Effect of antimony additions on the microstructure and performance of Zn–Mg–Al alloy coatings

Microscopy, electrochemical techniques and mechanical testing are used to investigate the effect of varying antimony additions (0.45–1.8 wt%) on the microstructure and corrosion properties of zinc-magnesium-aluminium coating alloys. Samples were produced by splat casting to produce high cooling rates similar to those seen in a continuous galvanising line. X-Ray Microscopy reveals that the Sb additions produce disk-shaped Mg3Sb2 intermetallics, subsequently reducing or eliminating the MgZn2 eutectic. Electrochemical testing in 1 wt% NaCl shows that the Mg3Sb2 phase is cathodic with respect to the bulk alloy with slower oxygen reduction kinetics. The decrease in eutectic content leads to less intense anodic activity. The combined effect is anodic and cathodic deactivation, which leads to a 43% reduction in corrosion rate as measured through LPR compared to the base alloy. This work shows that quaternary additions to ZMA coating alloys can be a potential route to improved corrosion resistance for galvanic protection.