Effect of hydrogen pre-annealing on high temperature oxidation of Fe-20Cr-5Al alloy

The effect of pre-annealing on the oxidation behavior of Fe-20 mass%Cr-5 mass%Al alloys containing up to 0.11 mass%La was examined by a cyclic oxidation test at 1423 K in air. SIMS and chemical analysis were carried out for thin oxide layers formed by pre-annealing and Al2O3 scales formed by oxidation. The oxidation rate of La-added Fe-20%Cr-5%Al alloys was decreased markedly by hydrogen annealing, while that of La-free alloy was not changed. La enrichment was found in both the thin oxide layer formed on the alloy annealed in hydrogen and the Al2O3 scale formed on that alloy. It is concluded that La enrichment in the oxide film formed during hydrogen annealing suppresses the inward diffusion of oxygen through the Al2O3 scale, resulting in a decrease in the oxidation rate.