Corrosion and Process Analysis of a Preoxidized MgO Recyclate‐Based Cermet Anode in Laboratory‐Scale Na‐Cryolite Molten Salt Electrolysis of Aluminum at 1000°C

The development of inert anodes for aluminum electrolysis remains challenging due to the high corrosivity of cryolite‐based melts at 950°C–1000°C. This study investigates the corrosion and process behavior of a carbon‐free MgO–steel cermet anode derived from refractory recycling during laboratory‐scale Na‐cryolite electrolysis at 1000°C, focusing on the effect of a pre‐oxidation treatment on its corrosion behavior. In the as‐sintered state, MgO grains are mechanically bonded to a 316L steel matrix with interfacial porosity that limits corrosion resistance. Pre‐oxidation at 900°C for 24 h forms a dense Mg–M–O (MFe, Cr) oxide‐rich surface layer, sealing pores and improving resistance to cryolite infiltration, albeit at the cost of reduced electrical conductivity, which translated into an overall higher cell voltage compared to the as‐sintered state. During electrolysis, this layer degrades under anodic polarization, O ₂ evolution, and fluorination. The surface transforms into a Fe–O‐rich magnetite layer (Fe ₃ O ₄ ), while a Cr‐containing oxide and an in situ Mg–Fe–O transition zone form beneath, maintaining partial protection. Overall, pre‐oxidation effectively delays electrolyte penetration by converting MgO into new oxide phases but introduces conductivity losses and surface instability. The results highlight both the potential and limitations of MgO‐based cermet anodes for sustainable aluminum production.