**4. Inhibitors for alkaline solution**

## **4.1 Boiler water inhibitors**

Supplemental water for boilers should be chemically treated in advance, for lowering the possibility of corrosion and scaling to increase the heat transfer effect. Steam is often needed for power generation, thus deposition of SiO2 on the turbine blades is inevitable if enough silicon component is contained in supplemental water. It is necessary to control the scaling by getting rid of the Ca2+ and Mg2+ with different kinds of methods, such as by ion-exchange resin and apposite precipitants. In order to control the corrosion process, the essential treatments include degassing, adding alkali and using corrosion inhibitor.

## *4.1.1 Extrude dissolved O2 and CO2*

If residual dissolved O2 in the supplemental water obeys stoichiometric ratio with the metal ions in the boiler system under the conditions of high pressure, it will cause serious pitting of the metal tubes. Degassing of the supplemental water and then adding of appropriate scavenger, such as Na2SO3 and N2H4, is the effective method for deoxygenation. The acceptable concentration of O2 should be kept lower than 0.005 ppm. Degassing may cause the reduction of CO2 content, especially if the supplemental water is pre-acidified to release H2CO3 from the dissolved carbonates. H2CO3 is aggressive to steel without dissolved O2, but adding of alkali into the boiler water will mitigate the corrosion raised by CO2.

Iron return pipe systems undergo critical corrosion; thus, if the concentration of CO2 in the boiler water is high enough, the iron carbonate will be formed, and it changes to ferrous hydroxide and CO2 further, which is the obvious corrosive medium. The Cu-Al cooling system also endures corrosion when both of the dissolved O2 and CO2 are together, but the Cu base alloys are not easily attacked when there is a lack of O2. As CO2 is not depleted at all during the corrosion stage, it will gather inevitably in the boiler during the process of adding supplemental water, and the concentration of CO2 may decrease if with occasional venting.

#### *4.1.2 Adding alkali*

Addition of alkali into boiler waters is a routine operation for many boilers under high pressure in the United States and abroad. Supplemental water for a boiler with high pressure is restricted to the minimal pH of 8.5 to reduce the corrosion of iron at room temperature, and the acceptable pH value should be at the range of 9.2–9.5. For Cu alloys, the optimal pH value is from 8.5 to 9.2. As both iron and Cu alloys are usually used in boiler systems, the balanced pH value range is suggested to be between 8.8 and 9.2. Adding buffer ions, such as phosphate, will restrict the rise of pH value. Such ions also have advantage of preventing high concentrations of hydroxide in the boiler waters, which can give rise to stress corrosion cracking of any section of the boiler under high external stress. It reported that when pH value of the boiler water is 9.5–10.0, phosphate with the concentration of 5–10 ppm was more efficient than either sodium hydroxide or ammonia in slowing down the corrosion rate of boiler tubes under high-pressure conditions.

#### *4.1.3 Adding inhibitors*

It is feasible to select suitable inhibitors for inhibiting two kinds of typical corrosion in boiler systems, mainly stress corrosion cracking and return pipe systems corrosion, and the former can be restricted by supplement of phosphate. Corrosion raised by dissolved CO2 in steam condensate can be limited by supplying volatile amines for the boiler water. Two kinds of characteristic volatile amines are chosen for neutralizing amines and film formation. The former group species contain benzylamine, cyclohexylamine, or morpholine. When one provided into the boiler water with adequate amount, it can neutralize CO2 and alkalinize the steam condensate, thus dropping the corrosive rate of the condensate. The latter group species include volatile hexadecylamine, octadecylamine, or dioctadecylamine, which is the typical film-forming inhibitor, which prevents corrosion from constructing the stable protective film on the surface of the condenser. The film-forming amine is regarded as more apposite to the circumscription of the inhibitor, while others are only used mainly for neutralizing actually.

#### **4.2 O2 scavenger**

O2 scavengers are reagents which are often used to get rid of the dissolved O2 from water through reduction reactions, thus prohibiting the corrosion ascribed to the O2 in water. For this purpose, the desirable characters of O2 scavengers should be (1) excellent reducing ability against O2, (2) no violently actions of the thermal decomposition products and the final reaction products with O2 countering the equipment.

**167**

**Table 14.**

*Some species of O2 scavengers.*

SO3

*Formulation of Corrosion Inhibitors*

*4.3.1.1 Neutralizing amines*

*DOI: http://dx.doi.org/10.5772/intechopen.88533*

Thus, the replacement for hydrazine was inevitable.

metals and the aggressive substances, for example, O2 and CO2.

*4.3.1 Inhibitors used in condensate lines of boilers under low pressure*

synergistic effect of using both of these two amines should be considered.

**Species Reagent names** N2H4-based O2 scavengers N2H4·H2O

<sup>2</sup><sup>−</sup>-based O2 scavengers Na2SO3

Other O2 scavengers Saccharides

**4.3 Corrosion inhibitors for condensate line**

The species and reagent names of O2 scavengers are displayed in **Table 14** [4]. The optimal reagent is hydrazine, but it is limited by its highly toxic properties.

