Quantification of aluminum outward diffusion during oxidation of FeCrAl alloys

The outward flux of aluminum during the oxidation of four similar alumina-forming alloys is determined using two types of experiments. One is 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 is 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 aluminum to inward diffusion of oxygen during oxidation. The latter experiment directly quantifies the outward flux of aluminum as a function of oxide thickness. Both the outward aluminum flux and the ratio of inward to outward diffusional fluxes are found to vary with the minor concentrations of reactive element alloying additions. Specifically, Y in solution in the alloy is found to limit outward aluminum diffusion more than Zr in solution, with Y2O3 limiting aluminum diffusion more than Zr, Y, and ZrO2.