The use of SO2 as a cover gas for molten magnesium

The two major problems with using sulphur hexafluoride (SF6) as a cover gas component for protecting molten magnesium are its high cost and extremely high Global Warming Potential of approximately 24,000. Sulphur dioxide (SO2) is a viable alternative to SF6. In comparison, SO2 is relatively inexpensive and has a Global Warming Potential of zero. However, there has been some concern regarding the safety of SO2 cover gases, particularly when used in die casting melting furnaces. During the 1940s and 1950s, numerous incidents were reported in magnesium die casting operations, where sulphur domes were used to protect the magnesium die casting alloys. This "Sulphur Dome Effect" was attributed to the presence of SO2 above the melt, with the suggestion that the incidents were associated with formation of magnesium sulphate. To optimise the conditions for use of SO2, experiments were conducted to assess the performance of various SO2-based cover gas mixtures for ingot casting. Ingots of pure and alloyed magnesium were cast under cover gases of SO2 diluted with air, carbon dioxide, argon and nitrogen. A wide range of SO2 concentrations was examined, and the ingots were ranked on the basis of visual appearance, dress formation and mould-coat interaction. To determine the significance of the Sulphur Dome Effect, melts of magnesium alloys were exposed to cover gas mixtures of 0.5% SO2 + dry air, CO2 and N-2 at 660, 680 and 700 degrees C. The scale that deposited on the crucible wall was then scraped with a mild steel spatula, simulating the de-sludging procedure used for crucible cleaning. Samples of the melt surface film and the crucible scale were collected for chemical analyses by X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD). For both pure and alloyed magnesium, scraping of the crucible scale produced significant sparking and burning, with little difference noted when changing the diluent gas or melt temperature. Chemical analyses revealed the presence of MgO and MgSO4 in the surface film and the crucible scale. The highly unstable compound Al-2(SO4)(3) was also detected in the crucible scale. These findings support the currently proposed mechanism for the "Sulphur Dome Effect", of a highly exothermic reaction between molten magnesium and magnesium sulphate.