Transient state in the selective oxidation of binary alloys with quasiparabolic kinetics: Solution for a fixed alloy-scale boundary

Abstract

Analytical solutions for the concentration profile and the surface concentration of an oxidizable component during the selective oxidation of a binary alloy following pseudoparabolic kinetics are examined, assuming a fixed alloy-scale boundary. A solution similar to that already proposed for rate control by metal transfer across the metal-scale interface can be obtained also assuming diffusion control. In the case of formation of a metal-deficit scale growing mainly by cation diffusion it is shown that oxidation is controlled by the diffusion in the alloy. The rate law is found to be pseudoparabolic, although different from that usually found in practice, leading to negative intercepts in a parabolic plot. Positive intercepts cannot be obtained if the regression of the alloy interface during oxidation is disregarded.