Quantification of Aluminum Outward Diffusion During Oxidation of FeCrAl Alloys

Abstract

The outward flux of aluminum during the oxidation of four similar aluminaforming 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 Y 2 O 3 limiting aluminum diffusion more than Zr, Y, and ZrO 2 .

Mass-Transfer Mechanism of Alumina Ceramics under Oxygen Potential Gradients at High Temperatures

“…For the growing Al 2 O 3 scale on the alumina forming alloy under a region of lower P O 2 , such as that exposed to flow of purified argon, 10) the absence of grain boundary ridges is considered to be mainly due to inward grain boundary diffusion of oxygen through oxygen vacancies. On the other hand, the scale in a region of higher P O 2 , such as that exposed to air flow, may exhibit a p-n junction behavior, where outward diffusion of aluminum through aluminum vacancies proceeds in addition to the inward diffusion of oxygen.…”

“…The ridge evolution has a similar P O 2 dependence as that on a growing Al 2 O 3 scale during oxidation of alumina forming alloys. 10) This thus suggests that mass-transfer through the Al 2 O 3 wafer is controlled by the P O 2 in the oxygen permeation technique.…”

“…By contrast, when oxidation of the alloys occurs through the -Al 2 O 3 scale in gases such as purified argon, the ridges are not present. 10) Ridge formation (i.e., mass-transfer through the Al 2 O 3 scale) is thought to be dependent on the P O 2 .…”

“…15,16) In this case, the appropriate tracer 26 Al has a very low specific activity and an extremely long half-life of 7:2 Â 10 5 years, making it very difficult to perform radiotracer diffusion experiments. Nychka et al, 10) then, deduced the aluminum grain boundary flux through an Al 2 O 3 scale during oxidation of alumina forming alloys by measuring the scale thickness, which consisted of double layers, such as equiaxial grains produced by outward diffusion of aluminum and underlying columnar grains developed by inward diffusion of oxygen.…”

Mutual grain-boundary transport of aluminum and oxygen in polycrystalline Al2O3 under oxygen potential gradients at high temperatures

“…2), 3) In contrast, these ridges do not form in a low P O2 environment, such as in a purified argon flow, in spite of progress of the scale growth. 3) To clarify the mass-transfer mechanism for the above-mentioned phenomena, oxygen permeation through polycrystalline alumina wafers exposed to a ÁP O2 at high temperatures was measured, with each surface of the wafer deliberately subjected to a different P O2 value.…”