Characterization of the Protective Surface Films Formed on Molten AZ91D Magnesium Alloy in SO2/Air Atmospheres in a Sealed Furnace

Surface films that formed on molten AZ91D magnesium alloy in SO2/air cover gases at 680 °C in a sealed furnace were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Auger electron spectroscopy. It is revealed that the film formed on molten AZ91D alloy surface in cover gas with high air content can prevent the molten AZ91D alloy from oxidation and ignition. The surface film contained three elements, namely magnesium, oxygen and sulfur, and was mainly composed of MgO and MgS. The properties of the film depended on air content in the cover gas and holding time. Thermodynamic calculation showed that MgSO4 was the stable phase, and it was concluded that the formation of MgSO4 was important for the formation of the protective surface film in SO2/air atmospheres.