Oxidation of molten AZ91D Mg alloy in hexafluoropropylene/air atmospheres

•The oxidation kinetics of molten AZ91D Mg alloy obeyed a parabolic rate law under most conditions, while in some cases it followed a linear rate law.•The film formed on the surface of molten AZ91D Mg alloy mainly consisted of MgF2, MgO, C and a small amount of AlF3, and MgF2 was a predominant product.•The thickness of the film changed with the concentration of HFP and temperature in the range of about 1–3 μm.•The apparent activation energy was 82.78 kJ mo1−1 for the reaction of molten AZ91D Mg alloy in 0.1 % HFP/air atmosphere at 670–800 °C. The oxidation behavior of molten AZ91D Mg alloy in the mixture of 0.05–5% hexafluoropropylene and air at 670–800 °C was investigated by TG, XRD, SEM, EDS and XPS. Results show that the oxidation of the melt in 0.05 % hexafluoropropylene/air atmosphere at 700 °C obeyed a linear rate rule, while under other conditions it followed a parabolic rate law. The apparent activation energy was 82.78 kJ mo1−1 for the reaction of the melt in 0.1 % hexafluoropropylene/air atmosphere at 670–800 °C. The film formed on the melt surface mainly consisted of MgF2, MgO, C and small amounts of AlF3.