Effects of Oxygen Pressure on the Oxidation Behavior of Ni–20Cr Alloy

Abstract

The Ni–20Cr alloy was oxidized at 1373 K at various oxygen partial pressures, ranging from 10⁻⁷ to 10⁴ Pa, which were controlled by Ar–O₂ mixtures. A large difference was observed between the oxidation behavior in a high P_O2 (≥10² Pa) range and that in a low P_O2 (≤1 Pa) range. Detailed experiments were carried out in P_O2 of 10⁴ and 1 Pa, the former being typical in high P_O2 and the latter in low P_O2. The mass gain-time curves showed that the growth rate of scale in the low P_O2 lie between the parabolic and linear law, while that in the high P_O2 obeys the parabolic law, which indicates that the scale formed in the low P_O2 is less protective than that in the high P_O2. It was found, on the other hand, that the oxide scales formed in the low P_O2 were more adherent to the alloy than those formed in the high P_O2. In the high P_O2 a large amount of spalling was observed on cooling even in short time oxidation. The oxide scales formed in the low P_O2 were uniform in thickness and rather porous, while those formed in the high P_O2 were dense and non-uniform in thickness. In the low P_O2 a number of small voids were found at the oxide-alloy interface. In the high P_O2, however, large voids were found; this suggests the occurrence of plastic deformation of the oxide and the alloy by the stress induced in the growing scale. The above difference in oxidation behavior can be ascribed to the difference between growth mechanisms of the oxide scale in the high and low P_O2.