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

The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H-2-H2O and Ar-O-2-H2O was studied at 1000degreesC. 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 p(H2O)/p(H2) ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of p(H2O)/p(H2) led to more rapid rates. At fixed p(H2O) values, the rate increased with increasing pH, Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H-2 mixtures, but not in reaction with O-2/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 p(H2) at fixed p(H2O). The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials