Industrial boilers like kraft recovery boilers experience stress assisted corrosion (SAC) cracks in their carbon steel tubes and other water touched surfaces. The performance of carbon steel in industrial boilers strongly depends upon the formation and stability of the protective magnetite film, Fe3O4, on the waterside surface of boiler tubes. Tests were carried out in a recirculation autoclave under industrial boiling water conditions. Boiler water chemistry was controlled during a series of tests to keep the dissolved oxygen in the range of 5 ppb - 10 ppm. Initial tests were conducted to develop the magnetite film on carbon steel tube samples under different test conditions. Results have indicated that the water chemistry and ratios of anode/cathode have an effect on the magnetite film morphology. Film characterization by atomic force microscopy (AFM) and scanning electron microscopy (SEM) has shown that the magnetite film changes from an irregular-grained compact and protective film to a fine-grained porous (non-protective) film with tetrahedral crystals at the surface when the anode to cathode area ratio decreases. Corrosion fatigue crack initiation and growth mechanisms involved in boiler water environments include magnetite film damage as an important step. Slow strain rate tests were carried out in simulated boiler water environments, using smooth carbon steel samples, to investigate the role of temperature and water chemistry on crack initiation. The mechanism of stress assisted corrosion was proposed here, and an interrupted slow strain rate test was designed and carried out in lab to validate the proposed SAC mechanism.

Subject
Steel surfaces, Corrosion fatigue, Water, Tubes, Protective films, Crack initiation, Water chemistry, Corrosion cracking, Stress cracking, Boilers, Magnetite, Carbon steel, Stress corrosion cracking
Keywords:
Corrosion Fatigue, Stress Assisted Corrosion, Water Chemistry, Industrial Boilers, Dissolved oxygen
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2006
Association for Materials Protection and Performance (AMPP)
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