Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

Abstract

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H2O was investigated. The oxidation kinetics of Ni and the alloys in air + H2O were very similar, but the mass gains of Ni and each alloy were smaller in air + H2O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al2O3 precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H2O. The NiO scale formed in air + H2O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H2O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H2O.