The early stages of high temperature oxidation of an Al-0.5wt% Mg alloy

Abstract

The high temperature oxidation of an Al-0.5 wt% Mg alloy in air at 723 and 823 K is investigated using atomic force microscopy, secondary ion mass spectrometry and transmission electron microscopy, with attention directed to the degradation and breakdown of the thin, protective amorphous Al 2O 3 layer covering the initial alloy surface. Following the commencement of oxidation, the amorphous Al 2O 3 layer thickens slightly. However, migration of Mg 2+ ions through the amorphous layer leads subsequently to formation of an outer layer of fine MgO crystals, accompanied by thinning of the Al 2O 3 layer and eventual complete transformation to less protective spinel MgAl 2O 4. The degradation of the Al 2O 3 layer is caused by either reaction between the layer and Mg in the alloy, with the formation of randomly oriented MgAl 2O 4 crystals of several nanometres size, or solid-state reaction between the inner and outer layers. Extensive local attack occurs upon breakdown of the oxide at sites where the cellular boundaries of faster Mg diffusion in the alloy intersect the surface, resulting in the formation of V-shaped cavities, filled with fine, cubic crystals of MgO, extending deep into the alloy along the cellular boundaries.