Oxidation Behaviour of Ni–Al and Ni–Cr–Al Alloys in Dry and Wet Oxygen at 750 °C

Abstract

The oxidation behaviour of binary Ni-(3, 5)Al and ternary Ni-(3, 4, 5)Al-(15, 20)Cr alloys (all in wt%) was investigated in both Ar-20%O₂ (dry oxygen) and Ar-20%O₂-20%H₂O (wet oxygen) at 750 °C. For Ni-3Al alloy, an external NiO and an internal oxidation zone (IOZ) were formed with an enhanced thickening kinetics in wet oxygen. Increasing Al to 5 wt% led to a partial protection of the surface by a thin alumina scale, together with non-protective NiO and IOZ as that for Ni-3Al. Adding Cr into Ni–Al alloys significantly increased oxidation protection. In dry oxygen, all ternary alloys formed mainly a thin protective alumina scale. This protective effect was significantly reduced in the presence of water vapour. For Ni-(3, 4)Al-15Cr, a complex multi-layered oxide structure was observed, with an external NiO, an inner oxide layer with a chromia band at the bottom, and an IOZ with alumina precipitates. Further increasing Cr to 20 wt% led to a dominant chromia band formation in the inner oxide region. For high Al (5 wt%) ternary alloys, a protective alumina scale was formed in most of the surface area. Oxide characterisation revealed that this thin layer of alumina was α-Al₂O₃. The effects of water vapour and alloy composition on oxide formation were discussed based on classic diffusion theory and oxidation kinetic analysis.