As mentioned before, volatile amines for neutralizing and amines for film formation are also two kinds of classic corrosion inhibitors for prohibiting the corrosion of condensate lines. Volatile amines prevent the erosion by adjusting the pH values of the condensates, while water barrier at the surface of metal can be formed attributed to the amines for film formation, which block the touching between

Under low pressure, the main aggressive substances in the pipelines of boilers are O2 and CO2. The existence of slight CO2 in the condensation product will decrease the pH, which will accelerate metal corrosion. Thus, volatile amines for neutralizing and amines for film formation are provided as the suitable corrosion inhibitors. The use of volatile amines for neutralizing displays the wonderful inhibition effect, but the efficiency is discounted at the presence of O2. Amines for film formation can adsorb on the surface of the metal and then construct the hydrophobic protective film even on low concentration. However, once the corrosion products are covered on the surface of the metal, it will take long time to form the protective film. Therefore, the

Cyclohexyl amine (C6H13N), monoisopropanol amine [NH2CH2CH(CH3)OH], morpholine (C4H9NO), and ammonium hydroxide (NH3·H2O) are usually used for neutralization reaction in boiler systems. They are also dosage by dose for condensate lines. The amine is complemented to the supplemental water and then mixed with the water vapor produced from the boiler. When the water vapor cools down in the condensate line, the amine will dissolve in condensation product and neutralize CO2 (H2CO3). Therefore, the pH value of the solution in the condensate line will be raised, and the crisis of metal corrosion will be eased. **Table 15** displays the needed

> N2H4·H2SO4 Hydrazine phosphate

NaHSO3 Na2S2O5

Tannins Hydrazide L-ascorbic acid Hydroquinone Alkanol amines

### *Formulation of Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.88533*

*Water Chemistry*

*4.1.2 Adding alkali*

high-pressure conditions.

only used mainly for neutralizing actually.

*4.1.3 Adding inhibitors*

**4.2 O2 scavenger**

carbonates. H2CO3 is aggressive to steel without dissolved O2, but adding of alkali

Iron return pipe systems undergo critical corrosion; thus, if the concentration of CO2 in the boiler water is high enough, the iron carbonate will be formed, and it changes to ferrous hydroxide and CO2 further, which is the obvious corrosive medium. The Cu-Al cooling system also endures corrosion when both of the dissolved O2 and CO2 are together, but the Cu base alloys are not easily attacked when there is a lack of O2. As CO2 is not depleted at all during the corrosion stage, it will gather inevitably in the boiler during the process of adding supplemental water, and

Addition of alkali into boiler waters is a routine operation for many boilers under high pressure in the United States and abroad. Supplemental water for a boiler with high pressure is restricted to the minimal pH of 8.5 to reduce the corrosion of iron at room temperature, and the acceptable pH value should be at the range of 9.2–9.5. For Cu alloys, the optimal pH value is from 8.5 to 9.2. As both iron and Cu alloys are usually used in boiler systems, the balanced pH value range is suggested to be between 8.8 and 9.2. Adding buffer ions, such as phosphate, will restrict the rise of pH value. Such ions also have advantage of preventing high concentrations of hydroxide in the boiler waters, which can give rise to stress corrosion cracking of any section of the boiler under high external stress. It reported that when pH value of the boiler water is 9.5–10.0, phosphate with the concentration of 5–10 ppm was more efficient than either sodium hydroxide or ammonia in slowing down the corrosion rate of boiler tubes under

It is feasible to select suitable inhibitors for inhibiting two kinds of typical corrosion in boiler systems, mainly stress corrosion cracking and return pipe systems corrosion, and the former can be restricted by supplement of phosphate. Corrosion raised by dissolved CO2 in steam condensate can be limited by supplying volatile amines for the boiler water. Two kinds of characteristic volatile amines are chosen for neutralizing amines and film formation. The former group species contain benzylamine, cyclohexylamine, or morpholine. When one provided into the boiler water with adequate amount, it can neutralize CO2 and alkalinize the steam condensate, thus dropping the corrosive rate of the condensate. The latter group species include volatile hexadecylamine, octadecylamine, or dioctadecylamine, which is the typical film-forming inhibitor, which prevents corrosion from constructing the stable protective film on the surface of the condenser. The film-forming amine is regarded as more apposite to the circumscription of the inhibitor, while others are

O2 scavengers are reagents which are often used to get rid of the dissolved O2 from water through reduction reactions, thus prohibiting the corrosion ascribed to the O2 in water. For this purpose, the desirable characters of O2 scavengers should be (1) excellent reducing ability against O2, (2) no violently actions of the thermal decomposition products and the final reaction products with O2 countering the

into the boiler water will mitigate the corrosion raised by CO2.

the concentration of CO2 may decrease if with occasional venting.

**166**

equipment.

The species and reagent names of O2 scavengers are displayed in **Table 14** [4]. The optimal reagent is hydrazine, but it is limited by its highly toxic properties. Thus, the replacement for hydrazine was inevitable.
