fig1

Figure 1. Schematic of the autonomous anodic protection mechanism. The Mg₃(Sb, Bi)₂ matrix, Mg₁₇Al₁₂ secondary phase, and the condensed water film collectively constitute a self-driven micro-galvanic cell, functioning as the cathode, sacrificial anode, and local electrolyte, respectively. Driven by the electrochemical potential gradient, the anodic Mg₁₇Al₁₂ undergoes preferential oxidation, releasing Mg²⁺ and Al³⁺ ions while concurrently transferring electrons to the cathodic matrix. This electron flux facilitates the reduction of water at the Mg₃(Sb, Bi)₂ surface into H₂ gas and OH^- ions. The resultant local supersaturation triggers the precipitation of dense Mg/Al-based oxides and hydroxides, which rapidly coalesce into a passivation film that seamlessly shields the Mg₃(Sb, Bi)₂ surface against further environmental corrosion.