Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.