Upcycled MgO–C recyclates as carbon-free anodes for primary aluminum production: effect of pre-oxidation treatments on corrosion resistance to cryolitic melts evaluated by hot-stage microscopy at 1000°C

The effect of thermal pre-oxidation on the microstructure and molten-salt interaction of MgO–316L composite anodes produced from decarbonized MgO–C refractory recyclate was investigated. Composite anodes containing 40 vol.% recycled MgO and 60 vol.% AISI 316L steel were fabricated by cold isostatic pressing and pre-oxidized in flowing air at 800, 900, or 1000°C for 24 h. The formation of Mg-containing spinel phases predicted by thermodynamic calculations was confirmed by Raman spectroscopy and SEM–EDX analysis. Pre-oxidation converted the initially physical MgO–steel contact into chemically bonded oxide interfaces and reduced molten-salt infiltration. The most pronounced reduction in Al-bearing salt penetration was observed for PO900, where infiltration was largely confined to a thin near-surface region due to its refined pore structure. Increasing the pre-oxidation temperature promoted outward oxide-scale growth while decreasing the depth of inward oxidation. Although Mg-containing spinels remained susceptible to interaction with the molten salt, pre-oxidation could substantially improve corrosion resistance compared with the as-sintered condition.