Effect of Cooling Rates on Microstructure and Cracking Resistance of Hot-Dipped Zn-12 wt% Al-5 wt% Mg Coatings

摘要

Conventional zinc hot -dip coating utilizes sacri ficial protection by preferentially corroding zinc ions over iron due to its higher ionization tendency, thereby acting as a sacrificial barrier to prevent the corrosion of iron. Research on ternary Zn-Al-Mg alloys is actively being conducted to enhance c orrosion resistance by incorporating Mg and Al to promote the formation of highly stable and dense corrosion products such as simonkolleite (Zn5(OH)8Cl2·H2O). This study delved into how different cooling rates affect the microstructure and cracking tendency of Zn-12Al- 5Mg (in wt%) coatings. In the case of Zn -12Al-5Mg (wt%, PosMAC Super) developed by POSCO, despite its excellent corrosion resistance, it was found that faster cooling rates lead to a more refined microstructure but also increase the likeliho od of cracking, mainly due to the presence of the hard and brittle MgZn 2 phase as primary phase particles. Interestingly, we observed an epitaxial relationship between the primary and binary eutectic MgZn 2 phases, shedding light on their crystallographic o rientation 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 Zn -Al-Mg alloy coatings with enhanced corrosion resistance and mechanical integrity. Overall, while faster cooling rates refine the mi crostructure, they also increase cracking susceptibility, emphasizing the need for a balanced approach in optimizing coating properties for different applications.