Unveiling the formation of oxide layers on Ni-16Cr and Ni-20Cr alloys by the addition of Ta

The effect of Ta on the oxidation behavior of Ni-16Cr and Ni-20Cr (wt%) alloys was investigated by isothermal oxidation tests at 1000 °C in air for 100 h. During the initial stages of oxidation, the oxide layer of the Ni-16Cr-0Ta alloy was mainly comprised of NiO, NiCr ₂O ₄, and C ₂O ₃ . The addition of a small amount of Ta (0.5 and 1 wt%) effectively inhibited NiO formation and reduced the oxidation rate. However, excessive Ta failed to inhibit the formation of NiO. For the Ni-20Cr-0Ta alloy, the oxide layer was mainly comprised of NiCr ₂O ₄ and Cr ₂O ₃ in the initial stages of oxidation, and the addition of Ta has no significant effect on the composition of the oxide layer for the alloy. The formation of CrTaO ₄ and Ta oxides increased with the amount of Ta and Cr. The appropriate amounts of CrTaO ₄ and Ta oxides contributed to inhibiting the diffusion of Cr ions, while excessive amounts of CrTaO ₄ and Ta oxides may decrease the adhesion of the oxide layer.