole) it was reduced from 38.7 minutes to 30.9 minutes. The rates of fuel oxidation were similar, both for the samples with and without methylindole. The induction period values were comparable in the first, second or third month of storage and identical in the fourth month of storage; a definitely shorter induction period was recorded only after 6 months of storage.

**Figure 13.** Induction period, as found according to EN 16091, for diesel oil samples containing 1.3%(V/V) FAME and methylindole.

The thermooxidation stability test according to ASTM D 5304 has demonstrated that, for a same duration of storage, more insolubles are formed in diesel oil samples with methylindole, compared with those without methylindole. After running the test for 3 and 4 months, sample AM was found to contain 2-3 more deposit, compared with sample A which had no content of indole. After 5 and 6 months, the amount of deposit was reduced although still higher than in sample A. No obvious relationship between the duration of storage and the content of filterable or adherent deposits was noted.

Determination of oxidation stability according to EN ISO 12205 was performed twice: at the beginning of tests and after 6 months of storage. Sample A was initially found to contain: 4 g/m3 of filterable deposits, 3 g/m3 of adherent deposits, a total of 7 g/m3 . After being stored for 6 months, the same sample contained 25 g/m3 of filterable deposits and 2 g/m3 of gum, a total of 27 g/m3 . Very similar values were obtained for sample AM with a content of methylindole: initially, the content of filterable deposits was 3 g/m3 , that of gum 2 g/m3 ; after 6 months, the sample contained 26 g/m3 of deposits and 3 g/m3 of gum. It was found that the amount of adherent deposits was not changed in the samples during storage,

initial 41.9 minutes to 32.8 minutes, for the sample AM (with indole) it was reduced from 38.7 minutes to 30.9 minutes. The rates of fuel oxidation were similar, both for the samples with and without methylindole. The induction period values were comparable in the first, second or third month of storage and identical in the fourth month of storage; a definitely shorter

0 1 2 3 4 5 6

**Time of storage [months]**

**Figure 13.** Induction period, as found according to EN 16091, for diesel oil samples containing 1.3%(V/V) FAME and

The thermooxidation stability test according to ASTM D 5304 has demonstrated that, for a same duration of storage, more insolubles are formed in diesel oil samples with methylindole, compared with those without methylindole. After running the test for 3 and 4 months, sample AM was found to contain 2-3 more deposit, compared with sample A which had no content of indole. After 5 and 6 months, the amount of deposit was reduced although still higher than in sample A. No obvious relationship between the duration of storage and the content of

Determination of oxidation stability according to EN ISO 12205 was performed twice: at the beginning of tests and after 6 months of storage. Sample A was initially found to contain:

stored for 6 months, the same sample contained 25 g/m3 of filterable deposits and 2 g/m3

after 6 months, the sample contained 26 g/m3 of deposits and 3 g/m3 of gum. It was found that the amount of adherent deposits was not changed in the samples during storage,

. Very similar values were obtained for sample AM with a content

4 g/m3 of filterable deposits, 3 g/m3 of adherent deposits, a total of 7 g/m3

of methylindole: initially, the content of filterable deposits was 3 g/m3

**37,5**

**40,3**

A AM

**30,9**

. After being

;

, that of gum 2 g/m3

induction period was recorded only after 6 months of storage.

filterable or adherent deposits was noted.

of gum, a total of 27 g/m3

methylindole.

**Induction period [min]**

180 Storage Stability of Fuels

**38,7 40,7 39,8 40,9**

**Figure 14.** Total insolubles, as found according to ASTM D 5304 for diesel oil samples with 1.3%(V/V) FAME and methylindole.

although that of filterable deposits was definitely increased, regardless of whether the sample contained any methylindole.

Tests of diesel oil samples with a content of 7.2% (V/V) FAME showed a certain relationship between the induction period and time of storage. The longer the time of storage, the shorter the induction period. The fuel oxidation process was running faster for the samples with a content of methylindole.

