Continuous Monitoring of Oxygen Chemical Potential at the Surface of Growing Oxide Scales during High Temperature Oxidation of Metals

Abstract

The continuous monitoring of the oxygen chemical potential at a surface of a growing oxide surface formed in high temperature oxidation of a metal was developed. During the oxidation of nickel and cobalt at 1373 K in Ar-21%O₂ gas, the oxygen chemical potentials at the surface of formed NiO scale and CoO scale were slightly smaller than that in the atmosphere, because the growths of these oxide scales were mainly rate-limited by the diffusion of the constituent ions in the scales. The difference of the oxygen chemical potential between at the surface of CoO scale and in the atmosphere was larger than that between at the surface of NiO scale and in the atmosphere under the same oxidation condition, since the oxygen consumption rate for the oxidation of cobalt was larger than that of nickel. In the case of the oxidation of iron at 1373 K in Ar-CO-CO₂ gas mixture, the oxygen chemical potential at the surface of formed FeO scale decreased largely from that in the atmosphere, since the growth of FeO scale was very fast and mainly rate-limited by the mass transport from gas phase to the scale surface. The oxygen chemical potential at the surface of FeO scale increased with increment of CO₂ concentration in the atmosphere.