Quantitative evaluation of the effect of the Al addition on the internal-external oxidation in Ni-Cr alloys

Internal oxidation of the alloy of Ni-5.49 mol%Cr-4.16 mol%Al (Ni-5.0 mass%Cr-2.0 mass%Al) was studied at 1273K under three oxygen partial pressures produced by Cr/Cr2O3, Fe/FeO or Ni/NiO mixtures. The growth of internal oxidation zone (IOZ) was controlled by the oxygen diffusion in IOZ. It was demonstrated that the enrichment of Cr appeared at the oxidation front in which only Al2O3 was precipitated. The Al2O3 precipitates in IOZ should be added to the critical volume fraction of oxides to transition from internal to external oxidation. These effects may be beneficial to develop the external Cr2O3 scale. The Wagner equation predicting the alloy composition required to form the external scale for binary alloys was modified for ternary alloys.