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

TLDR

Oxidation of nickel below 1000 °C proceeds at rates far exceeding bulk lattice diffusion predictions, suggesting a short‑circuit mechanism. The study aims to provide quantitative confirmation that grain‑boundary diffusion governs nickel oxidation. Oxidation of high‑purity nickel (500–800 °C) in pure O₂ was monitored by measuring oxide scale thickness and grain size over time, and the resulting parabolic rate constants were compared with values calculated from a grain‑boundary diffusion model. Measured and calculated oxidation rates agree closely, demonstrating that nickel diffusion along oxide grain boundaries controls the process and that the model also explains existing literature data.

Abstract

Abstract Below 1000°C 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. We report measurements 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 500–800°C. 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 our 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.