Fluctuating Sulfidizing-Oxidizing Environments and Their Effect on Stability of Protective Scales on Carbon Steel

Certain areas in the lower and mid-furnace of kraft recovery boilers have shown higher corrosion rates than others in the nearby areas. Local gaseous environments in boilers were characterized in areas with accelerated corrosion and equivalent areas with very low corrosion. Recovery boiler gases were analyzed using an on-line gas chromatograph. Results from this study have indicated that the areas with accelerated corrosion have large variations in the local gas composition at the water-wall surface. Environments were found to fluctuate between sulfidizing and oxidizing conditions under normal operation. Laboratory tests with SA210 carbon steel samples at high temperatures with gas composition cycling between oxidizing and reducing/sulfidizing led to an unstable, layered sulfide-oxide scale formation on the surface. Significant cracking of scale and subsequent mass loss was observed on carbon steel samples exposed to cyclic gaseous environments. Present work shows that cyclic gaseous environments can significantly destabilize the protective scale, leading to accelerated corrosion.