The protective surface film formed on molten ZK60 magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The composition, microstructure and growth kinetics of the surface film formed on molten ZK60 magnesium alloy in 1,1,1,2-tetrafluoroethane/air mixture were investigated by using SEM, EDS, XRD, XPS and TG. The results show that the film formed in 0.01% 1,1,1,2-tetrafluoroethane/air was non-protective, and its phase composition changed with melt temperature. At 760 °C, the film was mainly composed of MgO and some MgF2 and ZrO2, whereas at 660 °C it consisted mainly of MgO and Mg3N2. The film formed in air containing 0.1% 1,1,1,2-tetrafluoroethane or higher was comprised of MgF2 and C with small amounts of MgO, and it was protective.

Research highlights
► The surface film formed in 0.01% 1,1,1,2-tetrafluoroethane/air was non-protective. At 760 °C, the film was mainly composed of MgO and some MgF2 and ZrO2, whereas at 660 °C it consisted mainly of MgO and Mg3N2
► The surface film formed in air containing 0.1% 1,1,1,2-tetrafluoroethane or higher was comprised of MgF2 and C with small amounts of MgO, and it was protective
► High 1,1,1,2-tetrafluoroethane concentration facilitated the formation of MgF2-rich protective film on the melt
► Alloying element Zr had little effect on the oxidation of molten ZK60 magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres