Steam Effects on Oxidation Behavior of Alumina-Scale Forming Nickel-Based Alloys and a Kinetics Analysis of Complex Scale Evolution during Isothermal Oxidation

The first part of this study investigated steam effects on the oxidation behavior of Al2O3-scale forming Ni-based alloys. This detrimental effect was assessed in the context of Giggins and Pettit’s oxidation map for the Ni-Cr-Al system, which was determined under dry oxidizing conditions. Of particular relevance to the design of high-temperature alloys and coatings is the location in the oxide map of the kinetically-dictated critical Al content (NAl*) in a given alloy to form a continuous Al2O3-scale. It was found that NAl* increases when the air contains 30% steam. A rigorous quantitative analysis based on Maak’s modification of Wagner’s theory was carried out to determine the key parameter affecting an increase in NAl* under wet oxidizing conditions. By checking the sensitivity of NAl* with respect to each parameter in the theory, it was deduced that the only parameter which can give the necessarily large change in NAl* found in wet-air oxidation is the critical volume fraction of internal oxide, fv*, a parameter that is usually assumed to be a constant and independent of the reacting conditions. From experimental observation, it was found that the surface of NiO scale became unstable in wet oxidizing conditions. Unlike the dense and uniform surface formed scale in dry air, finger-like protrusions or a powder-like porous structure could form under wet conditions. The experimental conditions and possible mechanisms for this observed instability were assessed. In the second part of this study, a novel kinetics analysis method was established to quantify the kinetics of scale evolution based on analyzing the instantaneous growth rate constant ki and the time exponent ni determined from thermogravimetric (TG) data. A methodology is proposed to ascertain whether the oxidation kinetics has come to a steady state from a transient stage, and to accurately determine the growth rate constant. Both a fundamental analysis and an experimental validation of this methodology are presented. This kinetics analysis method was further extended to quantify the scale evolution during the initial oxidation stage. Specifically, a method to track the extent and kinetics of the metastable to stable Al2O3 phase transformation in thermally formed alumina scales was established