The melt protection mechanism of an SO2/CO2 gas mixture for a magnesium-rare-earth based alloy

The microstructure, chemical composition and phase constitution of the surface film formed on molten Mg-2.82Nd-0.45Zn-0.32Zr alloy in an open melting furnace under a protective gas mixture of 3% SO2 and 97% CO2 at temperatures 983-1053 K were investigated, using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) techniques. Based on the test results and thermodynamic calculations, the protection mechanism was presented in this paper. The gas mixture provided an effective protection for the Mg alloy melt through forming a continuous and dense surface film of MgO, MgSO4, Nd2O3 and ZrO2. While the formation of surface Nd2O3 and MgSO4 film provided protection to the melt at low temperatures (about 983 K), and the surface C and ZrO2 coupled with inner enrichment of Nd improved the protective effect of the film at high temperatures (around 1053 K).