Effect of La and Hf additions on the high-temperature oxidation resistance of high-purity Fe-20Cr-5Al alloy foils

The oxidation behavior of 30- or 50-mum thick high-purity Fe-20 w/o Cr-5 w/o Al alloy foil and similar alloy foils containing La and La-Hf was examined in cyclic-oxidation tests at 1373 and 1473 K in air. The oxidation process proceeded in three stages. In the first stage, an Al2O3 scale grew until all the Al in the foil had been removed. In the second stage, a Cr2O3 layer grew between the Al2O3 layer and the substrate on the alloys containing La or La-Hf, while a (Cr, Al)(2)O-3 layer formed on the alloy without La and La-HJ. In the third stage, breakaway oxidation occurred. The addition of La decreased the oxidation rate in both the first and the second stages. The addition of La-Hf decreased the rate further. The growth rate of alloys containing La or La-Hf in the second stage was found to be proportional to the diffusion rate of oxygen in the Al2O3 scale. Therefore, it is inferred that the inward oxygen diffusion rate in the Al2O3 scale on the alloy containing La-Hf was reduced compared with that on the alloy containing La, resulting in a decrease in the oxidation rate in the first stage.