The thermooxidation stability test according to ASTM D 5304 was carried out for samples with a higher content of FAME. The test showed, just like for the samples with 1.3%(V/V) FAME, that a higher amount of total insolubles was formed in the samples with methylindole, compared with those without methylindole. After storing the samples for 3 months, the maximum amounts of deposit were recorded in sample BM - 18.2 mg/100 cm3 and in the indolefree sample B - 11.5 mg/100 cm3 . This means a 6-7-fold increase, compared with the initial amount of deposits. The amount of total deposit depended on filterable deposit because the content of adherent deposits was not high-just between 0.1 and 1.9 mg/100 cm3 .

Measurement of the induction period by the Rancimat method did not yield the expected results. Fuel oxidation rates were comparable, regardless of storage times and sample com‐ positions.

Determination of oxidation stability according to EN ISO 12205, carried out for diesel oil containing 7.2% (V/V) FAME yielded the following results: for sample B, initially, the amount of filterable deposits was 7 g/m3 , adherent deposits was 1 g/m3 , giving a total of 8 g/m3 ; after being stored for 6 months, the same sample had 56.2 g/m3 of filterable deposits and 2 g/m3 of gum, a total of 58.2 g/m3 . Different values were obtained for sample BM with methylindole:

**Figure 15.** Induction period according to EN 16091 for diesel oil samples with a content of 7.2% (V/V) FAME and meth‐ ylindole.

**Figure 16.** Total insolubles, as found according to ASTM D 5304 for diesel oil samples with a content of 7.2%(V/V) FAME and methylindole.

**Figure 17.** Induction period, as found by the Rancimat method for diesel oil samples with a content of 7.2% (V/V) FAME and methylindole.

initially, 4 g/m3 of filterable deposits and 2 g/m3 of gum; after 6 months of storage - 336 g/m3 of filterabledepositsand13g/m3ofgum.Thevalue fortotaldeposits insampleBMalsowashigher, compared with sample B without indole. Samples B and BM had a positively higher contents oftotaldeposits, comparedwithsamplesAandAM,whichcontainedonly1.3%(V/V)ofFAME.

The findings indicate that addition of methylindole helps intensify the deposit formation process, especially filterable deposits and especially in the samples with a significant content of fatty acid methyl esters. The tests, performed in accodance with EN 16901 and EN 15751, did not confirm the effect of 2-methylindole on the induction period value for fuels during the period of time covered in the tests.

#### **3.3. IR spectral analysis**

**31,1**

0

**1,9**

FAME and methylindole.

**Content of deposits [mg/100ml]**

**2,4**

**3,7 3,5**

**3,6**

ylindole.

5

10

15

20

**Induction period [min]**

25

30

35

182 Storage Stability of Fuels

**29,9**

**27,5 26,8**

**25,5**

**23,8**

**26,5**

0 1 2 3 4 5 6

**Time of storage [months]**

**Figure 15.** Induction period according to EN 16091 for diesel oil samples with a content of 7.2% (V/V) FAME and meth‐

**11,5**

0 1 2 3 4 5 6

**Time of storage [months]**

**Figure 16.** Total insolubles, as found according to ASTM D 5304 for diesel oil samples with a content of 7.2%(V/V)

**4,8**

**12,8**

**14,2**

**18,2**

**8,6**

**9,5**

B BM

**11**

**10,2**

**27,8 26,9 26,7 26,6**

**24,2 24,3 24,2**

B BM

Identification of the products resulting from fuel oxidation was performed based on the analysis of FT-IR spectra. The spectra obtained for samples A, AM, B and BM before and after the oxidation process were subjected to a qualitative analysis.

When stored in laboratory conditions at a room temperature for 6 months, the fuel samples were not degraded enough to show any visible changes in their spectral analysis. On the contrary, the spectra of stored fuel samples after subjecting them to rapid ageing did show visible changes.

The fuel samples after oxidation showed changes in the spectral ranges 3600-3200 cm-1 and 1800-1600 cm-1. For samples A and AM, which had negligible ester contents, changes were less intense than for sample B and BM with a content of FAME of 7.2 % (V/V). In the spectral range 3600-3200 cm-1, there appears a wide absorption band, connected with valence vibrations for O-H groups. In the range 1800-1600 cm-1, there is a visible narrow carbonyl band, connected with the presence of fatty acid methyl esters (1745 cm-1) in the fuel. In samples A and AM, the peak is smaller than in samples B and BM. In the oxidized samples, in the carbonyl band 1745 cm-1 range, there appears a second band, at 1720 cm-1. The band is better discernible for samples with longer storage time and is well visible in the spectres for samples A and AM because of their low content of FAME.

