THE EARLY STAGES OF HIGH-TEMPERATURE OXIDATION OF AN AL-0.5WT-PERCENT-MG ALLOY

The high temperature oxidation of an Al-0.5wt% Mg alloy in air at 723 and 823 K is investigated using atomic force microscopy, secondary ion mass spectrometry and transmission electron microscopy, with attentio n directed to the degradation and breakdown of the thin, protective am orphous Al2O3 layer covering the initial alloy surface. Following the commencement of oxidation, the amorphous Al2O3 layer thickens slightly . However, migration of Mg2+ ions through the amorphous layer leads su bsequently to formation of an outer layer of fine MgO crystals, accomp anied by thinning of the Al2O3 layer and eventual complete transformat ion to less protective spinel MgAl2O4. The degradation of the Al2O3 la yer is caused by either reaction between the layer and Mg in the alloy , with the formation of randomly oriented MgAl2O4 crystals of several nanometres size, or solid-state reaction between the inner and outer l ayers. Extensive local attack occurs upon breakdown of the oxide at si tes 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 all oy along the cellular boundaries. (C) 1998 Published by Elsevier Scien ce Ltd. All rights reserved.