Effect of Chromium Content on Oxidation Behavior and Microstructure of Alumina Scale on Aluminum-Containing Ferritic Stainless Steel

The effect of Cr content on the oxidation behavior of Nb-modified Al2O3-forming ferritic stainless steels was investigated. All alloys formed an Al2O3 scale regardless of Cr content, and the Al2O3 scale had a duplex structure, with the outer layer containing Fe and Cr and the inner layer containing relatively pure Al rich oxide. The growth rate of the outer oxide scale increased with decreasing Cr content for shorter oxidation times because of the formation and growth of a Fe-rich oxide scale for the low-Cr alloys and because of a Cr-rich oxide scale for the high-Cr alloys. However, the growth rate of the inner Al-rich oxide scale increased with increasing Cr content. For longer oxidation periods, the oxidation kinetics of the inner oxide layer formed on low-Cr alloys decreased greatly owing to the formation of a Laves phase (Fe2Nb) at the scale/alloy interface.