12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Zinc coatings alloyed with low amount of aluminium and magnesium (Zn-Al-Mg) have been proved to provide better corrosion performance than conventional hot dip galvanized (HDG) steel in various atmospheres, which offers large applications in many industrial sectors such as in the automotive industry. Zn-Al-Mg coatings are characterized by a rather complex microstructure including a dendritic phase of zinc with small amount of aluminium, a binary eutectic MgZn2-Zn and a ternary eutectic phase MgZn2-Zn-Al. It is believed that the microstructure induced by the alloying might play a key role in the formation of the protective layers. Indeed, coarse microstructures shall be favorable to the formation of areas with extreme pH at anodic and cathodic sites, while a finer microstructure may indeed avoid such situations by averaging the surface pH, resulting in the creation of layers of improved protective quality. Yet, the relationship between microstructure and corrosion stability of Zn-Al-Mg coating is not fully demonstrated. In this study, model hot-dip Zn-Al-Mg coatings with various microstructures were produced in a galvanizing simulator by changing the cooling rate. The obtained coating variants were investigated in outdoor marine atmosphere and a number of laboratory tests with well controlled conditions. Upon testing conditions, 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 dentrites), the preferential dissolution of Mg-rich phase (eutectic) and the formation of a stable layer double hydroxide on the surface as detected by FTIR spectroscopy.