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 degrees C in air for 100 h. During the initial stages of oxidation, the oxide layer of the Ni-16Cr0Ta alloy was mainly comprised of NiO, NiCr2O4, and C2O3. 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 NiCr2O4 and Cr2O3 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 CrTaO4 and Ta oxides increased with the amount of Ta and Cr. The appropriate amounts of CrTaO4 and Ta oxides contributed to inhibiting the diffusion of Cr ions, while excessive amounts of CrTaO4 and Ta oxides may decrease the adhesion of the oxide layer.