Chemistry of corrosion products on Zn–Al–Mg alloy coated steel

Zn–Al–Mg alloy (ZM) coating provides a decisively enhanced corrosion resistance in a salt spray test according to DIN EN ISO 9227 (NSS) compared to conventional hot-dip galvanised zinc (Z) coating because of its ability to form a very stable, well adherent protecting layer of zinc aluminium carbonate hydroxide, Zn6Al2(CO3)(OH)16·4H2O on the steel substrate. This protecting layer is the main reason for the enhanced corrosion resistance of the ZM coating. Surface corrosion products on ZM coated steel consist mainly of Zn5(OH)6(CO3)2, ZnCO3 and Zn(OH)2 with additions of Zn5(OH)8Cl2 · H2O and a carbonate-containing magnesium species.