Microstructural heterogeneity and localized corrosion in magnesium alloy welded joint

Magnesium (Mg) alloy welded joints are highly susceptible to localized corrosion, which severely restricts their application in lightweight structural and multi-material systems. This susceptibility mainly arises from welding-induced microstructural heterogeneity. Variations in grain structure, intermetallic compounds, secondary phases, crystallographic texture, and welding defects redistribute local electrochemical activity and promote corrosion degradation. This review examines the role of heterogeneous weld microstructures in governing site-specific degradation initiation and propagation in both homogeneous and dissimilar Mg alloy joints. Particular attention is given to Mg/Al, Mg/steel, and Mg/Ti systems. In these joints, galvanic coupling, interfacial reactions, and defect-assisted occluded environments accelerate pitting corrosion, intergranular corrosion, and stress corrosion cracking. A unified microstructure–electrochemistry framework is proposed to interpret corrosion evolution across multiple length scales. Recent advances in corrosion mitigation are also critically reviewed, including alloy and interface design, welding process optimization, surface engineering, and post-weld treatments. This review provides an integrated mechanistic understanding of localized corrosion in Mg alloy welded joints and offers guidance for the design of corrosion-resistant lightweight joining systems.