Investigation on the corrosion behavior of Alloy 800H at various levels of deformation

Abstract

The corrosion behavior of Alloy 800H in deaerated supercritical water was investigated. Multi-pass cold rolling was used to achieve thickness reductions of up to 80%, and the weight gain vs. time (up to 1000 h) in supercritical water at 600 ℃ and 25 MPa were obtained for specimens in the polished vs. ground surface condition. Polished specimens without rolling showed the highest corrosion rates, and surface grinding or cold rolling significantly increased the corrosion resistance. A thick oxide film develops, with an outer moderately uniform layer of magnetite (Fe3O4) with crystallites along the surface, and areas of uneven oxidation into the metal that incorporate Cr (e.g., FeCr2O4). Very few Cr2O3 particles exist, and preferentially near the grain boundaries. Continuous Cr-rich oxide films were found in the thin oxide films on the surface of ground specimens and heavily rolled specimens, and an associated Cr depleted zone in the metal under the oxide of ground specimens, but not in heavily rolled specimens. This indicates that the high density of dislocations accelerates the diffusion of elements and enhance the formation of Cr-rich oxide film, which promotes passivity.