Characterization of the Surface Film Formed on Molten AZ91D Magnesium Alloy in Atmospheres Containing SO₂

The surface film formed on molten AZ91D magnesium alloy in an atmosphere containing SO₂ was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The surface film primarily contained MgO and MgS and had a network structure. MgS increased the Pilling–Bedworth ratio of the film and enhanced its protective capability. The films with a few pores at the surface consisted of two layers with an outer MgO layer and an inner layer of MgO and MgS. The film without pores at the surface also contained MgS and small amounts of MgSO₄ in the outer layer. Increasing the SO₂ content in the atmosphere promoted film growth and the formation of the protective film was prevented with the increased temperature.