**Figure 18.** Spectra for sample B with regions of change marked: blue – sample before oxidation, red – sample after oxidation.

It is hard to say without a doubt what products have been formed in the fuel samples as the result of oxidation. The carbonyl band region is connected with the presence of acids, esters, aldehydes or ketones. The band region which is characteristic for O-H groups is connected with the presence of alcohols, phenols, carboxylic acids, or water which may be formed in the oxidized fuel as the result of decomposition of methyl esters. A different measurement technique should be used to identify the resulting products of oxidation more reliably.

A rough quantitative analysis of the analytical band at 3440 cm-1, consisting in measuring the surface area below the peak, indicates that the surface area tended to grow with storage time, reaching its maximum after 3-4 months of storage, then was getting smaller. This is attributed to the radical mechanism of fuel degradation, where initiation of the reaction is followed by propagation, then by termination of oxidation.

**Figure 19.** Spectra for sample A in the carbonyl band region: blue – before oxidation, red – after oxidation.

Surface area below the carbonyl peak at 1745 cm-1 is seen to decrease with an increasing sample storage time; this may be indicative of decomposition of esters due to the pres‐ ence of high temperatures and oxygen and is particularly well visible for samples B and BM (they have a higher ester content, compared with samples A and AM). Although in the vicinity of the 1745 cm-1 peak, another one is formed at 1720 cm-1, indicating the forma‐ tion of products of fuel ageing, the total area below the carbonyl peak is only slightly reduced with the increasing storage time of the fuel samples.

Based on the comparative qualitative analysis of the spectra, no visible changes are seen in the spectra of fuel samples with or without methylindole.

#### **4. Conclusion**

intense than for sample B and BM with a content of FAME of 7.2 % (V/V). In the spectral range 3600-3200 cm-1, there appears a wide absorption band, connected with valence vibrations for O-H groups. In the range 1800-1600 cm-1, there is a visible narrow carbonyl band, connected with the presence of fatty acid methyl esters (1745 cm-1) in the fuel. In samples A and AM, the peak is smaller than in samples B and BM. In the oxidized samples, in the carbonyl band 1745 cm-1 range, there appears a second band, at 1720 cm-1. The band is better discernible for samples with longer storage time and is well visible in the spectres for samples A and AM because of

 4000 3500 3000 2500 2000 1500 1000 500 Wave numbers (cm-1)

**Figure 18.** Spectra for sample B with regions of change marked: blue – sample before oxidation, red – sample after

It is hard to say without a doubt what products have been formed in the fuel samples as the result of oxidation. The carbonyl band region is connected with the presence of acids, esters, aldehydes or ketones. The band region which is characteristic for O-H groups is connected with the presence of alcohols, phenols, carboxylic acids, or water which may be formed in the oxidized fuel as the result of decomposition of methyl esters. A different measurement technique should be used to identify the resulting products of oxidation more reliably.

A rough quantitative analysis of the analytical band at 3440 cm-1, consisting in measuring the surface area below the peak, indicates that the surface area tended to grow with storage time, reaching its maximum after 3-4 months of storage, then was getting smaller. This is attributed to the radical mechanism of fuel degradation, where initiation of the reaction is followed by

region

Carbonyl band

their low content of FAME.

184 Storage Stability of Fuels

 O-H groups region

propagation, then by termination of oxidation.

oxidation.

%T

IR spectra indicate that only slight changes occur in the chemical composition of the RON 95 and RON 98 gasolines during storage for 6 months, due to their autoxidation. Predominantly compounds which contain carbonyl and hydroxyl groups are formed, which is typical of hydrocarbon oxidation products. Addition of precursors of oxidation, such as peroxides or cyclohexene, to the stored fuels slightly intensified the changes. Subjecting the fuel to rapid ageing by exposing it to elevated temperatures and oxygen pressure, shows that oxidation of hydrocarbons is accompanied by other changes, leading to the formation of compounds with a triple bond.

