Influence of microstructure of zinc-aluminium-magnesium alloy coated steel on the corrosion behavior in outdoor marine atmosphere

Model hot-dip Zn-Al-Mg coatings with various microstructures were produced in a galvanising simulator by changing the cooling rate. Full characterization of the coatings was performed to identify the chemical composition of the different phases, the size of the grains and their distribution. The corrosion properties of the model microstructures were investigated in outdoor marine atmosphere up to 2 years. Weight loss measurements and cross section observations revealed a tendency for fine microstructures enriched in eutectic phases to be the most corrosion resistant. This was connected to a lower size of cathodic areas at the surface (zinc dendrites), the preferential dissolution of Mg-rich phase (eutectic) and the formation of a stable layered double hydroxide on the surface. •Model line Zn-2.7Al-1.5Mg coatings with various microstructures were produced.•Zinc dendrites size decreased with increasing the cooling rate.•Eutectic phases enriched for coating produced with a cooling rate of 30 °C/s.•Best corrosion resistance of fine microstructures enriched in eutectic phases.•Formation of a stable layered double hydroxide on the surface.