Contribution to understanding parabolic oxidation kinetics of dilute alloys. Part II: Oxides with metal deficit or oxygen excess

Abstract

Concerning the parabolic oxidation of alloys with a low content of a different valence element it is shown that the Wagner-Hauffe valence approach contains some inaccuracies. This paper is devoted to the growth of an oxide MO with metal deficit or oxygen excess. The case is developed where the doping is high enough to modify the defect concentrations over the entire oxide scale. It is shown that lower-valence metal additions decrease the scaling constant and modify the influence of oxygen pressure. For additions of higher-valence metals, as long as electronic conductivity remains over the whole scale, the limiting step might be the diffusion of ionic defects under only an electric field, with an increase of the scaling constant. When the conductivity becomes essentially ionic over the whole scale the limiting step might be the diffusion of electron holes, and the scaling constant will decrease with an increase in the doping level. Globally the parabolic constant might therefore increase and pass through a maximum before decreasing.