Interpretation of transient oxidation kinetics using modelling of grain boundary diffusion
Résumé
Diffusion through oxide scale is very often the limiting step in high temperature oxidation phenomena. Thus, reliable estimation of oxidation kinetics requires accurate understanding of the diffusion phenomena. Grain boundaries in the oxide scale can act as diffusion short circuits, especially for the low temperatures, and can have consequently a strong effect on the oxidation kinetics. In this study, the oxidation kinetics of chromia on Ni-30Cr alloy has been modelled, considering bulk and grain boundary diffusion in the oxide scale, by both analytical solutions and numerical resolutions. The chromia microstructure evolution has also been considered, with a growth of grain size over time, and a grain size gradient within the oxide scale. Hence, the influence of grain boundaries diffusion on oxidation kinetics has been quantitatively investigated in the particular case of chromia scales on Ni-30Cr alloy at 1173 K. In a first time, several historical analytical solutions proposed in the 60's have been supplemented by some new analytical models proposed for simple cases of parabolic or cubic grain growth law but also considering a grain size gradient within the oxide scale. In a second time, some more complex cases considering the combination of oxide grain growth and grain size gradient within the oxide scale have been investigated using numerical resolution. These calculations have been done thanks to new implementations in the EKINOX numerical code previously developed [1-3]. These simulations have then be compared to dedicated experiments of high temperature oxidation in which the chromia microstructure and its evolution over time have been characterized. Moreover, after discussion of grain boundary diffusion effect on oxidation kinetics, discussion has been carried out on the influence of diffusion short circuits in the alloys, considering grain boundaries but also the dislocation which can have a dual role of diffusion short circuits and annihilation sources for vacancies.