The outward flux of Al during the oxidation of 4 similar alumina-forming alloys was determined using two types of experiments. One was measurement of the total thickness of the oxide and the ratio of the thickness of the characteristic equiaxed layer to the total oxide thickness. The second was measurement of the new oxide formed along the grain boundaries of the oxide upon re-oxidation. The former provides information about the ratio of outward diffusion of Al to inward diffusion of oxygen during oxidation. The latter experiment directly quantifies the outward flux of Al as a function of oxide thickness. Both the outward Al flux and the ratio of inward to outward diffusional fluxes were found to vary with the minor concentrations of so-called reactive element alloying additions. Specifically, Y in solution in the alloy was found to limit outward Al diffusion more than Zr in solution, with Y2O3 limiting Al diffusion more than Zr, Y and ZrO2.

Quantification of Aluminum Outward Diffusion During Oxidation of FeCrAl Alloys. J.A.Nychka, D.R.Clarke: Oxidation of Metals, 2005, 63[5-6], 325-52