The Kinetics of Oxidation of High Purity Nickel

Abstract

The effect of time, temperature, and surface pretreatment on rate of oxidation of high purity nickel is studied for the temperature range of 400°–750°C using a vacuum microbalance technique. The data are compared to previous studies in the literature and with other metals. Oxidation data are interpreted in terms of the parabolic rate law and classical theory of diffusion.Large deviations from the parabolic rate law are found to occur during the initial stages of reaction and smaller deviations over long periods of time, especially at low temperatures. However, reasonable values of heat and entropy of activation for the over‐all reaction can be calculated; these are 41,200 cal/mole and −6.0 entropy units (eu), respectively. Parabolic rate law constants over the temperature range of 550°– 700°C are given by The negative value for entropy of activation for the over‐all reaction when corrected for entropy of formation of the vacancies leads to a value of 1.5 for entropy of activation for diffusion. Theoretical considerations suggest that the latter term should have a value of 1.7–3.3 eu. The good agreement between theoretical and experimental entropies of activation suggests that diffusion is occurring largely through the lattice of nickel oxide and not at grain boundaries, at least for the temperature and time region over which analyses were made.A comparison of present data with older studies in the literature shows a large variation in parabolic rate law constants. These variations are interpreted in terms of impurities increasing the concentration of nickel ion vacancies.