Kinetics and mechanism of the accelerated oxidation of low carbon scrap steels

Especially with accelerated oxidation, the kinetic curves of steel strips in CO₂-CO gas mixtures at 900° C do not adhere to either the parabolic or the linear growth law. Linear kinetics is essentially obeyed in the oxidation of untreated steel, whereas mixed parabolic or linparabolic laws determine the oxidation of steels treated with oxidation promoters. The observed kinetics are best explained with the Zener diffusion model, which assumes that the activation energy for diffusion is spent in deforming the crystal lattice elastically in the vicinity of the saddle point. The model results in a strong dependence of diffusivity on the melting point and elastic moduli of the diffusion medium (oxide scale). A diffusion coefficient that decreases exponentially with melting point and the Tammann differential relation for oxide growth account adequately for the observed kinetics.