A comparison of the effects of small additions of various second elements on the oxidation of nickel at 1200°C

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Abstract
The oxidation behaviour of Ni‐4.2% Mo, Ni‐4.0% W, Ni‐2.5% Al, Ni‐4.2% V and Ni‐5.0% Cr (all wt. %) at 1200 °C in flowing oxygen at 1 atm. pressure has been studied using various techniques. In particular, the solubility of the second element in NiO, its distribution across the NiO scale and the effects of these on the oxidation rates and scale morphologies have been examined. The oxidation rates of all the alloys are greater than that of nickel, although for Ni‐4.2% Mo, where incorporation of internal oxide into the scale does not occur and molybdenum does not dope the oxide, the small increase in weight gain during oxidation Compared with that for nickel is due to internal oxide formation only. As the internal oxide particles pileup at the alloy/oxide interface, they exert a blocking effect to outward diffusion of Ni2+ ions, especially in the later stages of oxidation. Ni‐4.0% W behaves similarly, although a few internal oxide particles are incorporated into the scale and a small amount of doping of the oxide ensures that it thickens at a slightly faster rate than the scale on nickel and for Ni‐4.2% Mo. The oxidation rates of the other alloys are significantly faster than that of nickel and increase in the order Ni‐2.5% Al, Ni‐4.2% V, Ni‐5.0% Cr. These increased rates are largely caused by increases in the total cation vacancy concentrations in the scales, although internal oxide formation can make a significant contribution to the oxidation kinetics. The influence on the oxidation behaviour of a number of factors, namely doping of the scale, internal oxidation, dissociation of NiO and transport of gaseous oxygen within the scale, blocking effects in the oxide and at the alloy/oxide interface, and grain growth of the oxide, are considered in detail.