Determination of induction period by the micro method, of the content of inherent and potential resins, indicate changes in the chemical stability of the gasoline samples, although the tendency of such changes cannot explicitly be identified nor can it be related to chemical changes, as shown by IR spectra.

From a detailed analysis of the results of stability tests, combined with an analysis of changes in the IR spectra, it follows expressly that the RON 95 gasoline with ethanol is more susceptible to autoxidation, compared with RON 98, which has no ethanol content.

In the case of diesel oil, the tests indicate that the formation of deposits is favored by acidic products of decomposition of fatty acid methyl esters; moreover, addition to the diesel oil of 2-methylindole, which is one of the compounds taking part in the autoxidation reaction chain, has led to the formation of a higher amount of deposits, chiefly filterable insolubles. In the 3rd and 4th months of testing, deposits developed in the fuels with the highest intensity. IR spectrophotometric analysis has shown that fuel oxidation products are formed in the fuel during its degradation; they include carboxylic acids, aldehydes, ketones or phenols. This confirms the adopted assumption that, as the result of oxidation of fuel, organic acids and other acidic compounds tend to catalyze the formation of indolephenalene salt complexes leading, eventually, to the development of deposits.

#### **Author details**

Joanna Czarnocka\* , Anna Matuszewska and Małgorzata Odziemkowska

\*Address all correspondence to: a.matuszewska@pimot.eu

Department of Fuels, Biofuels and Lubricants, Automotive Industry Institute, Poland

#### **References**


[6] Mortier R.M., Fox M.F., Orszulik S.T., editors. Chemistry and technology of lubri‐ cants. Springer; 2010.

Determination of induction period by the micro method, of the content of inherent and potential resins, indicate changes in the chemical stability of the gasoline samples, although the tendency of such changes cannot explicitly be identified nor can it be related to chemical

From a detailed analysis of the results of stability tests, combined with an analysis of changes in the IR spectra, it follows expressly that the RON 95 gasoline with ethanol is more susceptible

In the case of diesel oil, the tests indicate that the formation of deposits is favored by acidic products of decomposition of fatty acid methyl esters; moreover, addition to the diesel oil of 2-methylindole, which is one of the compounds taking part in the autoxidation reaction chain, has led to the formation of a higher amount of deposits, chiefly filterable insolubles. In the 3rd and 4th months of testing, deposits developed in the fuels with the highest intensity. IR spectrophotometric analysis has shown that fuel oxidation products are formed in the fuel during its degradation; they include carboxylic acids, aldehydes, ketones or phenols. This confirms the adopted assumption that, as the result of oxidation of fuel, organic acids and other acidic compounds tend to catalyze the formation of indolephenalene salt complexes leading,

, Anna Matuszewska and Małgorzata Odziemkowska

[1] Batts B.D., Zuhdan Fathoni A. A literature review on fuel stability studies with par‐

[2] Pedersen C.J. Mechanism of Antioxidant Action in Gasoline. Industrial & Engineer‐

[3] Totten G.E., editor. Fuels and Lubricants Handbook: Technology, Properties, Per‐

[4] Pedersen C.J. Inhibition of Deterioration of Cracked Gasoline during Storage. Indus‐

[5] Rudnick L.R., editor. Lubricant Additives: Chemistry and Applications. Boca Raton:

formance, and Testing. West Conshohocken: ASTM International; 2003.

Department of Fuels, Biofuels and Lubricants, Automotive Industry Institute, Poland

ticular emphasis on diesel oil. Energy Fuels 1991;5(1) 2-21.

trial & Engineering Chemistry 1949;41(5) 924-928.

to autoxidation, compared with RON 98, which has no ethanol content.

changes, as shown by IR spectra.

186 Storage Stability of Fuels

eventually, to the development of deposits.

\*Address all correspondence to: a.matuszewska@pimot.eu

ing Chemistry 1956;48(10) 1881-1884.

CRC Press; 2010.

**Author details**

Joanna Czarnocka\*

**References**

