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.
More recently the use of alternative cover gases with lower GWPs has been developed, leading to use restrictions in the EU for SF6 first implemented under the F-Gas Regulation 842/2006. Now, through Regulation 517/2014, the use of sulphur hexafluoride in magnesium die-casting and in the recycling of magnesium die-casting alloys is totally prohibited from 1 January 2018.
For further information see Magnesium smelting: bans on use of sulphur hexafluoride.
As well as SO2 and dilute SO2 mixtures, HFC-134a is being used as a cover gas. A study has shown that in die casting, on average 79% of the HFC-134a is destroyed over the melt during processing, reducing emissions of HFC-134a, and further reducing the environmental impact when compared to SF6. For further information see the Öko-Recherche 2009 report for the European Commission.