The effects of alloying additions on the microstructure and corrosion properties of an Al-Zn galvanizing alloy

The effects of minor additions on the micro structure and mechanical and corrosion behaviour of ZINCALUME (Zn-55%Al-1.6%Si) coatings are reported. Magnesium (0.1-2.5%), calcium (0.1-0.5%), gallium (0.05-1.0%) and copper (0.05-1.0%) additions were made singly to the ZINCALUME alloy and coatings with controlled mass were produced. Such coatings were used to analyse microstructural effects (using optical and scanning electron microscopy), mechanical properties (using microhardness measurement and visual estimation of the severity of coating cracking on bending deformation) and corrosion properties (using a humidifed salt spray test and outdoor exposure). It is found that magnesium additions segregate to the zinc-rich interdendritic regions, forming a divorced Zn-MgZn2 eutectic at approximately 0.5%Mg, and to the silicon-rich phase, forming large particles of Mg2Si at the highest addition level of 2.5%. Though magnesium additions are detrimental to coating ductility, magnesium is beneficial for corrosion resistance, resulting in significant improvement to salt spray resistance of unpainted ZINCALUME and edge undercutting of painted ZINCALUME.