Microstructure–corrosion–performance relationships in as-cast Mg–xAl–0.25Mn (x = 2–9 wt. %) alloys: effect of Al content on primary Mg–air battery anodes

Aqueous Mg–air batteries provide high theoretical energy density, but severe anode self-corrosion and unstable surface films in chloride electrolytes hinder their performance. This study investigates as-cast Mg–xAl–0.25Mn alloys (x=2, 5, 7, and 9 wt.%, i.e., AM20 toAM90) with pure Mg as a reference, focusing on the role of Al-controlled β-Mg₁₇Al₁₂ networks on microstructure, corrosion, and discharge behavior. Increasing Al refines grains, raises β-phase fraction (~2→~21 vol.%), and transforms its morphology from isolated precipitates to near-continuous networks with local agglomeration. Immersion, potentiodynamic polarization, and time-dependent EIS in 3.5 wt.% NaCl reveal an optimal β-connectivity: intermediate β levels in AM50 stabilize surface films and suppress self-corrosion, whereas highly connected β networks in AM90 destabilize the product layer and intensify micro-galvanic attack. X-ray diffraction pattern shows AM70 has the weakest basal texture, promoting sustained anodic dissolution. Half-cell discharge at 2–60 mA cm⁻² demonstrates that AM70 achieves the best high-rate performance, with 1.22 V mean voltage, ~1754 mW h g⁻¹ specific energy, and ~63.7% anode utilization at 60 mA cm⁻². These results highlight that controlling β-network connectivity and texture via Al alloying effectively balances corrosion resistance and electrochemical activity in as-cast Mg–air anodes.