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

The oxidation characteristics of Fe‐25Cr‐35Ni‐2.5Al‐ X Nb (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 Cr 2 O 3 ‐rich outer layer and Al 2 O 3 ‐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 Al 2 O 3 scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb 2 O 5 ) are formed on the surface of the 1.2Nb‐addition alloy and destroy the integrity of the Al 2 O 3 scale, which causes the formation of Cr‐rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr 2 O 4 , Cr 2 O 3 , and Fe 2 O 3 outer layers and Al 2 O 3 inner layer.