Oxidation of molten AZ91D Mg alloy in hexafluoropropylene/air atmospheres

Highlights: • 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.