A comprehensive review on microstructural evolution and corrosion behaviour of hot-dip Zn-Al-Mg coatings on Fe-based alloys

This review presents a systematic analysis of how alloying elements control microstructural phase evolution and corrosion mechanisms in hot-dip Zn-Al-Mg coatings on Fe-based alloys. This review examines how Al and Mg additions suppress thick Fe-Zn intermetallic growth and promote formation of thin Fe₂Al₅-based inhibition layer, governing interfacial stability, coating adhesion, and wettability. Increasing Al and Mg content to the Zn bath progressively transform coating microstructures from Zn-rich matrices to complex multiphase assemblages containing Al-rich dendrites, MgZn₂ intermetallics, and Zn-Al-MgZn₂ ternary eutectics. These phase distributions critically determine corrosion behaviour: conventional galvanized (GI) coatings rely on porous Zn(OH)₂ and basic Zn carbonate films providing limited long-term protection, whereas Zn-Al-Mg (ZAM) coatings develop dense, adherent layered double hydroxide (LDH), simonkolleite, and hydrozincite that supress ionic transport and provide superior corrosion resistance, particularly in chloride and atmospheric environments. The synergistic roles of Al (promoting LDH formation and Al₂O₃ passivation) and Mg (enabling preferential dissolution and self-healing corrosion products) are discussed in relation to phase-selective electrochemical mechanisms and environmental exposure. This review identifies critical Al/Mg ratio-phase assemblage-corrosion performance relationships and highlights LDH nucleation kinetics and cut-edge protection mechanisms.