Effect of oxygen partial pressure on the oxidation behaviour of an yttria dispersion strengthened NiCr-base alloy

An yttria dispersion strengthened NiCr-base alloy was studied with respect to isothermal oxidation behaviour at 1000 C and 1050 C in high- and low-pO2 gases, i.e. Ar–O2 and Ar(-H2)–H2O. The scale growth kinetics, morphology and composition were studied by thermogravimetry in combination with SEM/EDX and SNMS. Due to Y doping the surface scale is very protective and initially grows predominantly by inward oxygen diffusion. Local formation of mainly outwardly growing oxide nodules occurs after longer oxidation times and is related to metallic protrusions formed as a result of internal oxidation of the minor alloying addition aluminium. The differences in scale morphology in the various environments are related to the effect of the gas composition on scale grain size and on the relative amounts of inward scale growth. Possibly the pO2 dependence of the Ti-solubility in the chromia scale and/ or hydrogen doping of the oxide plays an additional role in the scale growth process.