Oxidation of Metals at High Temperatures

The available data on the oxidation rates of pure metals from 300° to 1000°C have been analyzed in terms of concentrations of defects in the oxide structures and defect diffusion coefficients. Quantitative agreement with the diffusion theory is obtained in the cases of p ‐type oxides such as cuprous oxide and nickel oxide, in which cationic vacancies arise by solution of oxygen at the oxide‐oxygen interface. The parabolic oxidation rates in the cases of some n ‐type oxides, such as zinc oxide and zirconium oxide, are characterized by low activation heats and large negative activation entropies. In some of these cases anionic diffusion seems likely. In the case of zinc oxide, the diffusion of radiozinc during oxidation supports the cationic diffusion mechanism. The data on linear oxidation rates fall into two classes; evidence is presented to show that one class represents reaction at the oxide‐oxygen interface, and the other, reaction at a metal‐oxygen interface.