Quantifying outward cation diffusion in thermally grown alumina scales

Thermally grown alumina scales were investigated by the high temperature oxidation of FeCrAl alloys. Four separate commercial FeCrAl alloys with different dopant elements, added as metals or oxides, were studied. The oxide scales formed on these alloys are thought to grow by countercurrent diffusion of oxygen inward, and aluminum outward grain boundary diffusion. Based on the type and form of dopant that incorporates into the growing scale, the oxide morphology and growth rate are altered. Such dopant effects are proposed in the literature to be related to modification of the grain boundary diffusion characteristics of aluminum, however there have been no reports quantifying such assertions.; This work describes innovative experiments developed to quantify outward aluminum grain boundary diffusion in alumina scales. The results show that outward aluminum grain boundary diffusion contributes significantly to the morphology and growth rates of scales; depending on the dopants, ∼15--50% of the scale thickness is attributed to outward aluminum grain boundary diffusion. Implications of these results are that aluminum grain boundary diffusion has been inappropriately neglected from current models describing alumina scale growth. New models of oxide growth should be developed, which incorporate outward cation grain boundary diffusion.