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One of the most frequently encountered corrosion problems results from exposure of boiler metal to dissolved oxygen. Since the oxides of iron are iron's natural, stable state, steels will spontaneously revert to this form if conditions are thermodynamically favorable. Generally, conditions are favorable if steel that is not covered by the protective form of iron oxide (magnetite) is exposed to water containing oxygen.
The fol1owing reactions occur:
2Fe + H2O + O2 => Fe2O3 + 2H
This reaction is the basis for the intensive mechanical and chemical deaeration practices that are typical of sound water-treatment programs. These practices are generally successful. In fact, occurrences of oxygen corrosion in boilers are generally confined to idle periods. For example, moisture condensing on the walls of an idle superheater tube will dissolve atmospheric oxygen. Fractures in the protective magnetite are caused by contraction stresses as the superheater is cooled to ambient temperatures. The fracture sites furnish anodic regions where oxygen-containing moisture can react with bare, unprotected metal. In addition to tube-wall perforation, oxygen corrosion is troublesome from another perspective. Oxygen pits can act as stress-concentration sites, thereby fostering the development of corrosion-fatigue cracks, caustic cracks, and other stress-related failures.