To investigate the impact of Al and Nb elements on the formation of a protective oxide layer on the surface of Fe-35Ni-20Cr-xAl-yNb (x = 0, 2, 4, 6 wt.%; y = 0, 1, 2 wt.%) alloys, their oxidation behavior was examined at 1000 °C, 10-17 atm. and 10-25 atm. oxygen pressure, and the oxidation mechanism was analyzed by Factsage and Pandat calculations. Enhancing the Al content at 10-17 atm. inhibited the generation of FeCr2O4 on the alloy surface and increased the Al content in the M2O3 layer. When the Al content exceeded 6 wt.%, the oxide film partially peeled off. It was found that the addition of Nb increased the activity of Cr and Al and decreased the activity of Ni and Fe and promoted the formation of Al2O3, and the appearance of Nb2O5 in the subsurface layer increased the density of the oxide film. In addition, under an oxygen pressure of 10-25 atm., the only protective layer on the surface of the alloy comprised of Al2O3. The experimental results demonstrated that the Fe-35Ni-20Cr-4Al-2Nb alloy generated a continuous and dense Al2O3 protective film, and the reduction in oxygen pressure and the addition of Nb elements were favorable for selective external oxidation of Al2O3.
Keywords: Fe-Ni-Cr-based alloys; low oxygen pressure; oxidation behavior; phase diagram of oxidation; pre-oxidation.