Sulphur hexafluoride (SF6) is used in metal casting processes in magnesium foundries as well as in aluminium foundries. The function of SF6 in both processes is different:
Magnesium is a highly reactive metal. As temperatures of up to 800 °C can occur during the casting process, the surface of the melt has therefore to be protected against ignition, oxidation and the formation of nitrides. In the early years of industrial magnesium production only salt fluxes or powdered sulphur were known as protective cover for this purpose. In order to prevent contamination of the cast products, the salt fluxes were replaced by protective gas mixtures. Sulphur dioxide (SO2) was the first cover gas used in magnesium foundries to protect the molten metal, and plays an important role still today. Due to its toxic and corrosive properties (especially humid SO2 enhances corrosion of steel equipment) it was replaced by SF6 in most foundries.
Porosity caused by the hydrogen content of the aluminium melt is a fundamental problem in the production of aluminium cast components, as it leads to a reduction in strength. Such an effect can only be avoided by pre-treating the aluminium melt. During this pre-treatment also oxides and solid impurities should be removed. This can be achieved by injecting chlorine (pure or in mixture with inert gas) or an SF6/inert gas mixture.