The effect of lanthanum on the scales developed on thin foils of Fe-20Cr-5Al at very high temperatures

A study has been undertaken of the effects of lanthanum on the oxidation of thin foils of Fe-20Cr-5Al in air at 1150 degrees C. The addition of lanthanum causes the time to breakaway to increase front about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidation of the lanthanum-containing alloy occurs in three stages. During the first stage, an alpha-Al2O3 layer is established and thickens with time until the aluminum in the foil is depleted sufficiently for a layer of Cr2O3 to become stable and develop at the scale-alloy interface. This continues to thicken at a relatively slow rate until breakaway occurs. The main emphasis in the present paper has been an examination and analysis of the scale established on the lanthanum-containing alloy in cross section in the analytical transmission electron microscope (TEM), after an exposure period that coincides with the second stage of oxidation, prior to breakaway. The scale at that time consists of three layers. The outer layer is composed of equiaxed Al2O3 grains. The intermediate and inner layers consist of columnar Al2O3 grains min equiaxed Cr2O3 grains, respectively. Numerous voids are observed in the oxide grain boundaries and at the intermediate-inner layer interface. Lanthanum segregates in the oxide grain boundaries and its concentration increases toward the outermost surface of the scales. These results are consistent with the "dynamic segregation model" to account for the effects of reactive elements on the growth of Al2O3 scales.