Model to simulate parabolic followed by linear oxidation kinetics

Abstract

A model is proposed to describe parabolic followed by linear oxidation kinetics of zirconium and zircaloy. The one-dimensional description presented here for parabolic kinetics considers the superficial oxide layer as formed by a succession of diffusion layers of crystalline oxide alternating with short-circuit paths for diffusion. Within this framework, the parabolic-linear transition can be viewed as the result of the rupture of the thin diffusion laters, leaving behind long channels connecting the oxidizing environment with the diffusion region adjacent to the oxide-metal interface. These channels represent easy paths for oxygen diffusion. This model unifies the description of parabolic oxidation of Zr with that of linear kinetics and can be also connected with the description of cubic oxidation kinetics. The latter can be thought of as the result of a recrystallization process where the oxidation rate diminishes more than during a parabolic regime as time progresses due to the growing influence of volume diffusion.