**3. Theoretical background**

It is known that one of the basic approaches to promote the corrosion resistance of alloys is to enhance their passivity. It has been established that upon introducing a new component with higher inclination to passive state into the metal or into the alloy, it transfers this property to the main metal or to the alloy. The formation of a system, which is more stable to corrosion (i.e more easily passivated system) could be achieved through promoting the effectiveness of the cathodic process. At first glance this is a self-contradictory statement, however it can be easily explained in the following way. It can be seen from the corrosion diagram, represented in Fig. 1, that if the anodic potentiodynamic curve of the alloy remains one and the same, the rate of corrosion can be changed considerably at the expense of the changing effectiveness of the occurring cathodic process. It is important to note that in the case of non-passivating systems (i.e. systems characterized by anodic behavior until point B of the anodic curve) the corrosion rate is always increasing with the increase in the cathodic efficiency (for example the transition from cathodic curve K1 to K2 in Fig.1). In the cases when the corrosion systems are passivating ones and the anodic polarization curve is not a monotonous dependence between the potential and the current and when there exists a

The chemical composition and state of the surfaces being formed was studied using X-ray photoelectron spectroscopy (XPS). The XPS studies were performed on an Escalab MkII system (England) with Al K radiation (*hν* = 1486.6 eV) and total instrumental resolution of ~ 1 eV. The pressure in the chamber was 10–8 Pa. The binding energy (BE) was referred to the C1s line (of the adventitious carbon) at 285.0 eV. The element concentrations were evaluated from the integrated peak areas after Shirley-type of linear background subtraction

The electrochemical behaviour of the samples (plates 10 x 10 x 0.05mm) was studied in a standard three-electrode thermostated cell (100 ml volume). The model corrosion medium was 0.1 N H2SO4 ("p.a." Merck) after deaeration with additionally purified argon at 25C*.* A counter electrode, representing a platinum plate (10x10x0.6mm), and a mercury/mercurous sulfate reference electrode (MSE), (EHg/Hg2SO4= +0.642V versus SHE) were used. All potentials in the text are related to MSE. The anodic and cathodic polarization curves were obtained using a 273 EG&G potentiostat/galvanostat and computer-aided processing of the results according to an "Echem" programme, with a potential sweeping rate of 10 mV/s within a potential range from –1.500 to +1.500V*.* The recording of the potentiodynamic curves was carried out starting from the stationary corrosion potential (Est), measured in the absence of external current (at open circuit) in the anode and cathode directions. We used a separate electrode for each recorded curve. The stationary corrosion potential of the samples under investigation was determined by direct measurement of the function "Est–time" at open circuit (with respect to the same reference electrode) after immersing the samples in 0.1 N H2SO4 in the absence and in the presence of Ce4+. The Est was established after a sufficiently long time interval - from few minutes to several decades of minutes until the

moment, when the corrosion potential change did not exceed 1-3mV for 5 min.

It is known that one of the basic approaches to promote the corrosion resistance of alloys is to enhance their passivity. It has been established that upon introducing a new component with higher inclination to passive state into the metal or into the alloy, it transfers this property to the main metal or to the alloy. The formation of a system, which is more stable to corrosion (i.e more easily passivated system) could be achieved through promoting the effectiveness of the cathodic process. At first glance this is a self-contradictory statement, however it can be easily explained in the following way. It can be seen from the corrosion diagram, represented in Fig. 1, that if the anodic potentiodynamic curve of the alloy remains one and the same, the rate of corrosion can be changed considerably at the expense of the changing effectiveness of the occurring cathodic process. It is important to note that in the case of non-passivating systems (i.e. systems characterized by anodic behavior until point B of the anodic curve) the corrosion rate is always increasing with the increase in the cathodic efficiency (for example the transition from cathodic curve K1 to K2 in Fig.1). In the cases when the corrosion systems are passivating ones and the anodic polarization curve is not a monotonous dependence between the potential and the current and when there exists a

**2.2 Chemical characterization** 

**3. Theoretical background** 

using theoretical Scofield`s photoionization cross-sections.

**2.3 Electrochemical (corrosion) characterization** 

region of potentials somewhere between the passivation potential (Еp) and the potential of transpassivity (Еt) (or the potential of pitting formation (Еpit)), in which the increase of the effectiveness of the cathodic process is leading not to enhancement but rather to abatement of the corrosion rate (for example in the course of the transition from one cathodic process К2 to another one К3) one can observe a system more stable to corrosion (easily passivating system). Obviously in this case of minimal corrosion currents there will appear a cross-point between the anodic and the cathodic curves of the corrosion diagram within the zone of stable passive state. Under these conditions it is quite possible that a smaller corrosion current is corresponding to a more efficient cathodic process in comparison to the system displaying a lower cathodic efficiency (if we exclude the conditions where the potential of the system is reaching the value of the potential of transpassivation and the potential of pitting formation - К4). Therefore one can conclude that during the occurring of an efficient cathodic process the system will pass over spontaneously to a stable passive state and it will be corroding at a much lower rate, corresponding to the current of complete passivation. The stationary corrosion potential of such a system will be more positive than the potential of complete passivation (Ecp) and at the same time more negative than the potential of break through the passive film and the potential of transpassivation. In this way the rate of corrosion can be reduced to a considerable extent by the correct use of the phenomenon "passivation".

Fig. 1. Schematic polarization diagram explaining the action of the effective cathodic coatings on the steel corrosion: ip, icp., ipit, it- respectively currents of initial passivation, complete passivation, pitting formation and corrosion in transpassive state.

So, in order to create a system stable to corrosion and to decrease the rate of corrosion, it is necessary to find a way to promote the cathodic effectiveness (for example as it is in this specific case of investigations, carried out by us, to modify the steel surface with СеО2-Ce2O3

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 247

become commensurable with that of the pure steel. At concentration of the cerium oxides, corresponding to the cathodic curve С2 it is possible to establish two corrosion potentials of the steel, located respectively in the passive and active regions of dissolution of the anodic potentiodynamic curve. At the higher concentrations of the cerium oxides, represented in the corrosion diagram by means of the respective cathodic potentiodynamic curves of the system cerium oxides/steel - С3 and С4, the steel is characterized by a stable passive state and under these conditions the rate of corrosion of the steel is no longer dependent on the surface concentration of the cerium oxides. The influence of the further increase in the concentration of the cerium oxides can be illustrated through the corrosion diagram by means of the cathodic curve С5 – the rate of steel corrosion will grow up due to the fact that the stationary corrosion potential of the steel will be shifted to the region of transpassivity (Tomashov & Chernova, 1963; 1993). The experimental results, obtained by us, confirm these

Figure 3 shows a typical experimentally obtained corrosion diagram Е-lg i, illustrating the kinetics of the cathodic and anodic processes on the studied steel in the absence of electrochemically deposited cerium oxides film (the curves 2) and after the deposition of

1 1 - Ce as a metal

10-9 10-8 10-7 10-6 10-5 10-4

i, mA.cm-2

Fig. 3a. Potentiodynamic E–lgi curves for Ce (1), for SS (2) and for the systems CeO2- Ce2O3/SS containing different concentrations of Ce (3-5), obtained in 0.1 N H2SO4.

5

a)

2

4

3

thin oxide films with different surface concentrations of Ce (curves 3-5).

3 - Ce oxides/SS (3.6 at.%) 4 - Ce oxides/SS (13.1 at.%) 5 - Ce oxides/SS (25.7 at.%)

theoretical concepts.

**4. Experimental results and discussion 4.1 Potentiodynamic polarization studies** 

2 - SS



0.0

E, V vs. MSE

0.5

oxides as cathodic coating). This theoretical approach has been used in this work, with a view to explain the obtained results, in view of stabilization and restoration of the passive state of ОС 404 steel as a consequence of electrochemical formation of the surface СеО2- Ce2O3 layers. We agree with the assumption that for a similar type of modified systems there exists only one passive state of the system (even without applying any external anodic current). Or, in other words, the result is a spontaneously self-passivating system and if in some way it is being led away from its passive state (for example in the case of cathodic polarization or by exerting a mechanical impact), after the termination of the external effect, again the system will pass over to its passive state.

As a matter of fact the increase in the effectiveness of the cathodic process is connected with the character of the cathodic process. The dilemma is whether the promotion of the cathodic efficiency is due only to the process of hydrogen depolarization, on the cerium oxide cathodic coating (in case of steel corrosion in acidic medium) or it is possible that there exists another cathodic process, owing to the oxidative properties of the electrochemically active CeO2.

Figure 2 represents an example of a corrosion diagram, illustrating the changes in the behavior of the corrosion system upon increasing the surface concentration of the effective cathodic coating (for example in the case of modifying the steel surface with cerium oxides). It follows from the diagram that the shift in the corrosion potential of the system is associated with the increase in the concentration of the cathodic depolarizer (the cerium oxides), which will facilitate the transition from active state into passive state of the system, under the conditions of disturbed passivity. At concentration of the cerium oxides, corresponding to the cathodic curve С1 (Fig. 2), the rate of dissolution of the steel will

Fig. 2. Schematic polarization diagram illustraiting the influence of the effective cathodic coating on the steel corrosion, respective cathodic curves : c1, c2, c3, c4, c5 in case of increasing the surface concentration of cerium oxides.

become commensurable with that of the pure steel. At concentration of the cerium oxides, corresponding to the cathodic curve С2 it is possible to establish two corrosion potentials of the steel, located respectively in the passive and active regions of dissolution of the anodic potentiodynamic curve. At the higher concentrations of the cerium oxides, represented in the corrosion diagram by means of the respective cathodic potentiodynamic curves of the system cerium oxides/steel - С3 and С4, the steel is characterized by a stable passive state and under these conditions the rate of corrosion of the steel is no longer dependent on the surface concentration of the cerium oxides. The influence of the further increase in the concentration of the cerium oxides can be illustrated through the corrosion diagram by means of the cathodic curve С5 – the rate of steel corrosion will grow up due to the fact that the stationary corrosion potential of the steel will be shifted to the region of transpassivity (Tomashov & Chernova, 1963; 1993). The experimental results, obtained by us, confirm these theoretical concepts.

