Abstract
The relationship between alloy chromium content and oxidation rate has been determined for nickel-chromium alloys containing up to 17At% chromium. The tests were conducted at 1096C, under one atmosphere of oxygen. The observed structural and phase changes in the oxide were related to the oxidation rate. It has been found that increasing amounts of chromium in the NiO phase govern the oxidation rate for alloys containing less than 2At% chromium. For alloys of greater than 2At% chromium, a complex exchange of chromium among three phases--NiO, Cr2O3 and the spinel, NiCr2O4--has been detected. This chromium exchange was reflected in the lattice parameter of the NiO phase, and in the rate of increase of oxidation rate with chromium addition to the alloy. The present work indicates that an interstitial diffusion mechanism may become dominant in the oxides of high chromium content. Within the scope of the present work, the high oxidation stability of nickel-chromium alloys has been correlated with the presence of Cr2O3 in the oxide layer. Oxidation rates for high purity nickel have been determined at temperatures of 980, 1096, and 1260C, under one atmosphere of oxygen. The present data, combined with lower temperature data obtained by Gulbransen and Andrew, yielded an activation energy of 45.1Kcal/mole for the oxidation of nickel.
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