On High Temperature Oxidation of Chromium: II . Properties of and the Oxidation Mechanism of Chromium

Abstract

Chromium specimens oxidized at 1000°–1100°C and at different oxygen pressures have been treated in high vacuum at temperature as a direct continuation of the oxidation runs. All specimens lost weight during the high vacuum treatment. During an initial period the rate of weight loss slowly decreased, but after longer time the loss rate became essentially linear. It is concluded that the weight loss is due to chromium evaporation due to chromium transport through the scales by lattice diffusion and transport along paths of easy diffusion and microcracks. On the basis of the properties of scales and available literature data on defect‐dependent properties of , a defect structure model has been proposed for the oxide. It is proposed that interstitial chromium ions are the predominating point defects at partial pressures of oxygen near the phase boundary. At near‐atmospheric pressures the oxide is an intrinsic electronic conductor. Parabolic rate constants calculated from the diffusion data and the proposed defect structure model are compared with experimental values. The oxidation mechanism of chromium is discussed. Important factors are lattice diffusion, transport along paths of easy diffusion and microcracks through the scale, growth stresses in the scale, the ability of the scale to deform, and vapor transport from the metal substrate to detached scales.