Two-phase layered scales comprising CoO and Co 3O4 formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (∼10 μ) and the spinel comprised small grains (⪝3 μ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co3O4.
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Hsu, H.S., Yurek, G.J. Kinetics and mechanisms of the oxidation of cobalt at 600–800°C. Oxid Met 17, 55–76 (1982). https://doi.org/10.1007/BF00606193
- grain boundaries