Characterisation of protective surface films formed on molten magnesium protected by air/SF6 atmospheres

Abstract

Magnesium metal in its molten state will oxidise rapidly in air, leading to burning on the melt surface. In order to process molten magnesium safely, the melt is protected from oxidation by blanketing the surface with a cover gas. The cover gas reacts with the metal and forms a protective film on the melt surface. Sulphur hexafluoride (SF₆) is widely used by the magnesium industry as a component of cover gas mixtures. However, the use of SF₆ has come under increased scrutiny due to its high cost and, more importantly, its significant impact on the greenhouse effect. To optimise the use of SF₆, a detailed knowledge of its role in protecting molten magnesium is required. This paper describes the characterisation of the surface films formed on molten magnesium protected by cover gas mixtures containing SF₆. Sensitive surface analysis techniques have been used to quantitatively determine the chemical attributes of the films. XPS showed that the film contains only MgO and MgF₂. No sulphur compounds were detected. AES depth profiling indicated film thickness to be in the range 0.1–1 μm.