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

Abstract 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, Zn 6 Al 2 (CO 3 )(OH) 16 ·4H 2 O 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 Zn 5 (OH) 6 (CO 3 ) 2 , ZnCO 3 and Zn(OH) 2 with additions of Zn 5 (OH) 8 Cl 2  · H 2 O and a carbonate-containing magnesium species.