Effects of Oxygen Pressure on the Oxidation Kinetics of a Ni-20Cr Alloy at High Temperatures

Abstract

The oxidation of a Ni-20Cr alloy was studied at temperatures between 1273 and 1423K under various oxygen partial pressures by mass gain measurements, microstructure observations with optical and scanning electron microscopies, X-ray diffraction and electron probe microanalysis. Oxygen pressures were obtained using air and CO-CO₂ mixtures with various ratios, ranging from 2.1×10⁴Pa to the pressures below the dissociation pressure of NiO. Above the dissociation pressure of NiO, the oxidation rate was affected by oxygen pressure in the early stage. The amount of NiO initially formed on the alloy decreased with lowering oxygen pressure. In the later stage, the growth of the oxide scale obeyed the parabolic rate law, and optical micrographs showed the formation of the compact oxide layers. These two results suggested that the oxidation was controlled by the diffusion of Cr ions through the Cr₂O₃ scale. The parabolic rate constants were independent of oxygen pressure, which was explained by the n-type or intrinsic defect structure of Cr₂O₃.