Molten magnesium exposed to an ambient air atmosphere will oxidise rapidly, resulting in burning on the metal surface. If magnesium is to be used as a casting metal, the melt must be protected from this severe oxidation.
The current method of magnesium melt protection utilises a cover gas mixture that forms a thin, coherent and stable film on the melt surface. The cover gas usually contains air and/or CO2, combined with a small amount of an inhibiting agent. Sulphur hexafluoride (SF6) has proved to be a successful inhibitor. It is non-toxic, non-corrosive and provides effective protection of molten magnesium at concentrations as low as 0.1%. However, the increasing cost of SF6 and concern regarding the contribution of SFe to the Green House Effect has resulted in the need for a more detailed examination of these cover gas techniques.
Despite commercial acceptance of this method, the action of SF6-based cover gases is not well understood, and the composition and structure of the film formed on magnesium melts has not been fully characterised. This research project was initiated with the major aim of gaining an understanding of the fundamental mechanisms by which SF6 gas mixtures protect molten magnesium from oxidation.
Samples of the protective film formed on molten magnesium were produced under cover gas mixtures of air containing 0.1%, 0.3% and 1% SF6, for exposure times of 1, 10 and 100 minutes. Specialised surface analysis techniques determined that the film is composed of MgO and MgF2. No chemical species containing sulphur were detected in the film. These observations are consistent with thermodynamic modelling of O2 (g) / SF6 (g) / Mg (1) interactions using the FACT software package. The concentration of fluorine in the reaction film increases with the level of SF6 in the cover gas mixture. The increase in the film thickness follows a parabolic growth rate. The topography of the film is representative of a dense fibrous network.
A mechanism of film formation is proposed, where the principal reaction is between molten magnesium and oxygen, forming MgO solid. The SF6 decomposes to yield SF4 and fluorine. The released fluorine combines with magnesium to give solid MgF2. In the presence of moisture, gaseous HF and SO2 are also formed.
Experimental trials indicate that, in an SF6 environment, molten magnesium will wet magnesium oxide. This observation led to a proposed mechanism of protection, whereby the oxide particles on the molten metal surface are drawn together by capillary forces. The net effect is a dense and coherent film which greatly reduces the vaporisation of magnesium. The presence of magnesium liquid between the film particles allows the film to remain flexible with movement of the molten metal surface.