Corrosion behaviour of Zn–Al–Mg coated steel sheet in sodium chloride-containing environment

Conventional hot-dip galvanised zinc coated (Z) and novel hot-dip galvanised Zn–Al–Mg alloy coated (ZM) steel sheet samples with a coating thickness of 7 μm each were exposed to standardised salt spray test and cross-sections of the corrosion samples were analysed by using SEM and EDS. On Z corrosion proceeds very fast and the steel substrate is attacked even after 100 h of exposure. ZM samples showed a different behaviour. The entire metallic ZM coating is converted into a stable, adherent aluminium-rich oxide layer, which protects the steel substrate against corrosive attacks. This layer is the main reason for the enhanced corrosion resistance of the ZM coating in sodium chloride-containing environment.