The kinetics and morphological development of oxide scales on cobalt-iron alloys (0–70 wt.% Fe) at 1200°C

The oxidation kinetics of Co-Fe alloys containing up to 70 wt.% Fe have been determined at 1200°C in oxygen at pressures ranging from 10^–4 to 1 atm. These kinetics, which were parabolic, resulted from metal diffusion through the scales as the rate-controlling process. The scales were either one-phase cobalto-wustite (CoFe)O or two-phase, containing growth of spinel cobalt ferrite (CoFe)₃O₄ in the cobalto-wustite matrix. The particular shape of the relationship, exhibiting a maximum for the parabolic oxidation constant vs. the iron alloy composition, is interpreted as being caused by the combined opposing effects of more rapid metal diffusion in cobalto-wustite and diffusional resistance of the spinel growths in cobalto-wustite with increasing iron alloy content.