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

Abstract

AbstractThe 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.