Role of Water Vapor in Chromia-Scale Growth at Low Oxygen Partial Pressure

Abstract

The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H2-H2O and Ar-O2-H2O was studied at 1000°C. At high oxygen potentials, the addition of H2O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the pH2O/pH2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of pH2O/pH2 led to more rapid rates. At fixed pH2O values, the rate increased with increasing pH2. Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H2 mixtures, but not in reaction with O2/H2O. These blades did not form when Y2O3 dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H2O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with pH2 at fixed pH2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.