High Temperature Oxidation of Co-10 w/o Cr Alloys

Thermogravimetric studies of the oxidation kinetics of Co‐10 w/o Cr have been carried out in the temperature range 800°–1300°C at oxygen pressures from 0.05 to 760 Torr. The over‐all oxidation is parabolic, and the oxygen pressure dependence of the rate constant can be expressed by , where varies from 3 at 900° to 2.5 at 1300°C. The oxidation is faster than that of pure cobalt. The oxide scale is double‐layered and consists of an outer layer and an inner layer of and particles embedded in a matrix. The inner layer also contains 30–35 v/o (volume per cent) porosity. It is concluded that the oxidation is controlled by Co‐vacancy diffusion in the . The spinel inclusions inhibit the oxidation by decreasing the effective diffusion area in the scale. The pores partially short‐circuit the solid‐state diffusion through the scale due to oxygen transport across the pores; the porosity then serves to increase the oxidation. Semiconductor valence effects due to dissolution of chromium in the phase are concluded to be of minor importance as regards the relative oxidation behavior of pure cobalt and Co‐10 w/o Cr alloy.