Stress and oxidation of nickel according to its purity

Abstract

The effect of impurities on the oxidation mechanism of nickel and on mechanical characteristics of the NiO scale was studied on two industrial grades and one pure nickel.The oxidation mechanism at 800°C was clarified using kinetics approach, microstructure observations, EDX and XPS analyses, profilometry, oxygen isotopic exchange and SIMS.The mechanical characteristics were determined mainly by three point bending tests performed in a scanning microscope.Whatever the nickel grade, the oxide toughness varies with the scale thickness and tends towards the value of massive NiO. The main difference related to Ni purity consists in the fact that spalling occurs at an oxide/oxide interface for the industrial grades, while it appears at the metal/oxide interface for pure Ni.Indeed, due to the presence of impurities, internal oxidation, extrusion of metallic nickel along grain boundaries of the substrate and formation of an inner equiaxed oxide film are observed in industrial grades. This induces mechanical keying of the oxide, and therefore crack propagation in a mixed mode is easier at the oxide/oxide interface.With pure nickel, only a single oxide film is formed by outward diffusion of Ni and there is no internal oxidation. Thus crack propagation occurs along the metal/oxide interface.