The oxidation behaviour of an AZ91D magnesium alloy at high temperatures

As-cast AZ91D magnesium alloy was exposed to air in the temperature range from 470 to 800 K for time intervals up to 10 h. Thermogravimetric measurements revealed three distinct stages of the reaction where an initial formation of protective oxide was followed by an incubation period with a subsequent transient to non-protective oxidation, at a rate either constant or sharply increasing over time. The approximate temperature and time frames for an onset of each stage were identified. A strong link was found between the oxidation kinetics and the scale morphology. The initial protective film, grown anisotropically over the microstructural features of the substrate, was transformed to oxide ridges. The non-protective oxidation was associated with a formation of oxide nodules and their further coalescence into a fine-grained scale of a loose structure. All the morphologies were comprised of a randomly oriented magnesium oxide MgO with traces of MgAl₂O₄ spinel. The oxidation mechanism represented a complex reaction where morphological and phase transformations within the alloy substrate were accompanied by magnesium sublimation/evaporation and subsequent condensation within the scale pores or cracks, and superimposed on the reaction with oxygen. Some implications for the high temperature processing of magnesium alloys are discussed.