A quantitative demonstration of the grain boundary diffusion mechanism for the oxidation of metals

Below 10000C the oxidation of nickel cannot be controlled by the diffusion of ions through the bulk crystal lattice of the pure oxide, because the measured oxidation rates are several orders of magnitude faster than would be predicted on this basis. Short-circuit diffusion through oxide grain boundaries or dislocations has usually been held responsible, but there has hitherto been no proper quantitative confirmation of this mechanism. Measurements are reported of the oxide scale thickness and oxide grain size as a function of time during the oxidation of high-purity nickel in the temperature range 500 to 8000C. All the oxidation experiments were carried out in pure oxygen at a pressure of one atmosphere. The measured parabolic oxidation rate constants have been compared with those calculated from grain boundary diffusion data obtained in previous work, using a grain boundary diffusion model for the oxidation process. The quantitative agreement between measured and calculated oxidation rates shows convincingly that the diffusion of nickel along oxide grain boundaries controls the oxidation of nickel in these experiments. Oxidation data in the literature can also be accounted for with this model. (author)