Promoting exclusive (alpha)-Al2O3 growth upon high-temperature oxidation of NiCrAl alloys: experiment versus model predictions

The relationships between the compositional changes in the alloy and the morphology and phase constitution of the developing oxide scale are established as a function of oxidation time for the thermal oxidation of a (gamma)-Ni-27Cr-9Al (at.%) alloy at 1373 K and a high and low partial oxygen pressure (pO2) of 2 × 10^4 and 0.1 Pa, respectively. Based on experimental observations, and model calculations using a coupled thermodynamic-kinetic oxidation model, four successive growth stages can be identified for the oxidation of a (gamma)-Ni-27Cr-9Al alloy at 1373 K. It is shown that the exclusive growth of (alpha)-Al2O3 is promoted for oxidation at low pO2 by maintaining a sufficiently high concentration of Al in the alloy at the oxide/metal interface during the initial stages of fast oxide-scale growth.