Corrosion in Boiler Steam and Condensate

Ordinary paints may be permeable to oxygen and water vapor (see Chap. 13), and though they may slow the rate of corrosion, they cannot prevent it completely. Hence, special paints with chromates or red lead (Pb304) have been used for many years as a protective coating for steel. Polymeric resins, though more expensive than the linseed oil-based paints, last longer and thus are more effective. 

Steam lines with air and CO2 entrained can be very corrosive. To reduce corrosion, oxygen can be removed by the addition of hydrazine (N2H4). 

Other additives which are commonly added are basic amines which neutralize the acids (H2CO3) present in the water. One important property of the amine besides the pH of its solution is the distribution ratio (DR) which is the ratio of amine in steam to that dissolved in the condensate. A high 

DR value means that the metal is readily coated with a thin protective film of the amine. Some amines commonly used and their pH and DR are given in Table 10.5. The amine is slowly lost, and it must be replaced continuously. Steam lines invariably have these amines, and the use of brass, bronze, or copper results in the corrosive removal of copper. 

Stored metallic equipment or parts are subject to corrosion. Sodium nitrite is an inhibitor which is often included in the enclosure or packaging. However, vapor phase corrosion inhibitors (VCI) such as dicyclohexylammonium nitrite and ammonium benzoate are superior corrosion inhibitors because o/the film formed on the metal surfaces. 

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