Characterisation of the surface films formed on molten magnesium in different protective atmospheres

Molten magnesium oxidises rapidly during casting and handling unless it is protected by an atmosphere that stabilises the surface. In this article, results from the analysis of magnesium melt surfaces exposed to SO2 and different fluorine-containing atmospheres are reported. The microstructure of the surface films, formed during controlled exposure in laboratory scale experiments, have been characterised using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). Both SO2 and the fluorine-containing gases were found to protect the melt from burning and vaporisation in oxidising atmospheres. The protected surfaces generally had a shiny metallic appearance, but turned dull grey after extended exposure to high concentrations of fluorine containing gases. All the surface films initially consisted of small crystallites of MgO forming a thin continuous film. This film was found to contain some sulphur when the melt was protected by SO2, while fluorine was the only element detected in the oxide when SF6 or other fluorine-containing gases were used for protection. With increasing exposure time, the films gradually grew thicker and the fluorine/oxygen-ratio of the films formed in fluorine-containing atmospheres increased. Finally, after long term exposure to fluorine containing atmospheres, the thermodynamically stable MgF2-phase was formed. In a N2 atmosphere, SO2 and SF6-additions did not protect the magnesium, indicating that a rapid initial formation of MgO is necessary to obtain protective films.