The effect of Nb content on the oxidation behavior of Al2O3-forming ferritic stainless steels with different Al contents was investigated. The Al2O3 scales formed on all alloys exhibited a duplex structure, with the outer layer containing Fe and Cr and the inner layer consisting of relatively pure Al2O3 oxide. No apparent influence of Nb on the growth rate of the outer oxide scale was observed among alloys with the same Al content, but the growth rate of the inner oxide layer increased with increasing Nb content. This effect of Nb was more pronounced in alloys with lower Al content. In this study, it was also observed that the growth rates of both the inner and outer oxide scales increased with increasing Al content. Fe and/or Nb segregation was confirmed in the Al2O3 scale formed on 1Al and 3Al alloys with Nb addition. However, neither Fe nor Nb segregation was detected in the Al2O3 scales formed on 5Al alloys, with or without Nb addition. EELS analysis suggested that Nb4+ segregated along the grain boundaries of the inner Al2O3 layer formed on 1Al and 3Al alloys, which could explain the enhanced inward diffusion of oxygen, resulting in the faster growth of the inner Al2O3 layer.
Li et al. (2026) studied this question.