The Effects of Nb Addition on the Oxidation Behavior of Ni–Fe–Cr Alloys at 800 °C

High temperature oxidation of Ni–18Fe–19Cr alloys with different Nb contents (based on the chemical composition of Alloy718) was investigated to elucidate the effect of Nb on oxidation kinetics and scale formation after exposure at 800 °C in air. The experimental results indicated that the oxidation kinetics decreased with increasing niobium content. Rapid establishment of an external Cr2O3 scale was interpreted by the formation of a continuous CrNbO4 layer at the metal-oxide interface, which acts as a diffusion barrier for inward oxygen and outward Fe and Ni diffusion and is of benefit for the establishment of an external Cr2O3 scale. The early establishment of this diffusion barrier prevented internal oxidation of chromium and reduced the growth of the outer Ni2FeO4 oxide scale. The beneficial effect of Nb could be attributed to effectively increasing the volume fraction of internal oxide precipitates, which are favorable for rapid establishment of an external Cr2O3 scale.