Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

Abstract

The oxidation behavior of 30- or 50-μm thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al2O3 scale grew until all the Alin the foil had been removed. In the second stage, a Cr2O3layer grew between the Al2O3 layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)2O3layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al2O3 scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al2O3scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.