## **4. Experimental results and discussion**

#### **4.1 Potentiodynamic polarization studies**

246 Corrosion Resistance

oxides as cathodic coating). This theoretical approach has been used in this work, with a view to explain the obtained results, in view of stabilization and restoration of the passive state of ОС 404 steel as a consequence of electrochemical formation of the surface СеО2- Ce2O3 layers. We agree with the assumption that for a similar type of modified systems there exists only one passive state of the system (even without applying any external anodic current). Or, in other words, the result is a spontaneously self-passivating system and if in some way it is being led away from its passive state (for example in the case of cathodic polarization or by exerting a mechanical impact), after the termination of the external effect,

As a matter of fact the increase in the effectiveness of the cathodic process is connected with the character of the cathodic process. The dilemma is whether the promotion of the cathodic efficiency is due only to the process of hydrogen depolarization, on the cerium oxide cathodic coating (in case of steel corrosion in acidic medium) or it is possible that there exists another cathodic process, owing to the oxidative properties of the electrochemically

Figure 2 represents an example of a corrosion diagram, illustrating the changes in the behavior of the corrosion system upon increasing the surface concentration of the effective cathodic coating (for example in the case of modifying the steel surface with cerium oxides). It follows from the diagram that the shift in the corrosion potential of the system is associated with the increase in the concentration of the cathodic depolarizer (the cerium oxides), which will facilitate the transition from active state into passive state of the system, under the conditions of disturbed passivity. At concentration of the cerium oxides, corresponding to the cathodic curve С1 (Fig. 2), the rate of dissolution of the steel will

<sup>A</sup> C5

lg i

Fig. 2. Schematic polarization diagram illustraiting the influence of the effective cathodic coating on the steel corrosion, respective cathodic curves : c1, c2, c3, c4, c5 in case of increasing

C4

C3


the surface concentration of cerium oxides.

C2

C1

Eo A

again the system will pass over to its passive state.

active CeO2.

Figure 3 shows a typical experimentally obtained corrosion diagram Е-lg i, illustrating the kinetics of the cathodic and anodic processes on the studied steel in the absence of electrochemically deposited cerium oxides film (the curves 2) and after the deposition of thin oxide films with different surface concentrations of Ce (curves 3-5).

Fig. 3a. Potentiodynamic E–lgi curves for Ce (1), for SS (2) and for the systems CeO2- Ce2O3/SS containing different concentrations of Ce (3-5), obtained in 0.1 N H2SO4.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 249

The strong shifting of Еcorr of the steel surface, covered with cerium oxides in the positive direction depending on the surface concentration of cerium could also be associated with the occurrence of another cathodic process. The values of the corrosion potential in the presence of Ce are more positive than the Flade potential and more negative than the potential of transpassivity of the steel under consideration. Therefore we can conclude that even at surface concentration of Ce about 4% the corrosion potential of the steel is shifted in positive direction reaching potentials more positive than the potential of complete passivation. The improvement of the corrosion stability of the steel as a result of the action of the effective cathodic coating is expressed in the stabilization of the passive state of the steel. One can conclude from Figure 3a that the steel samples with electrochemically deposited cerium oxide film will corrode under the conditions of passivity. Thereupon with the increase in the surface concentration of Ce from 0 up to 3.6 at % a tendency is observed – a decrease in the currents of complete passivation (ic.p.,see Table 1). At 13.1%concentration of the cerium oxides the current of complete passivation is of the same order with that of the non-covered steel, while at 25,7% concentration as a result of the strong shifting of Еcorr in positive direction and its approaching the values within the zone of potentials of pitting formation and transpassivity the anodic potentiodynamic curve is not characterized by a well expressed region of passivity. The change in the currents of complete passivation depends to a considerable extent on the composition of the passive film on the steel. Therefore for the system Ce2O3-CeO2/SS we can assume that it will pass over spontaneously into a stable passive state and that it will be dissolved at a much lower rate of corrosion, corresponding to the values of the anodic

If in one way or another the system is artificially taken out of its state of passivity (for example by means of cathodic polarization or by some mechanical impact), after discontinuing the external impact, it will restore again its passive state, i.e. what we obtain is

A similar effect, expressed to an even greater extent, is also observed with the samples of thermally treated system Ce2O3-CeO2/SSt.t (Fig. 3b), in which case it was established that as a result of disruption of the integrity of the oxide film the passive state of the steel is disturbed (Guergova et al., 2008) and conditions are created to increase the rates of the total and the local corrosion. The presence of electrochemically deposited cerium oxide film (in a way analogous to that for the samples of non-treated thermally system Ce2O3-CeO2/SS) shifts strongly the corrosion potential of the system in positive direction (see curves 3–5). This effect determines the restoration of the passive state of the steel, disturbed as a result of its thermal treatment. Upon increasing the surface concentration of the cerium one can observe not only shifting of the corrosion potential of the samples in the positive direction, but also a tendency of decrease in the currents of complete passivation. An exception in this respect is observed at very high concentrations of the cerium oxides (≥ ~29%). Obviously in these cases the corrosion potential of the system CeO2-Ce2O3/SSt.t, which is still in the process of being established, starts approaching the value of the reversible redox potential of the couple Ce4+/Ce3+, whereupon the reaction of oxidation of Ce3 to Ce4+ is taking place. As a result of this the character of the anodic curve will be changed (Fig. 3b, curve 5) and the determination of the current of complete passivation of the steel based on this curve would

currents in the passive state (Fig. 3a).

a spontaneously self-passivating system.

be incorrect.

The plotting of the model polarization curves enabled us to follow the changes in Еcorr , estimated on the basis of the cross-point of the anodic polarization curve of the studied steel (SS) with the cathodic polarization curves of the studied systems, having different surface concentrations of Ce (ranging from 4% up to 30%). Such an approach of considering the partial polarization curves allows us to make the connection between the occurring anodic and cathodic corrosion processes, localizing the cathodic reaction on the rich in cerium zones on the electrode surface. For the sake of comparison the figure represents also the anodic and cathodic potentiodynamic curves of the metallic Ce (curves1). It follows from Fig. 3а that with the increase in the surface concentration of Ce (curves 3-5) the values of the corrosion potential Еcorr are shifted strongly in the positive direction - from ~ –0.900V (for the non-coated with cerium oxides steel) up to ~ + 0.160 V. Obviously for the non-coated with CeO2-Ce2O3 steel surface the cathodic depolarizing reaction, occurring at voltage ~ –0.900V, is connected with the evolution of hydrogen. As far as the respective cathodic branches of the potentiodynamic curves of the system Ce2O3-CeO2/ SS are concerned, they are also shifted strongly in positive direction in the zone of passivity of the investigated steel.

The slope of the cathodic Taffel's curves grows up considerably from 0.250V up to 0.319 V with the increase in the surface concentration of cerium (Table 1). The change in the slope of these curves confirms the supposition about the occurring of another cathodic reaction, different from that of hydrogen evolution on the heterogeneous electrode surface.


Table 1. Electrochemical characteristics of coated steel before and after thermal treatment compared to bare steel.

The plotting of the model polarization curves enabled us to follow the changes in Еcorr , estimated on the basis of the cross-point of the anodic polarization curve of the studied steel (SS) with the cathodic polarization curves of the studied systems, having different surface concentrations of Ce (ranging from 4% up to 30%). Such an approach of considering the partial polarization curves allows us to make the connection between the occurring anodic and cathodic corrosion processes, localizing the cathodic reaction on the rich in cerium zones on the electrode surface. For the sake of comparison the figure represents also the anodic and cathodic potentiodynamic curves of the metallic Ce (curves1). It follows from Fig. 3а that with the increase in the surface concentration of Ce (curves 3-5) the values of the corrosion potential Еcorr are shifted strongly in the positive direction - from ~ –0.900V (for the non-coated with cerium oxides steel) up to ~ + 0.160 V. Obviously for the non-coated with CeO2-Ce2O3 steel surface the cathodic depolarizing reaction, occurring at voltage ~ –0.900V, is connected with the evolution of hydrogen. As far as the respective cathodic branches of the potentiodynamic curves of the system Ce2O3-CeO2/ SS are concerned, they are also shifted strongly in positive direction in the

The slope of the cathodic Taffel's curves grows up considerably from 0.250V up to 0.319 V with the increase in the surface concentration of cerium (Table 1). The change in the slope of these curves confirms the supposition about the occurring of another cathodic reaction,

**SS** 

**Samples Ecorr, V icorr, mA.cm-2 ic.p, mA.cm-2 b, V** 

SS non-covered with cerium oxides -0.890 2.24 x 10-7 1.89 x 10-6 0.107

SS covered with 3.6 at. % cerium oxides - 0.432 2.85 x 10-7 9.10 x 10-7 0.250

SS covered with 13.1 at. % cerium oxides - 0.371 2.93 x 10-7 3.63 x 10-6 0.276

SS covered with 25.7 at. % cerium oxides +0.161 1.36 x 10-7 - 0.319

**SSt.t.**

**Samples Ecorr, V icorr, mA.cm-2 ic.p., mA.cm-2 b, V** 

SS non-covered with cerium oxides - 0.975 1.74 x 10-7 3.21 x 10-6 0.074

SS covered with 4.2 at. % cerium oxides - 0.486 7.96 x 10-7 2.96 x 10-6 0.151

SS covered with 20.7 at. % cerium oxides - 0.269 7.98 x 10-8 1.33 x 10-6 0.176

SS covered with 29.6 at. % cerium oxides + 0.090 8.19 x 10-7 - 0.304

Table 1. Electrochemical characteristics of coated steel before and after thermal treatment

different from that of hydrogen evolution on the heterogeneous electrode surface.

zone of passivity of the investigated steel.

compared to bare steel.

The strong shifting of Еcorr of the steel surface, covered with cerium oxides in the positive direction depending on the surface concentration of cerium could also be associated with the occurrence of another cathodic process. The values of the corrosion potential in the presence of Ce are more positive than the Flade potential and more negative than the potential of transpassivity of the steel under consideration. Therefore we can conclude that even at surface concentration of Ce about 4% the corrosion potential of the steel is shifted in positive direction reaching potentials more positive than the potential of complete passivation. The improvement of the corrosion stability of the steel as a result of the action of the effective cathodic coating is expressed in the stabilization of the passive state of the steel. One can conclude from Figure 3a that the steel samples with electrochemically deposited cerium oxide film will corrode under the conditions of passivity. Thereupon with the increase in the surface concentration of Ce from 0 up to 3.6 at % a tendency is observed – a decrease in the currents of complete passivation (ic.p.,see Table 1). At 13.1%concentration of the cerium oxides the current of complete passivation is of the same order with that of the non-covered steel, while at 25,7% concentration as a result of the strong shifting of Еcorr in positive direction and its approaching the values within the zone of potentials of pitting formation and transpassivity the anodic potentiodynamic curve is not characterized by a well expressed region of passivity. The change in the currents of complete passivation depends to a considerable extent on the composition of the passive film on the steel. Therefore for the system Ce2O3-CeO2/SS we can assume that it will pass over spontaneously into a stable passive state and that it will be dissolved at a much lower rate of corrosion, corresponding to the values of the anodic currents in the passive state (Fig. 3a).

