Effect of Cr Content on Microstructure and Growth of External Al2O3 Scale Formed on Fe–Cr–Al Alloys

The effect of Cr on the microstructure and growth of Al2O3 scale that forms on Fe–4Cr–6Al and Fe–24Cr–6Al alloys during long-term oxidation at 1000 °C was investigated. The Al2O3 scale that formed on both alloys consisted of a duplex layer comprising columnar grains. The grain size of Al2O3 scale was found to be smaller for the Fe–24Cr–6Al alloy. The smaller grain size of Al2O3 scale formed on the alloy with higher Cr content was inferred to be due to the formation of an initial transient Cr-rich oxide scale with finer grain structure for the outer layer, and that a higher number density of internal Al2O3 precipitates formed during the initial stage of oxidation for the inner layer in the alloy with higher Cr content. The columnar grains grew larger laterally accompanying the growth of both the outer and inner layers at the gas/scale and scale/substrate interface during the steady-state oxidation period. Differences in the growth rate of Al2O3 scale formed on Fe–4Cr–6Al and Fe–24Cr–6Al can be explained by differences in the Al2O3-scale grain size, which indicates that the initial transient oxidation behavior strongly affects the microstructure and, therefore, the growth rate of Al2O3 scale.