Alloying effects of Al, Si and Fe on Ni-20Cr alloy oxidation in water vapour at 650 °C

Ni-20Cr alloys with or without minor elements (Al, Si, Fe) were oxidized in Ar-5 H2O at 650°C. The undoped alloy formed a feathery NiO external scale and a thick internal oxidation zone (IOZ). Additions of Al, Si and Fe improved oxidation resistance by changing oxide morphologies and reducing the IOZ. For Al and Si additions, more surface area formed a chromia scale with Al2O3 or SiO2 precipitates underneath. In addition, needle-shaped precipitates with a lamellar structure of chromia mixed with alumina or silica were observed. For Fe addition, a multi-layered oxide scale was formed with additional Fe-rich spinel, improving oxidation resistance.