If in one way or another the system is artificially taken out of its state of passivity (for example by means of cathodic polarization or by some mechanical impact), after discontinuing the external impact, it will restore again its passive state, i.e. what we obtain is a spontaneously self-passivating system.

A similar effect, expressed to an even greater extent, is also observed with the samples of thermally treated system Ce2O3-CeO2/SSt.t (Fig. 3b), in which case it was established that as a result of disruption of the integrity of the oxide film the passive state of the steel is disturbed (Guergova et al., 2008) and conditions are created to increase the rates of the total and the local corrosion. The presence of electrochemically deposited cerium oxide film (in a way analogous to that for the samples of non-treated thermally system Ce2O3-CeO2/SS) shifts strongly the corrosion potential of the system in positive direction (see curves 3–5). This effect determines the restoration of the passive state of the steel, disturbed as a result of its thermal treatment. Upon increasing the surface concentration of the cerium one can observe not only shifting of the corrosion potential of the samples in the positive direction, but also a tendency of decrease in the currents of complete passivation. An exception in this respect is observed at very high concentrations of the cerium oxides (≥ ~29%). Obviously in these cases the corrosion potential of the system CeO2-Ce2O3/SSt.t, which is still in the process of being established, starts approaching the value of the reversible redox potential of the couple Ce4+/Ce3+, whereupon the reaction of oxidation of Ce3 to Ce4+ is taking place. As a result of this the character of the anodic curve will be changed (Fig. 3b, curve 5) and the determination of the current of complete passivation of the steel based on this curve would be incorrect.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 251

Ce2O3-CeO2/SS (25.7 at.%)

Ce2O3-CeO2/SS (13.1 at.%)

Ce2O3-CeO2/SS (3.6 at.%)

CeO2/SS (6.8 at.%)

CeO2/SS (57.4 at.%)

**thermal treated samples**

CeO2/SS (20.7 at.%)

CeO2/SS (4.2 at.%)

b)

a)

0 20 40 60 80 100 120 140 160

time, min

0 20 40 60 80 100 120

time, min

Fig. 4. Open circuit potential vs. time curves for SS (a) and SSt.t.(b), as well as for the systems CeO2-Ce2O3/SS (a) and CeO2-Ce2O3/SSt.t..(b), containing different concentrations of Ce,

SSt.t.

SS



obtained in 0.1 N H2SO4.





E, V vs. MSE

0.0

0.2

0.4

0.6


0.0

E, V vs. MSE

0.2

0.4

Fig. 3b. Potentiodynamic E–lgi curves for Ce(1), for SSt.t. (2) and for the systems CeO2- Ce2O3/SSt.t. containing different concentrations of Ce (3-5), obtained in 0.1 N H2SO4.

#### **4.2 Chronopotentiometric investigation**

Fig. 4a and 4b illustrate the altering of the stationary corrosion potentials in the case of open circuit (open circuit potentials) with the SS and SSt.t. samples and with the systems Ce2O3- CeO2/SS and Ce2O3-CeO2/SSt.t. The juxtaposition of the values of the stationary corrosion potentials with the anodic potentiodynamic curves of SS and SSt.t. shows that in the presence of cerium oxide film on the surface of the steel one can observe a strongly manifested tendency to self-assivation. In the cases of non-thermally treated steel its high corrosion resistance and its ability to passivate itself is connected also with the high content of Cr, while the role of the cerium oxides is reduced to promoting the passivation ability and stabilization of its passive state in weakly acidic medium (Stoyanova et al, 2006). In the case of thermally treated steel, however, due to the cracking of the surface passive film, as a result of the thermal treatment its stationary corrosion potential reaches values (Еst.= - 0.975V), characteristic of the corrosion in the active state (Fig. 4b). The disrupted passive state is also a prerequisite for the development of local corrosion in the active anodic sections – pitting and/or inter-crystalline, which is characteristic for this type of steel. It is also seen in Fig. 4b that the electrochemically formed cerium oxide films on the surface of the steel samples lead to strong shifting of the stationary corrosion potential of the steel in the positive direction – to potentials more positive than the potential of complete passivation and more negative than the potential of transpassivity. The established experimental facts unanimously indicate that the electrochemically deposited oxide films on the surface of the steel lead to restoration of its passive state, due to promoted ability of the system to passivate itself under the conditions of the real corrosion process.

**thermal treated samples** 1

1 - Ce as a metal

3 - CeO2/SS (4.2 at.%) 4 - CeO2/SS (20.7 at.%)

5 - CeO2/SS (29.6 at.5%)

2 - SS

10-9 10-8 10-7 10-6 10-5 10-4 10-3

i, mA.cm-2

Fig. 4a and 4b illustrate the altering of the stationary corrosion potentials in the case of open circuit (open circuit potentials) with the SS and SSt.t. samples and with the systems Ce2O3- CeO2/SS and Ce2O3-CeO2/SSt.t. The juxtaposition of the values of the stationary corrosion potentials with the anodic potentiodynamic curves of SS and SSt.t. shows that in the presence of cerium oxide film on the surface of the steel one can observe a strongly manifested tendency to self-assivation. In the cases of non-thermally treated steel its high corrosion resistance and its ability to passivate itself is connected also with the high content of Cr, while the role of the cerium oxides is reduced to promoting the passivation ability and stabilization of its passive state in weakly acidic medium (Stoyanova et al, 2006). In the case of thermally treated steel, however, due to the cracking of the surface passive film, as a result of the thermal treatment its stationary corrosion potential reaches values (Еst.= - 0.975V), characteristic of the corrosion in the active state (Fig. 4b). The disrupted passive state is also a prerequisite for the development of local corrosion in the active anodic sections – pitting and/or inter-crystalline, which is characteristic for this type of steel. It is also seen in Fig. 4b that the electrochemically formed cerium oxide films on the surface of the steel samples lead to strong shifting of the stationary corrosion potential of the steel in the positive direction – to potentials more positive than the potential of complete passivation and more negative than the potential of transpassivity. The established experimental facts unanimously indicate that the electrochemically deposited oxide films on the surface of the steel lead to restoration of its passive state, due to promoted ability of the

Fig. 3b. Potentiodynamic E–lgi curves for Ce(1), for SSt.t. (2) and for the systems CeO2- Ce2O3/SSt.t. containing different concentrations of Ce (3-5), obtained in 0.1 N H2SO4.

system to passivate itself under the conditions of the real corrosion process.

2

4

3

5

b)


**4.2 Chronopotentiometric investigation** 


E, V vs. MSE

0.0

0.5

Fig. 4. Open circuit potential vs. time curves for SS (a) and SSt.t.(b), as well as for the systems CeO2-Ce2O3/SS (a) and CeO2-Ce2O3/SSt.t..(b), containing different concentrations of Ce, obtained in 0.1 N H2SO4.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 253

Figure 5 illustrates the analogous Е-τ dependences at open circuit, obtained upon immersion of SStt in 0.1 N H2SO4 solution, to which various concentrations of Се4+ ions have been added. It was important to find out what is the influence of cerium ions on the corrosion behavior of the samples of thermally treated steel, when the character of the corrosion process is changed as a consequence of the thermal treatment of the steel. It should be reminded at this point that the stationary corrosion potentials (Еst) of SS and SSt.t.in 0.1N H2SO4 solution have values for the non-thermally treated steel Еst= ~ -0.300 V, while for the thermally treated steel this value is Еst= ~ -0.980 V (Fig. 4)). The registered negative values of Еst in the absence of cerium ions for the SSt.t. samples, in our opinion, are connected with the strong cracking of the natural passive film on the surface of SS (Fig. 6). The most probable reason for this loss of the "stainless character" of the steel surface are the revealed sections, determining a several times higher concentration of iron-containing agglomerates in the

0 20 40 60 80 100

Time, min

Fig. 5. Evolution of the open-circuit potential for SSt.t. at different concentrations of Ce4+ in

**a)**

Fig. 6. SEM images of stainless steel before (a) and after thermally treatment (b).

0.075m

500 ppm 1000 ppm

25.3 ppm 15.4 ppm 0.9 ppm 0.3 ppm

0.1 ppm

0.075m

**b)**

surface layer (Table 2).

0.1 N H2SO4 solution.


E, V/MSE

The strong shifting of Еcorr of the steel sample, modified with cerium oxide film, in the positive direction, depending on the surface concentration of cerium, can be attributed to the occurrence of another cathodic process in addition to the reaction of hydrogen evolution. We can assume that the effective cathodic sections of CeO2-Ce2O3 will participate in the occurring cathodic depolarizing reaction according to the equations (1 and 2) given below:

$$\rm{CeO\_2 + 2H^+ = Ce(OH)\_2^{2+}} \tag{1}$$

$$2\text{Ce}(\text{OH})\_2\text{}^{2+} + 2\text{e}^- = \text{Ce}\_2\text{O}\_3 + \text{H}\_2\text{O} + 2\text{H}^+\tag{2}$$

The above indicated reactions occur as a result of the extraordinary oxidation-reduction capability of the couple CeO2-Ce2O3. The occurring of these reactions means that in the course of the corrosion process the surface film will be changing, enriching itself in Ce2O3. On the other side, the reactions (3) and (4) will also take place on the anodic sections of the steel surface and the latter one will lead to passivation:

$$\text{Me} \rightarrow \text{Me}^{\bullet \ast} \text{\textbullet} \tag{3}$$

$$\text{Me} + \text{H}\_2\text{O} \rightarrow \text{Me}\_2\text{O}\_n \text{ + ne} \tag{4}$$

Taking into account the fact that the oxides of Се3+ of the type Ce2O3 are soluble in acids*,*  the reaction reported in (Achmetov, 1988) will also occur:

$$\rm Ce\_2O\_3 + 6H^+ = 2Ce^{3\*} + 3H\_2O \tag{5}$$

As well as the respective conjugated reaction of oxidation:

$$\rm{Ce^{\cdot+} + 2H\_2O = Ce(OH)\_2^{\cdot \cdot \cdot} + 2H^+ + e^-} \tag{6}$$

Obviously, the cathodic reaction of reduction of CeO2, which occurs at the corrosion potentials, established for the systems CeO2-Ce2O3/SSt.t., is the main reason for restoring and preserving the passive state of the thermally treated steel samples (in accordance with equation 4) during their corrosion in solutions of sulfuric acid.

