Effect and role of alloyed Nb on the air oxidation behaviour of Ni-Cr-Fe alloys at 1000 °C

Ni-25Cr alloys alloyed with Fe (5 wt.% and 15 wt.%) and Nb (0, 1 and 2 wt.%) were designed and their oxidation behaviour was evaluated at 1000 °C. Results reveal that alloyed Nb has a positive impact on oxidation in both the kinetics and oxide scale structures. The parabolic rate constant kp of Nb-alloyed Ni-25Cr-15Fe alloys are nearly one-fourth the value for Nb-free alloy. The oxide scales change from spinel-included multi-layers in the Nb-free alloy to a much thinner Cr2O3 single-layer in the Nb-alloyed alloys. NbC precipitates and slightly dissolved Nb account for these changes.