The improvement in high-temperature oxidation resistance of Ni_0.25Co_0.25Cr_0.22Mo_0.14Re_0.14 high entropy alloy via industrial chemical vapor aluminizing process

The effects of the protective coating (NiAl coating) on the high temperature isothermal oxidation resistance of Ni_0.25Co_0.25Cr_0.22Mo_0.14Re_0.14 high entropy alloy (HEA) was discussed in this work. A nickel aluminide (NiAl) coating was obtained on the Ni_0.25Co_0.25Cr_0.22Mo_0.14Re_0.14 HEA via the industrial chemical vapor aluminizing (CVA) process. Isothermal oxidation tests were applied to NiAl-coated and uncoated samples in ambient laboratory air for durations of up to 200 h at 1050 °C to investigate the effects of the NiAl coating on high-temperature isothermal oxidation resistance of Ni_0.25Co_0.25Cr_0.22Mo_0.14Re_0.14 HEA. The results revealed that a protective and slowly growing α-Al₂O₃ oxide layer formed on the surface of the NiAl-coated HEA samples and remained stable even after 200 h of isothermal oxidation. For uncoated HEA samples, not only Cr₂O₃ but also non-protective spinel oxides (CoCr₂O₄ and NiCr₂O₄) were detected on the surface. Weight gain was observed in the NiAl-coated HEA samples due to the formation of a protective and slowly growing α-Al₂O₃ oxide layer on the NiAl coating. In contrast, the uncoated HEA samples experienced weight loss even after 6 h of isothermal oxidation, as Cr₂O₃ lost its stability at 1050 °C and non-protective spinel oxides formed. Therefore, it has been determined that the NiAl coating significantly enhance the isothermal oxidation resistance of the Ni_0.25Co_0.25Cr_0.22Mo_0.14Re_0.14 HEA. The α-Al₂O₃ oxide layer has prevented severe oxidation of NiAl-coated samples due to its stability at 1050 °C over 200 h and its slow growth rate.