#### **4.3 The inhibiting effect of cerium ions**

In connection with the above statements a next step has been made in the investigations, namely studying the influence of the Ce3+ and Ce4+ ions as components of the corrosion medium (0.1N H2SO4) on the anodic behavior of stainless steel. These investigations were provoked by the observed occurrence of cathodic depolarization reaction of Ce4+ (CeO2) reduction, as a result of which the surface concentration of cerium is decreasing and theoretically it should approach zero value (Stoyanova et al., 2010). For this purpose an inverse experiment was carried out at different concentrations of Ce4+ ions in the corrosion medium we monitored the changes in the stationary corrosion potential of the thermally treated steel by the chronopotentiometric method. The aim of this experiment was to prove the occurrence of a reversible reaction of reduction of Ce4+: Ce4+- e ↔ Ce3+, (instead of the reaction of hydrogen depolarization), which in its turn creates also the option to form a film (chemically insoluble) of cerium hydroxides/oxides on the active sections of the steel surface.

The strong shifting of Еcorr of the steel sample, modified with cerium oxide film, in the positive direction, depending on the surface concentration of cerium, can be attributed to the occurrence of another cathodic process in addition to the reaction of hydrogen evolution. We can assume that the effective cathodic sections of CeO2-Ce2O3 will participate in the occurring cathodic depolarizing reaction according to the equations (1 and 2) given below:

CeO2 + 2H+ =Ce(ОH)22+ (1)

 2Ce(OH)22+ +2e- =Ce2O3 +H2O +2H+ (2) The above indicated reactions occur as a result of the extraordinary oxidation-reduction capability of the couple CeO2-Ce2O3. The occurring of these reactions means that in the course of the corrosion process the surface film will be changing, enriching itself in Ce2O3. On the other side, the reactions (3) and (4) will also take place on the anodic sections of the

Taking into account the fact that the oxides of Се3+ of the type Ce2O3 are soluble in acids*,* 

Ce2O3 + 6H+ = 2Ce3+ +3H2O (5)

 Ce3++ 2H2O = Ce(OH)22+ + 2H+ + e- (6) Obviously, the cathodic reaction of reduction of CeO2, which occurs at the corrosion potentials, established for the systems CeO2-Ce2O3/SSt.t., is the main reason for restoring and preserving the passive state of the thermally treated steel samples (in accordance with

In connection with the above statements a next step has been made in the investigations, namely studying the influence of the Ce3+ and Ce4+ ions as components of the corrosion medium (0.1N H2SO4) on the anodic behavior of stainless steel. These investigations were provoked by the observed occurrence of cathodic depolarization reaction of Ce4+ (CeO2) reduction, as a result of which the surface concentration of cerium is decreasing and theoretically it should approach zero value (Stoyanova et al., 2010). For this purpose an inverse experiment was carried out at different concentrations of Ce4+ ions in the corrosion medium we monitored the changes in the stationary corrosion potential of the thermally treated steel by the chronopotentiometric method. The aim of this experiment was to prove the occurrence of a reversible reaction of reduction of Ce4+: Ce4+- e ↔ Ce3+, (instead of the reaction of hydrogen depolarization), which in its turn creates also the option to form a film (chemically insoluble) of cerium hydroxides/oxides on the active sections of the steel

Ме → Меn++ ne (3)

Me + H2O → Me2On + ne (4)

steel surface and the latter one will lead to passivation:

the reaction reported in (Achmetov, 1988) will also occur:

As well as the respective conjugated reaction of oxidation:

equation 4) during their corrosion in solutions of sulfuric acid.

**4.3 The inhibiting effect of cerium ions**

surface.

Figure 5 illustrates the analogous Е-τ dependences at open circuit, obtained upon immersion of SStt in 0.1 N H2SO4 solution, to which various concentrations of Се4+ ions have been added. It was important to find out what is the influence of cerium ions on the corrosion behavior of the samples of thermally treated steel, when the character of the corrosion process is changed as a consequence of the thermal treatment of the steel. It should be reminded at this point that the stationary corrosion potentials (Еst) of SS and SSt.t.in 0.1N H2SO4 solution have values for the non-thermally treated steel Еst= ~ -0.300 V, while for the thermally treated steel this value is Еst= ~ -0.980 V (Fig. 4)). The registered negative values of Еst in the absence of cerium ions for the SSt.t. samples, in our opinion, are connected with the strong cracking of the natural passive film on the surface of SS (Fig. 6). The most probable reason for this loss of the "stainless character" of the steel surface are the revealed sections, determining a several times higher concentration of iron-containing agglomerates in the surface layer (Table 2).

Fig. 5. Evolution of the open-circuit potential for SSt.t. at different concentrations of Ce4+ in 0.1 N H2SO4 solution.

Fig. 6. SEM images of stainless steel before (a) and after thermally treatment (b).

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 255

of a reduction reaction Ce4+ ↔ Ce3+ in the redox system Ce4+/ Ce3+. Therefore the Ce4+ ions, as component of the corrosion medium, are acting as inhibitor, exerting an oxidative effect. It follows from (Fig. 7), that the increase in the concentration of the inhibitor in the corrosion environment leads to a substantial decrease in the corrosion current - from 1.10-6 (at inhibitor concentration of 0.1 ppm) - to 1. 10-8 А.сm-2 (at inhibitor concentration ~0.9 ppm). What is making impression is the fact that the further increase in the concentration of the Ce4+ ions in the corrosion medium from ~0.9 ppm (which could be accepted as "critical") up to 1000 ррm influences to a smaller extent the rate of corrosion. It is necessary also to point out that with the increase in the concentration of cerium ions the corrosion potential is shifted in positive direction, whereupon its values remain more positive than the potential of complete passivation and more negative than the potential of depassivation of the steel within the entire interval of studied concentrations – an effect analogous to the one already

established for the systems Ce2O3-CeO2/SSt.t.in 0.1 N H2SO4 solution.

0.3 ppm

10-8 10-7 10-6 10-5

0.1 ppm

SSt.t.

Z% = (iocorr.- icorr.) / iocorr.x 100 (7)

i, A cm-2

Fig. 7. Potentiodynamic E-lgi curves of SSt.t. at different concentrations of Ce4+ in 0.1 N

Тable 3 lists the electrochemical kinetic parameters: corrosion current density (icorr), corrosion potential (Еcorr.) and degree of inhibition efficiency (Z, %), characterizing the corrosion process in the presence and in the absence of cerium ions, determined on the basis of the results represented in Fig. 7. The degree of inhibition efficiency has been calculated on

where iocorr and icorr are the values of the corrosion current density in the absence and in the


H2SO4 solution.

the basis of the equation:

presence of cerium ions.


E, V/MSE

0,0

0,5

1000 ppm

0.9 ppm


Table 2. Distribution of the elements (in at. %) on the surface SS and SSt.t. before and after 50h immersion in 0.1N H2SO4.

For this reason this cycle of investigations has been carried out using samples of thermally treated steel, since the thermal treatment leads to change in the character of the corrosion process of steel. It should be taken into account that such kind of excessive heat treatment happen to take place both in the formation of catalytic converters, as well as in the course of their operation. In the latter case in the presence of cerium ions in the corrosion medium (Fig. 5), one observes a strong shifting of Еst in positive direction (from -0.942 V to -0.286 V), even at relatively low concentrations of Се4+ (0.3 ppm) in the corrosion medium. The further increase in the concentration of Се4+ ions (from 0.3 to 0.9 ppm) results in insignificant changes in Еst . Thereupon for SStt this shift jumps from -0.942 V (at Се4+ ions concentration 0.1 ppm) up to -0.175 V (at Се4+ ions concentration 0.9 ppm). In the consecutive 20 – 30 fold increase in the concentration of Се4+ ions (15-25 ppm) a preservation of the Еst value is observed, whereupon it manifests values ~ -0.150 - -0.120 V. The consecutive 20-50 fold increase in the concentration of Се4+ ions (500-1000 ppm) leads to strong shifting of Еst in positive direction reaching values of about +0.510 - +0.570V.

These results prove that in the case of samples of thermally treated steel non-coated with Ce2O3-CeO2 one observes analogous changes in the stationary corrosion potential of the steel electrode, which have already been registered for the system Ce2O3-CeO2/SSt.t.. The juxtaposition of the above-mentioned changes in Еst at open circuit (conditions of selfdissolution) with the characteristic zones (corrosion potential, Flade potential, zone of passivity, transpassivity region), defined by the cathodic and anodic potentiodynamic E-lgi polarization curves (conditions of external cathodic and anodic polarization) for SStt (Fig. 7.) in 0.1 N H2SO4 solution not-containing Се4+, shows the following. The addition of cerium ions causes shifting and establishing stationary corrosion potential (Fig. 7) in the zone of passivity of the steel. Evidently, this effect will lead also to improvement of the passivation ability, respectively to improvement of the stability to corrosion, of the steel in sulfuric acid medium, which is of great importance for the specific case of thermally treated steel, when the inhibitory action of the Се4+ ions eliminates the negative influence of the cracking of the natural passive film on the steel.

