Perturbation of Parabolic Kinetics Resulting from the Accumulation of Stress in Protective Oxide Layers

Abstract

A frequent observation in metal oxidation is the development of subparabolic kinetics, variously described as cubic or quartic. Although a number of detailed mechanisms have been proposed to account for this effect, none seem generally applicable. This paper presents a model of the oxidation process which is divorced from such restrictions. It is argued that deviations from parabolic behavior occur as a result of the concurrent development of stresses within the oxide. It is shown that the presence of stress fields can influence significantly the rate of transport of vacancy defects within the oxide such that tensile stresses produce positive deviations and compressive stresses, negative deviations from parabolic behavior. The model is applied in detail to Zircaloy‐2 oxidation at 773°K. It is predicted that the kinetics should be insensitve to the oxygen potential of the environment and this has been confirmed by previous experimental work. In addition, the absolute value of the oxidation rate is closely predicted using measured values of diffusion coefficients and the observed gradual departure from parabolic kinetics with increasing oxide thickness is accounted for.