Impact of cooling rates on the microstructure and cracking susceptibility of hot-dip galvanized Zn-12 wt% Al-5 wt% Mg coatings

Abstract

Zinc-Aluminum-Magnesium (ZnAlMg) alloy coatings, applied through the hot-dip galvanizing process, are renowned for their superior corrosion resistance. This study investigated how different cooling rates affect the microstructure and cracking tendency of Zn-12Al-5Mg (in wt%) coatings. It was found that faster cooling rates lead to a more refined microstructure with primary MgZn2 grains but also increase the likelihood of cracking. This is primarily attributed to the finer distribution of primary MgZn2 grains and ternary eutectic (Zn/MgZn2/Al) regions, which allows micro-cracks originating from hard and brittle MgZn2 grains to easily propagate through the surrounding eutectic regions and develop into macro-cracks. Notably, we observed an epitaxial relationship between the primary and binary eutectic MgZn2 phases, shedding light on their crystallographic orientation and morphology. Additionally, our investigation revealed that cracking susceptibility depends on the crystallographic orientation of MgZn2 grains relative to the tensile axis. Fiber-like MgZn2 grains tend to crack along their basal planes, while hexagonal grains crack along other crystallographic planes. This insight into cracking behavior is crucial for designing ZnAlMg coatings with enhanced corrosion resistance and mechanical integrity. Overall, while faster cooling rates refine the microstructure, they can also increase the susceptibility to cracking in ZnAlMg coatings with Mg contents exceeding 3 wt%, emphasizing the need for a balanced approach in optimizing coating properties for different applications.