The recovery of the passive state of SStt, characterized by disrupted passive film, in our opinion, is brought about also by some other reasons. It is caused by the flow of internal cathodic current (instead of external anodic current), which is determined by the occurring

Ce, at. %

66.8 2.7 3.9 26.6 - 1.44 9.85 6.82 -0.209

65.4 9.1 12.2 13.3 - 1.34 1.46 1.09 -0.300

Cr/Fe, %

Al/Fe, %

Al/Cr, %

Est, V

Al, at. %

SS 58.3 3.1 3.4 35.2 - 1.09 11.35 10.35 \_

SSt.t. 64.9 7.2 7.0 20.9 - 0.97 2.90 2.99 \_

Table 2. Distribution of the elements (in at. %) on the surface SS and SSt.t. before and after

For this reason this cycle of investigations has been carried out using samples of thermally treated steel, since the thermal treatment leads to change in the character of the corrosion process of steel. It should be taken into account that such kind of excessive heat treatment happen to take place both in the formation of catalytic converters, as well as in the course of their operation. In the latter case in the presence of cerium ions in the corrosion medium (Fig. 5), one observes a strong shifting of Еst in positive direction (from -0.942 V to -0.286 V), even at relatively low concentrations of Се4+ (0.3 ppm) in the corrosion medium. The further increase in the concentration of Се4+ ions (from 0.3 to 0.9 ppm) results in insignificant changes in Еst . Thereupon for SStt this shift jumps from -0.942 V (at Се4+ ions concentration 0.1 ppm) up to -0.175 V (at Се4+ ions concentration 0.9 ppm). In the consecutive 20 – 30 fold increase in the concentration of Се4+ ions (15-25 ppm) a preservation of the Еst value is observed, whereupon it manifests values ~ -0.150 - -0.120 V. The consecutive 20-50 fold increase in the concentration of Се4+ ions (500-1000 ppm) leads to strong shifting of Еst in

These results prove that in the case of samples of thermally treated steel non-coated with Ce2O3-CeO2 one observes analogous changes in the stationary corrosion potential of the steel electrode, which have already been registered for the system Ce2O3-CeO2/SSt.t.. The juxtaposition of the above-mentioned changes in Еst at open circuit (conditions of selfdissolution) with the characteristic zones (corrosion potential, Flade potential, zone of passivity, transpassivity region), defined by the cathodic and anodic potentiodynamic E-lgi polarization curves (conditions of external cathodic and anodic polarization) for SStt (Fig. 7.) in 0.1 N H2SO4 solution not-containing Се4+, shows the following. The addition of cerium ions causes shifting and establishing stationary corrosion potential (Fig. 7) in the zone of passivity of the steel. Evidently, this effect will lead also to improvement of the passivation ability, respectively to improvement of the stability to corrosion, of the steel in sulfuric acid medium, which is of great importance for the specific case of thermally treated steel, when the inhibitory action of the Се4+ ions eliminates the negative influence of the cracking of the

The recovery of the passive state of SStt, characterized by disrupted passive film, in our opinion, is brought about also by some other reasons. It is caused by the flow of internal cathodic current (instead of external anodic current), which is determined by the occurring

Samples O,

SS 50h in 0.1N H2SO4

SSt.t. 50h in 0.1N H2SO4 at. %

50h immersion in 0.1N H2SO4.

natural passive film on the steel.

Fe, at. %

Cr, at. %

positive direction reaching values of about +0.510 - +0.570V.

of a reduction reaction Ce4+ ↔ Ce3+ in the redox system Ce4+/ Ce3+. Therefore the Ce4+ ions, as component of the corrosion medium, are acting as inhibitor, exerting an oxidative effect. It follows from (Fig. 7), that the increase in the concentration of the inhibitor in the corrosion environment leads to a substantial decrease in the corrosion current - from 1.10-6 (at inhibitor concentration of 0.1 ppm) - to 1. 10-8 А.сm-2 (at inhibitor concentration ~0.9 ppm). What is making impression is the fact that the further increase in the concentration of the Ce4+ ions in the corrosion medium from ~0.9 ppm (which could be accepted as "critical") up to 1000 ррm influences to a smaller extent the rate of corrosion. It is necessary also to point out that with the increase in the concentration of cerium ions the corrosion potential is shifted in positive direction, whereupon its values remain more positive than the potential of complete passivation and more negative than the potential of depassivation of the steel within the entire interval of studied concentrations – an effect analogous to the one already established for the systems Ce2O3-CeO2/SSt.t.in 0.1 N H2SO4 solution.

Fig. 7. Potentiodynamic E-lgi curves of SSt.t. at different concentrations of Ce4+ in 0.1 N H2SO4 solution.

Тable 3 lists the electrochemical kinetic parameters: corrosion current density (icorr), corrosion potential (Еcorr.) and degree of inhibition efficiency (Z, %), characterizing the corrosion process in the presence and in the absence of cerium ions, determined on the basis of the results represented in Fig. 7. The degree of inhibition efficiency has been calculated on the basis of the equation:

$$\mathbf{Z}\% = \left(\mathbf{i}^{\text{o}}\_{\text{corr.}} \mathbf{:} \mathbf{i}\_{\text{corr.}}\right) / \left(\mathbf{i}^{\text{o}}\_{\text{corr.}} \mathbf{x} \, 100\right) \tag{7}$$

where iocorr and icorr are the values of the corrosion current density in the absence and in the presence of cerium ions.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 257

and in the absence of inhibitor and judging from the measurements of the stationary corrosion potential of the steel, depending on the concentration of the inhibitor at open circuit we could suppose that under the conditions of internal anodic polarization in the presence of inhibitor the nature of the passive layers remains the same as in the case of external anodic polarization. As far as we can judge the specific action of the inhibitor is manifested in the formation of an adsorption layer, which is transformed into bulk phase, on the active anodic sections of the surface of the metal. In view of the XPS analyses ( Guergova, 2011) after 500-hour interval of staying of the thermally treated steel in the aggressive medium in the presence of Ce4+ ions (25 ppm), on the surface of the studied film in the region of the Ce3d XPS band one can observe the appearance of a certain amount of cerium (1.5 at. %) in the form of Ce2O3. The cerium is most probably incorporated into the surface film as a result of the stay of the steel sample in the inhibited corrosion medium, which leads to its modifying as a consequence of the formation of mixed oxides of the type of cerium aluminates and chromates (Burroughs et al., 1976; Hoang et al., 1993). In support of such hypotheses comes the absence of visible corrosion damages on the surface of SSt.t. exposed for 500 h in 0.1N H2SO4 solution in the presence of Ce4+ ions (Fig. 9). Of course, from purely electrochemical point of view, the ability of the inhibitor to define strongly positive oxidation-reduction potential of the steel is connected with the proceeding of reduction of Ce4+ into Ce3+. In order to investigate the kinetics of reduction of the Ce4+ ions to Ce3+, in the region of potentials, characteristic of the passive state of the steel under consideration, we plotted the anodic and the cathodic potentiodynamic curves, characterizing the behavior of the oxidativereductive couple Ce4+/Ce3+ at various concentrations of Ce4+ in 0.1 N H2SO4 solution, on an inert support of platinum (Fig. 10). Such an approach (Tomashov & Chernova, 1965), to our mind, enables the complete elucidation of the mechanism of inhibitory action of the cerium ions. It allows direct juxtaposition of the changes in the values of the corrosion potentials (respectively the corrosion currents) of the steel in their presence with the values of the reversible redox potentials (respectively the exchange currents) of the couple Ce4+/ Ce3+ at

0 200 400 600 800 1000

Cinh, ppm

Fig. 8. Langmuir adsorption plots for SSt.t. in 0.1 N H2SO4 solution at different concentrations

comparable concentration levels.

0

200

400

600

C / Q

of Ce4+ ions.

800

1000


Table 3. Electrochemical parameters characterizing corrosion behaviour of SSt.t..

It is seen from the table that upon increasing the concentration of Ce4+ ions in the corrosion medium the degree of protection reaches values up to 99% for the samples of thermally treated steel. The obtained data about the promotion in the efficiency of the inhibiting action with the increase in the concentration of Ce4+ in the corrosion medium for the thermally treated steel (Table 3) supposes an interconnection between the inhibitor concentration and the degree of surface coverage, Q following the equation (8):

$$\mathbf{Q} = (\mathbf{i}\_{\text{corr}} - \mathbf{i}\_{\text{corr}}) / \,\mathrm{i}\mathbf{e}\_{\text{corr}} \tag{8}$$

where iocorr and icorr are respectively the corrosion current density, obtained by extrapolation of anodic and cathodic potentiodynamic curves in the absence and in the presence of various concentrations of the inhibitor in the corrosion medium. On the basis of the obtained data about the fraction of surface coverage of steel electrode as a function of the concentration of the inhibitor, one can accept that the adsorption process obeys Langmuir's isotherm. According to this isotherm the interconnection between the fraction of surface coverage and the concentration of the inhibitor is the following:

$$\mathbf{Q} = \text{KC.} (1 + \text{KC}), \text{ and respectively:} \tag{9}$$

$$\mathbf{C}/\mathbf{Q} = \mathbf{1}/\mathbf{K} + \mathbf{C} \tag{10}$$

where К is the adsorption constant and С is the concentration of the inhibitor. The dependence C/Q as a function of С for the thermally treated steel is represented in Fig. 8. It is seen that the experimental data describe a linear dependence, whereupon the coefficient of the linear regression and the slope of the straight line of this dependence approach a value of 1, which proves the validity of Langmuir's isotherm in our case.

The constant К in the equation (9) is connected with the standard free energy of adsorption (∆ G) in accordance with the equation:

$$\text{IK} = \text{(1/55.5)} \exp \text{ (-} \Delta \text{ DG} \text{ }^{\circ} \text{ads} \text{ / RT)} \tag{11}$$

The value of К, determined graphically based on the plot of the dependence C/Q as a function of С, is 44,6x106М-1, while the value of (-∆Goads ) amounts to 10.35 к call.mol-1.The relatively low value of ∆Goads, is indicative of electrostatic forces of interaction between the ions of the inhibitor and the steel surface. Or in other words the interaction of the inhibitor with the surface of the thermally treated steel has physical nature. On the basis of the obtained electrochemical corrosion data from the potentiodynamic curves in the presence

Samples E, V i corr, A m-2 Z, % SS after thermal treatment SSt.t. -0.900 1.5 x 10-6 with 0.1 ppm -0.435 5.8 x 10-7 37.8 with 0.3 ppm -0.238 2.2 x 10-7 86.8 with 0.9 ppm -0.156 3.2 x 10-8 98.2 with 1000 ppm 0.212 1.6 x 10-8 99.4

It is seen from the table that upon increasing the concentration of Ce4+ ions in the corrosion medium the degree of protection reaches values up to 99% for the samples of thermally treated steel. The obtained data about the promotion in the efficiency of the inhibiting action with the increase in the concentration of Ce4+ in the corrosion medium for the thermally treated steel (Table 3) supposes an interconnection between the inhibitor concentration and

where iocorr and icorr are respectively the corrosion current density, obtained by extrapolation of anodic and cathodic potentiodynamic curves in the absence and in the presence of various concentrations of the inhibitor in the corrosion medium. On the basis of the obtained data about the fraction of surface coverage of steel electrode as a function of the concentration of the inhibitor, one can accept that the adsorption process obeys Langmuir's isotherm. According to this isotherm the interconnection between the fraction of surface

where К is the adsorption constant and С is the concentration of the inhibitor. The dependence C/Q as a function of С for the thermally treated steel is represented in Fig. 8. It is seen that the experimental data describe a linear dependence, whereupon the coefficient of the linear regression and the slope of the straight line of this dependence approach a

The constant К in the equation (9) is connected with the standard free energy of adsorption

The value of К, determined graphically based on the plot of the dependence C/Q as a function of С, is 44,6x106М-1, while the value of (-∆Goads ) amounts to 10.35 к call.mol-1.The relatively low value of ∆Goads, is indicative of electrostatic forces of interaction between the ions of the inhibitor and the steel surface. Or in other words the interaction of the inhibitor with the surface of the thermally treated steel has physical nature. On the basis of the obtained electrochemical corrosion data from the potentiodynamic curves in the presence

Q = (iocorr - icorr)/ iocorr (8)

Q =KC.(1+KC), and respectively: (9)

K= (1/55.5) exp (-∆ DG oads / RT) (11)

C/Q = 1/K +C (10)

Table 3. Electrochemical parameters characterizing corrosion behaviour of SSt.t..

the degree of surface coverage, Q following the equation (8):

coverage and the concentration of the inhibitor is the following:

value of 1, which proves the validity of Langmuir's isotherm in our case.

