Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr₂O₃

Abstract

In most cases, chromia scales are assumed to grow by predominant chromium diffusion. However, results of Atkinson and Taylor indicated that chromium bulk diffusion could not account for the growth rate of chromia scales. Moreover, recent results of Parket al. showed that oxygen diffusion in chromia was faster than chromium diffusion. So, at this date, the controlling process of the growth of chromia scales is not elucidated.

To interpret such a phenomenon, oxygen and chromium self-diffusion coefficients in Cr₂O₃ single crystals and polycrystals were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. Tracers were introduced by ion implantation, thick film methods, and isotopic exchange, using the⁵⁴Cr,⁵⁰Cr and¹⁸O isotopes. Depth profiling was made by secondary ion mass spectroscopy (SIMS). The bulk diffusion coefficients were computed by using a general solution of the Fick's law taking into account evaporation and exchange at the surface. Grain-boundary diffusion coefficients were computed by using the Whipple-Le Claire equation for type B intergranular diffusion. Lattice and grain boundary self-diffusien coefficients were determined as a function of temperature and oxygen pressure.

The diffusion coefficients are lower than results given in the literature and do not depend on the oxygen pressure. Moreover, it is found that oxygen diffusion is faster than chromium diffusion. These results are compared to the oxidation constants of chromia-forming alloys and it is shown that neither lattice self-diffusion, nor grain-boundary self-diffusion can justify the growth rate of chromia scales. Such a situation is compared to NiO case, for which authors found important differences in grain-boundary diffusivity, according to the elaboration mode of NiO (thermal oxidation or growth from the melt).