The improvement in high-temperature oxidation resistance of Ni0.25Co0.25Cr0.22Mo0.14Re0.14 high entropy alloy via industrial chemical vapor aluminizing process

Abstract

The effects of the protective coating (NiAl coating) on the high temperature isothermal oxidation resistance of Ni 0.25 Co 0.25 Cr 0.22 Mo 0.14 Re 0.14 high entropy alloy (HEA) was discussed in this work. A nickel aluminide (NiAl) coating was obtained on the Ni 0.25 Co 0.25 Cr 0.22 Mo 0.14 Re 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 degrees C to investigate the effects of the NiAl coating on high- temperature isothermal oxidation resistance of Ni 0.25 Co 0.25 Cr 0.22 Mo 0.14 Re 0.14 HEA. The results revealed that a protective and slowly growing alpha-Al2O3 2 O 3 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 2 O 3 but also non-protective spinel oxides (CoCr2O4 2 O 4 and NiCr2O4) 2 O 4 ) 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 alpha-Al2O3 2 O 3 oxide layer on the NiAl coating. In contrast, the uncoated HEA samples experienced weight loss even after 6 h of isothermal oxidation, as Cr2O3 2 O 3 lost its stability at 1050 degrees C and non-protective spinel oxides formed. Therefore, it has been determined that the NiAl coating significantly enhance the isothermal oxidation resistance of the Ni0.25C- 0.25 C- o 0.25 Cr 0.22 Mo 0.14 Re 0.14 HEA. The alpha-Al2O3 2 O 3 oxide layer has prevented severe oxidation of NiAl-coated samples due to its stability at 1050 degrees C over 200 h and its slow growth rate.