To investigate possible reactions between differently coated carbon-bonded alumina filters and an AZ91 magnesium alloy melt, immersion tests at a melt temperature of 680 °C under SF6 cover gas were carried out. These experiments are to be the first step in evaluating the applicability of carbon-bonded alumina filters for the filtration of magnesium alloy melts, with the goal of lowering the inclusion content of primary as well as recycled magnesium. Uncoated as well as Al2O3-, MgAl2O4-, nano- (CNT/alumina nano sheets) and MgO-C-coated filters were tested.
Thermodynamic calculations showed that only magnesia (MgO) and carbon are stable against liquid magnesium; alumina and spinel will be reduced under the formation of magnesia. The results of the thermodynamic calculations were confirmed by the experiments. The MgO-C-coated filter was the only one that did not show an in situ-formed layer on its surface after being in contact with the magnesium alloy melt. The alumina- or spinel-containing filter surfaces displayed platelet-like in situ layers after their contact with the molten AZ91. Magnesium and oxygen were detected in the in situ layers by means of EDX analysis, suggesting that they consist of MgO, as predicted by the thermodynamic calculation. In future filtration experiments, the finely structured in situ-formed layers may prove useful for the attraction and removal of impurities, such as magnesium oxide particles, from magnesium melts.