(∆ G) in accordance with the equation:

and in the absence of inhibitor and judging from the measurements of the stationary corrosion potential of the steel, depending on the concentration of the inhibitor at open circuit we could suppose that under the conditions of internal anodic polarization in the presence of inhibitor the nature of the passive layers remains the same as in the case of

external anodic polarization. As far as we can judge the specific action of the inhibitor is manifested in the formation of an adsorption layer, which is transformed into bulk phase, on the active anodic sections of the surface of the metal. In view of the XPS analyses ( Guergova, 2011) after 500-hour interval of staying of the thermally treated steel in the aggressive medium in the presence of Ce4+ ions (25 ppm), on the surface of the studied film in the region of the Ce3d XPS band one can observe the appearance of a certain amount of cerium (1.5 at. %) in the form of Ce2O3. The cerium is most probably incorporated into the surface film as a result of the stay of the steel sample in the inhibited corrosion medium, which leads to its modifying as a consequence of the formation of mixed oxides of the type of cerium aluminates and chromates (Burroughs et al., 1976; Hoang et al., 1993). In support of such hypotheses comes the absence of visible corrosion damages on the surface of SSt.t. exposed for 500 h in 0.1N H2SO4 solution in the presence of Ce4+ ions (Fig. 9). Of course, from purely electrochemical point of view, the ability of the inhibitor to define strongly positive oxidation-reduction potential of the steel is connected with the proceeding of reduction of Ce4+ into Ce3+. In order to investigate the kinetics of reduction of the Ce4+ ions to Ce3+, in the region of potentials, characteristic of the passive state of the steel under consideration, we plotted the anodic and the cathodic potentiodynamic curves, characterizing the behavior of the oxidativereductive couple Ce4+/Ce3+ at various concentrations of Ce4+ in 0.1 N H2SO4 solution, on an inert support of platinum (Fig. 10). Such an approach (Tomashov & Chernova, 1965), to our mind, enables the complete elucidation of the mechanism of inhibitory action of the cerium ions. It allows direct juxtaposition of the changes in the values of the corrosion potentials (respectively the corrosion currents) of the steel in their presence with the values of the reversible redox potentials (respectively the exchange currents) of the couple Ce4+/ Ce3+ at comparable concentration levels.

Fig. 8. Langmuir adsorption plots for SSt.t. in 0.1 N H2SO4 solution at different concentrations of Ce4+ ions.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 259

corrosion process (i.e. oxidative depolarization), respectively in the establishment of oxidative-reductive potential of the medium more positive than the potential of complete passivation of the steel. This effect, in its turn, defines the value of the stationary corrosion

Samples Eo, V io, A cm-2 Pt metal 0.070 2.17 x 10-7 Pt with 0.3 ppm 0.107 8.99 x 10-6 Pt with 25 ppm 0.135 6.35 x 10-7 Pt with 1000 ppm 0.331 6.08 x 10-7 Pt with 1500 ppm 0.722 7.67 x 10-7

Table 4. Reversible redox potentials Eo, and equilibrium currents io, of the system Ce4+ /Ce3+

In order to prove the integral nature of cerium oxides films as efficient cathodic coating, involved directly in the corrosion process and the role of cerium ions as inhibitors possessing oxidative effect, participating also directly in the corrosion process and leading to the formation of phase layer of cerium oxides on the active cathodic sections of the steel surface, we compared the dependences Е-lgi for the systems Ce2O3-CeO2/SSt.t. as well as for the system Ce4+/Ce3+/Pt. – Fig. 11. It is seen in Fig. 11 that the corrosion currents of the systems Ce2O3-CeO2/SSt.t.are close in value to the exchange current of the oxidationreduction current of the couple Ce4+/Ce3+ on Pt. The differences between the corrosion potential of the systems Ce2O3-CeO2/SSt.t and of the equilibrium oxidation-reduction potential of the couple Ce4+/Ce3+ can be explained by the discrepancies in the surface and

10-7 10-6 10-5 10-4

i, A cm-2

Fig. 11. Potentiodynamic E–lgi curves for SSt.t (1)., for Pt (2) and for the systems CeO2-

4

3

<sup>1</sup> b)

2

on Pt at different concentrations of the Ce4+ in the corrosion medium.

**thermal treated samples**

3- CeO2/SS (29.6 at.%) 4 - Pt with 1000ppm

Ce2O3/Pt (3); Ce4+/Ce3+/Pt (4); obtained in 0.1 N H2SO4 solution.

bulk phase concentrations of the components.



E, V vs. MSE

0.0

1- SS 2 - Pt

0.5

potential of the steel to be more positive than the potential of complete passivation.

Fig. 9. SEM images on thermally treated stainless steel after 500 h immersion in 0.1 N H2SO4 without Ce4+(a) and in the same media with 25 ppm Ce4+ (b).

Fig. 10. Potentiodynamic E-lgi curves of Pt at different concentrations of Ce4+ in 0.1 N H2SO4 solution.

The comparison of the obtained results gives evidence that with the increase in the concentration of the cerium ions in 0.1 N H2SO4 solution the equilibrium oxidationreduction potential of the system Ce4+/ Ce3+ is shifted in positive direction (Fig.10 and Table 4), in correspondence with the equation of Nernst, whereupon at all the studied concentrations it is located in the zone of potentials, characteristic of the passive state of steel (Fig. 7). Thereupon the corrosion potentials of the steel are more negative than the equilibrium oxidation-reduction potentials of the system Ce4+/Ce3+. At the same time, the juxtaposition of the corrosion currents for the steel in the presence of cerium ions with the exchange currents for the system Ce4+/Ce3+on Pt, at comparable concentration levels of the cerium ions, shows that they have quite close values.

The so obtained data give us the reason to classify the studied oxidation-reduction couple as an inhibitor having an oxidative effect, which does not influence directly the kinetics of the anodic process. Its action is expressed in its participation in the depolarizing reaction of the

Fig. 9. SEM images on thermally treated stainless steel after 500 h immersion in 0.1 N H2SO4

10-7 10-6 10-5 10-4

Fig. 10. Potentiodynamic E-lgi curves of Pt at different concentrations of Ce4+ in 0.1 N H2SO4

The comparison of the obtained results gives evidence that with the increase in the concentration of the cerium ions in 0.1 N H2SO4 solution the equilibrium oxidationreduction potential of the system Ce4+/ Ce3+ is shifted in positive direction (Fig.10 and Table 4), in correspondence with the equation of Nernst, whereupon at all the studied concentrations it is located in the zone of potentials, characteristic of the passive state of steel (Fig. 7). Thereupon the corrosion potentials of the steel are more negative than the equilibrium oxidation-reduction potentials of the system Ce4+/Ce3+. At the same time, the juxtaposition of the corrosion currents for the steel in the presence of cerium ions with the exchange currents for the system Ce4+/Ce3+on Pt, at comparable concentration levels of the

The so obtained data give us the reason to classify the studied oxidation-reduction couple as an inhibitor having an oxidative effect, which does not influence directly the kinetics of the anodic process. Its action is expressed in its participation in the depolarizing reaction of the

**Pt**

i, A cm-2

without Ce4+(a) and in the same media with 25 ppm Ce4+ (b).

25 ppm

1000 ppm

1500 ppm


cerium ions, shows that they have quite close values.

0,0

0,2

0,4

E, V/MSE

solution.

0,6

0,8

1,0

corrosion process (i.e. oxidative depolarization), respectively in the establishment of oxidative-reductive potential of the medium more positive than the potential of complete passivation of the steel. This effect, in its turn, defines the value of the stationary corrosion potential of the steel to be more positive than the potential of complete passivation.


Table 4. Reversible redox potentials Eo, and equilibrium currents io, of the system Ce4+ /Ce3+ on Pt at different concentrations of the Ce4+ in the corrosion medium.

In order to prove the integral nature of cerium oxides films as efficient cathodic coating, involved directly in the corrosion process and the role of cerium ions as inhibitors possessing oxidative effect, participating also directly in the corrosion process and leading to the formation of phase layer of cerium oxides on the active cathodic sections of the steel surface, we compared the dependences Е-lgi for the systems Ce2O3-CeO2/SSt.t. as well as for the system Ce4+/Ce3+/Pt. – Fig. 11. It is seen in Fig. 11 that the corrosion currents of the systems Ce2O3-CeO2/SSt.t.are close in value to the exchange current of the oxidationreduction current of the couple Ce4+/Ce3+ on Pt. The differences between the corrosion potential of the systems Ce2O3-CeO2/SSt.t and of the equilibrium oxidation-reduction potential of the couple Ce4+/Ce3+ can be explained by the discrepancies in the surface and bulk phase concentrations of the components.

Fig. 11. Potentiodynamic E–lgi curves for SSt.t (1)., for Pt (2) and for the systems CeO2- Ce2O3/Pt (3); Ce4+/Ce3+/Pt (4); obtained in 0.1 N H2SO4 solution.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 261

obvious that the chemical state of Ce could be evaluated based on the percentage of the area of the u''' peak, located at 916.8 eV with respect to the total Ce3d area. So if the percentage of the u''' peak related to the total Ce3d area varies from 0 to 14%, then the Ce4+ percentage related to the total amount of Ce varies from 0 to 100%. In our case the u'''% amounts change as a result of dipping the as-deposited sample into 0.1 N H2SO4 solution, so we observed also change in the percentage of Ce4+ and Ce3+ on the surface (Table 6). The obtained O1s X-ray photoelectron spectra, recorded after different time intervals of exposure, are quite complicated. These spectra had to undergo de-convolution procedure to

Fig. 12. Ce3d and O1s XPS spectra of CeO2-Ce2O3/SS sample after 250 h exposition to 0.1 N H2SO4, Panel A and B correspond to high BE and high BE portion of Ce3d and O1s spectral regions taken at different step of corrosion test. (1) as-deposited; (2) after 18 h ; (3) after 200

**528 532 536 530 535 Binding Energy, eV**

**O1s B**

The chemical composition of the surface film of the CeO2-Ce2O3/SSt.t. systems under consideration is shown in Тable 5. It is important to note that on the surface of the 'asdeposited" samples, covered with cerium oxide film, we detected also the presence of iron, chromium and aluminum in addition to the cerium. The latter is mainly in the valence state Ce4+ i.e. in the form of CeO2. After 18 hours of exposure to 0.1 N H2SO4 solution the chemical composition on the surface of the system has changed. The surface concentration of cerium drops down from 45.1 to 11.7 at. %, whereupon Ce3+ appears in the form of Ce(OH)3 and Ce2O3. After continuing the exposure further (200-400 hours) one observes evolution of the spectra with respect to cerium and oxygen (Fig.12). The main peak in the spectrum of oxygen, having binding energy of about ~529.5 eV, is attributed to the presence of O2- ions, which exist basically as Ce-O bonds in the crystal lattice of the cerium oxide being formed. The second peak, located at 531.7 eV, is associated with the existence of OH- groups on the surface, while the presence of a peak at 533 eV shows that there is adsorbed water on the surface of the studied passive films (533 eV) (Hoang et al., 1993; Paparazzo, 1990). It can be seen in the spectra that after exposure of the samples in corrosion medium for 200-400 hours

h; (4) after 250 h; (5) after 400 h; (6) after 1000 h exposition to 0.1 N H2SO4 solution.

analyze the contribution of the separate components in them.

**III A**

**III u**

**880 890 900 910 920 915 920 Binding Energy, eV**

**u**

**Ce3d**

**v v I v II** **v III u u Iu II**

#### **4.4 XPS and SEM results**

In confirmation of these results and the conclusions come also the data of the ХPS analyses of the samples, having electrochemically deposited cerium oxide films, characterizing the changes in the chemical state and in the composition of the surface film, depending on the time interval of the immersion stay of the samples in 0.1NH2SO4 solution (Stoyanova et al., 2010). Table 5 represents the results for the sample with surface concentration of electrochemically deposited cerium oxide layer 45.1 at.% .It is seen that after 1000 hours of exposure to the corrosion medium the surface concentration of cerium is decreased from 45.1ат.% down to 0.2 at.%. This result is convincing evidence for the occurring of depolarizing reaction involving the participation of the rich in CeO2 sections of the surface, acting as effective cathodes, in accordance with the equations (1-4). It becomes evident that the presence of cerium oxide film determines the establishment of more positive stationary corrosion potential of the system, due to the proceeding of the reactions 1- 6, the surface passive film will become modified, whereupon its composition, respectively the ratio Cr/Fe, will become different.


Table 5. Concentration of the elements (in at. %) on the surface layers of the system CeO2- Ce2O3/SS after thermal treatment and after corrosion test in 0.1 N H2SO4.

In this cycle of experimental runs, using the XPS method, the changes were monitored, occurring in the chemical composition of the passive film of the system CeOx /SS, during prolonged exposure of the samples in 0.1 N H2SO4 solutions (Тable 5). The analyses were carried out after the 18th, 200 th, 250 th , 400 th , and 1000 th hour - time intervals of exposure. Within the interval 200-400 hours Est remains practically the same, while after 1000 hours of exposure it is shifted strongly in the negative direction, reaching a value of about ~ +0.060 V. To obtain further information about the influence of ceria on the corrosion behaviour of as-deposited sample we analyzed in depth the Ce3d and O1s XPS spectra. As it has already been discussed in our previous papers (Nikolova et al., 2006; Stoyanova et al, 2006), the Ce3d spectrum is a complex one, due to the fact that the peak is spin-orbital split into a doublet, each doublet showing extra structure due to the effect of the final state. There are 8 peaks assignments in the spectra labelled according to Burroughs (Burroughs et al., 1976), where the peaks V, VII, VIII and U, UII, UIII refer to the 3d5/2 and 3d3/2 respectively and they are characteristic of Ce(IV) 3d final states. The peaks labelled as VI and UI refer to 3d5/2 and 3d3/2 they are characteristic of Ce(III)3d final state (Fig.12).The literature data make it

In confirmation of these results and the conclusions come also the data of the ХPS analyses of the samples, having electrochemically deposited cerium oxide films, characterizing the changes in the chemical state and in the composition of the surface film, depending on the time interval of the immersion stay of the samples in 0.1NH2SO4 solution (Stoyanova et al., 2010). Table 5 represents the results for the sample with surface concentration of electrochemically deposited cerium oxide layer 45.1 at.% .It is seen that after 1000 hours of exposure to the corrosion medium the surface concentration of cerium is decreased from 45.1ат.% down to 0.2 at.%. This result is convincing evidence for the occurring of depolarizing reaction involving the participation of the rich in CeO2 sections of the surface, acting as effective cathodes, in accordance with the equations (1-4). It becomes evident that the presence of cerium oxide film determines the establishment of more positive stationary corrosion potential of the system, due to the proceeding of the reactions 1- 6, the surface passive film will become modified, whereupon its composition, respectively the ratio Cr/Fe,

Est, V C, at. % O, at. % Al, at. % Fe, at. % Cr, at. % Ce, at. %

deposited 0.151 15.6 37.6 1.0 0.1 0.6 45.1

Table 5. Concentration of the elements (in at. %) on the surface layers of the system CeO2-

In this cycle of experimental runs, using the XPS method, the changes were monitored, occurring in the chemical composition of the passive film of the system CeOx /SS, during prolonged exposure of the samples in 0.1 N H2SO4 solutions (Тable 5). The analyses were carried out after the 18th, 200 th, 250 th , 400 th , and 1000 th hour - time intervals of exposure. Within the interval 200-400 hours Est remains practically the same, while after 1000 hours of exposure it is shifted strongly in the negative direction, reaching a value of about ~ +0.060 V. To obtain further information about the influence of ceria on the corrosion behaviour of as-deposited sample we analyzed in depth the Ce3d and O1s XPS spectra. As it has already been discussed in our previous papers (Nikolova et al., 2006; Stoyanova et al, 2006), the Ce3d spectrum is a complex one, due to the fact that the peak is spin-orbital split into a doublet, each doublet showing extra structure due to the effect of the final state. There are 8 peaks assignments in the spectra labelled according to Burroughs (Burroughs et al., 1976), where the peaks V, VII, VIII and U, UII, UIII refer to the 3d5/2 and 3d3/2 respectively and they are characteristic of Ce(IV) 3d final states. The peaks labelled as VI and UI refer to 3d5/2 and 3d3/2 they are characteristic of Ce(III)3d final state (Fig.12).The literature data make it

Ce2O3/SS after thermal treatment and after corrosion test in 0.1 N H2SO4.

18 0.169 50.1 35.7 1.6 0.5 0.4 11.7 200 0.249 64.4 31.5 0 0 0 4.1 250 0.236 63.7 31.5 0 0 0.2 4.6 400 0.239 68.5 27.4 0 0 0 4.1 1000 0.060 31.4 42.9 19.9 0.7 4.9 0.2

**4.4 XPS and SEM results** 

will become different.

Time of exposure, h

as

obvious that the chemical state of Ce could be evaluated based on the percentage of the area of the u''' peak, located at 916.8 eV with respect to the total Ce3d area. So if the percentage of the u''' peak related to the total Ce3d area varies from 0 to 14%, then the Ce4+ percentage related to the total amount of Ce varies from 0 to 100%. In our case the u'''% amounts change as a result of dipping the as-deposited sample into 0.1 N H2SO4 solution, so we observed also change in the percentage of Ce4+ and Ce3+ on the surface (Table 6). The obtained O1s X-ray photoelectron spectra, recorded after different time intervals of exposure, are quite complicated. These spectra had to undergo de-convolution procedure to analyze the contribution of the separate components in them.

Fig. 12. Ce3d and O1s XPS spectra of CeO2-Ce2O3/SS sample after 250 h exposition to 0.1 N H2SO4, Panel A and B correspond to high BE and high BE portion of Ce3d and O1s spectral regions taken at different step of corrosion test. (1) as-deposited; (2) after 18 h ; (3) after 200 h; (4) after 250 h; (5) after 400 h; (6) after 1000 h exposition to 0.1 N H2SO4 solution.

The chemical composition of the surface film of the CeO2-Ce2O3/SSt.t. systems under consideration is shown in Тable 5. It is important to note that on the surface of the 'asdeposited" samples, covered with cerium oxide film, we detected also the presence of iron, chromium and aluminum in addition to the cerium. The latter is mainly in the valence state Ce4+ i.e. in the form of CeO2. After 18 hours of exposure to 0.1 N H2SO4 solution the chemical composition on the surface of the system has changed. The surface concentration of cerium drops down from 45.1 to 11.7 at. %, whereupon Ce3+ appears in the form of Ce(OH)3 and Ce2O3. After continuing the exposure further (200-400 hours) one observes evolution of the spectra with respect to cerium and oxygen (Fig.12). The main peak in the spectrum of oxygen, having binding energy of about ~529.5 eV, is attributed to the presence of O2- ions, which exist basically as Ce-O bonds in the crystal lattice of the cerium oxide being formed. The second peak, located at 531.7 eV, is associated with the existence of OH- groups on the surface, while the presence of a peak at 533 eV shows that there is adsorbed water on the surface of the studied passive films (533 eV) (Hoang et al., 1993; Paparazzo, 1990). It can be seen in the spectra that after exposure of the samples in corrosion medium for 200-400 hours

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 263

propagation and spreading further to give total corrosion (Fig.13 а,b). For the sake of comparison Fig. 13 shows the same surface in the presence of cerium oxide coating after 50 hours of exposure to the corrosion medium. In this case no corrosion damages can be observed on the surface not only after the 50th hour, but even after 1000 hours of exposure (Fig.13) to the corrosion medium, as a result of its modifying, already discussed above.

