The re-oxidation of pre-formed nickel oxide scales

Abstract

The effect of a pre-existing thermal nickel oxide scale on the subsequent re-oxidation of nickel at a different temperature has been investigated. The scale microstructure, oxidation kinetics and through-scale transport of molecular oxygen during re-oxidation have been determined. Grain growth during re-oxidation is consistent with a simple parabolic rate law. The activation energy for grain growth is approximately 4 eV, suggesting control of grain growth by an anion vacancy diffusion mechanism. Reoxidation kinetics are described by a parabolic rate law. The re-oxidation rate is reduced, relative to isothermal oxidation at the same temperature, when the initial oxidation is at a higher temperature than the re-oxidation. The through-scale transport of oxygen during re-oxidation is determined by the preoxidation temperature and is independent of both re-oxidation temperature and time. It is proposed that the amount of oxide formed at the scale-metal interface is determined by the structure of the interface formed during the initial oxidation. Subsequent annealing at higher temperatures does not substantially alter the interface structure although the scale microstructure is changed.