**a) b)**

**c) d)**

Fig. 13. SEM images for the samples: a) SSt.t.; b) SSt.t. after 50 h exposition in 0.1 N H2SO4 (O – areas of local corrosion); c) SSt.t. after 1000 h exposition; d) CeO2-Ce2O3/SSt.t.; e) CeO2- Ce2O3/SSt.t. after 50 h exposition in 0.1 N H2SO4; f) CeO2-Ce2O3/SSt.t. after 1000 h exposition.

**e)**

**f)**

the respectively detected high-energy peak in the spectrum of oxygen for these samples is growing up initially, while afterwards it decreases its intensity. This effect, in our opinion, is owing to consecutive enrichment and then impoverishment of the surface layer of the film in OH groups i.e. adsorbed water molecules (Fig.12, Tablе 6). The obtained results gives us the reason to draw the conclusion that the surface passive film under these conditions at this stage consists of CeO2, Ce2O3, Ce(OH)4 as well as CeO(OH)2 and Ce(OH)3, whose existence has been ascertained also by some other authors (Huang et al., 2008).


Table 6. Calculated contribution of oxygen and percentage of Ce4+, depending of the exposure time in 0.1 N H2SO4. Types of the chemical bonds and values of binding energy.

After 1000 hours of exposure the quantity of cerium is drastically decreased, as a consequence of the occurring reactions 1-4 and only some insignificant amounts of cerium have been registered in the valence state Ce3+, i.e. in the form of Ce2O3. Only a single peak has been detected in the spectrum of oxygen, having a binding energy of 531,2 eV. Chromium, aluminum and iron have also been detected (Table 5). On the basis of the values of their binding energies (Table 5), including also the location of the O1s peak (Fig.12), we can also conclude that they exist in the form of oxides and hydroxides: Cr2O3, Cr(OH)3, Fe2O3, FeOOH, Al2O3 and Al(OH)3. The high concentration of carbon registered in the surface film is owing to the considerable amount of carbonates adsorbed during the thermal treatment in a high-temperature oven.

In support of the conclusions, drawn on the basis of the above results, evidence is also given by the direct SEM observations carried out. It follows from the electron microscopic studies of the samples, exposed to the corrosion medium, that the disruption of the passive state of steel at the initial stages (until the 50th hour) leads to appearance of local corrosion and its

the respectively detected high-energy peak in the spectrum of oxygen for these samples is growing up initially, while afterwards it decreases its intensity. This effect, in our opinion, is owing to consecutive enrichment and then impoverishment of the surface layer of the film in OH- groups i.e. adsorbed water molecules (Fig.12, Tablе 6). The obtained results gives us the reason to draw the conclusion that the surface passive film under these conditions at this stage consists of CeO2, Ce2O3, Ce(OH)4 as well as CeO(OH)2 and Ce(OH)3, whose existence

Bonds Ce3d, eV

Ce-O Ce-OH

Ce-O Ce-OH Others

Ce-O Ce-OH Others

Ce-O Ce-OH Others

Ce-O Ce-OH Others

Percentage of Ce4+ to the total Ce

883.0 100

882.9 85.35

882.4 76

882.5 76

882.5 76

has been ascertained also by some other authors (Huang et al., 2008).

Percentage of oxygen contribution to the total

> 59 41

43 42.5 14.5

21.4 33.6 45

> 23 32 45

> 23 45 32

1000 531.2 100 OH 882.0 -

After 1000 hours of exposure the quantity of cerium is drastically decreased, as a consequence of the occurring reactions 1-4 and only some insignificant amounts of cerium have been registered in the valence state Ce3+, i.e. in the form of Ce2O3. Only a single peak has been detected in the spectrum of oxygen, having a binding energy of 531,2 eV. Chromium, aluminum and iron have also been detected (Table 5). On the basis of the values of their binding energies (Table 5), including also the location of the O1s peak (Fig.12), we can also conclude that they exist in the form of oxides and hydroxides: Cr2O3, Cr(OH)3, Fe2O3, FeOOH, Al2O3 and Al(OH)3. The high concentration of carbon registered in the surface film is owing to the considerable amount of carbonates adsorbed during the thermal

In support of the conclusions, drawn on the basis of the above results, evidence is also given by the direct SEM observations carried out. It follows from the electron microscopic studies of the samples, exposed to the corrosion medium, that the disruption of the passive state of steel at the initial stages (until the 50th hour) leads to appearance of local corrosion and its

Table 6. Calculated contribution of oxygen and percentage of Ce4+, depending of the exposure time in 0.1 N H2SO4. Types of the chemical bonds and values of binding energy.

Time of

18

200

250

400

exposure, h O1s, eV

as deposited 529.2

531.4

529.2 531.5 533.2

529.4 531.9 533.3

529.5 531.9 533.4

529.5 532.1 533.4

treatment in a high-temperature oven.

propagation and spreading further to give total corrosion (Fig.13 а,b). For the sake of comparison Fig. 13 shows the same surface in the presence of cerium oxide coating after 50 hours of exposure to the corrosion medium. In this case no corrosion damages can be observed on the surface not only after the 50th hour, but even after 1000 hours of exposure (Fig.13) to the corrosion medium, as a result of its modifying, already discussed above.

Fig. 13. SEM images for the samples: a) SSt.t.; b) SSt.t. after 50 h exposition in 0.1 N H2SO4 (O – areas of local corrosion); c) SSt.t. after 1000 h exposition; d) CeO2-Ce2O3/SSt.t.; e) CeO2- Ce2O3/SSt.t. after 50 h exposition in 0.1 N H2SO4; f) CeO2-Ce2O3/SSt.t. after 1000 h exposition.

Corrosion Behavior of Stainless Steels Modified by Cerium Oxides Layers 265

Almeida, E., Diamantino, T.C, Figueiredo M.O., Sa C. (1998). Oxidizing alternative species to

Almeida, E., Fedrizzi, L., Diamantino, T.C. (1998). Oxidizing alternative species to

study. *Surface and Coating Technology*, Vol. 105**,** pp. 97-101, ISSN: 0257-8972 Amelinckx, L., Kamrunnahar, M., Chou P, Macdonald D., (2006). Figure of merit for the

Aramaki, K. (2001). The inhibition effects of cation inhibitors on corrosion of zinc in aerated 0.5M NaCl. *Corrosion Science,* Vol. 43**,** (June, 2001), pp.1573-1588, ISSN: 0010-938X Aramaki K. (2001). Treatment of zinc surface with cerium(III) nitrate to prevent zinc

Aramaki, K. (2002). Preparation of chromate-free, self-healing polymer films containing

Aramaki, K. (2002). Cerium (III) chloride and sodium octylthiopropionate as an effective

Aramaki, K.,(2002). Self-healing protective films prepared on zinc by treatment with cerium

Arenas, M.A., Conde A., de Damborenea, J., (2002). Cerium: a suitable green corrosion inhibitor for tinplate. *Corrosion Science,* Vol.44, No.3, pp. 511-520, ISSN: 0010-938X Arenas, M.A., de Damborenea, J.,(2003). Growth mechanism of cerium layers on galvanised

Arenas, M.A, Garcia, I., de Damborenea, J., (2004). X-ray photoelectron spectroscopy study

Arnott, D.R., Hinton, B.R.W & Ryan N.E, (1989). Cationic-film-forming inhibitors for the

Avramova, I., Stefanov, P., Nicolova, D., Stoychev, D., Marinova, T., (2005). Characterization

*Composites Science and Technology*, Vol. 65, pp. 1663-1667 ISSN: 0266 - 3538 Avramova, I., Stoychev, D., Marinova, T., (2006). Characterization of thin CeO2-ZrO2-Y2O3

Balasubramanian, M., Melendres, C.A., Mansour, A. N., (1999). An X-ray absorption study

*Films,* Vol. 347, No. 1-2, (June 1999), pp. 178-183, ISSN: 0040-6090

*Science*, Vol. 46, No. 4, pp. 1033-1049, ISSN: 0010-938X

solution. *Corrosion,* Vol. 45, No. 1, pp. 12-18

pp. 1365-1370, ISSN: 0169-4332

*Science,* Vol. 48*,* (April, 2006) pp. 3646-3667, ISSN: 0010-938X

8972

ISSN: 0010-938X

ISSN: 0010-938X

4686

pp. 1375-1389, ISSN: 0010-938X

1361-1374, ISSN: 0010-938X

chromium VI in zinc - galvanized steel surface treatment. Part I-A morphological and chemical study. *Surface and Coating Technology*, Vol. 106**,** pp**.** 8-17. ISSN: 0257-

chromium VI in zinc - galvanized steel surface treatment Part 2-An electrochemical

quality of ZrO2 coatings on stainless steel and nickel-based alloy surfaces. *Corrosion* 

corrosion in aerated 0.5 M NaCl, *Corrosion Science,* Vol. 43, No.11 pp. 1201-1215,

sodium silicate on zinc pretreated in a cerium (III) nitrate solution for preventing zinc corrosion at scratches in 0.1 M NaCl. *Corrosion Science,* Vol. 44 (August 2001),

inhibitor mixture for zinc in 0.1M NaCl. *Corrosion Science,* Vol.. 44, (June 2001), pp.

(III) nitrate and sodium phosphate. *Corrosion Science*, Vol. 44, No.11, pp**.**2621-2634.

steel. *Electrochimica Acta*, Vol. 48, No. 24, (October, 2003) pp. 3693-3698, ISSN: 0013-

of the corrosion behaviour of galvanised steel implanted with rare earths. *Corrosion* 

protection of the AA 7075 aluminum alloy against corrosion in aqueous chloride

of nanocomposite CeO2-Al2O3 coatings electrodeposited on stainless steel.

films electrochemical deposited on stainless steel. *Applied Surface Science*, Vol. 235,

of the local structure of cerium in electrochemically deposited thin films. *Thin Solid* 
