Wetting of liquid Zinc-aluminum-magnesium alloy on steel substrate during hot-dipping: Understanding the role of the flux

The surface quality of hot-dipping Zn-Al-Mg coatings was significantly influenced by the composition of fluxes. In this study, five different fluxes were chosen and named as F1 (ZnCl 2 , NH 4 Cl), F2 (ZnCl 2 , NaF), F3 (ZnCl 2 , NH 4 Cl, KCl), F4 (ZnCl 2 , NH 4 Cl, KCl, SnCl 2 , HCl), and F5 (ZnCl 2 , NH 4 Cl, KCl, BiCl 3 , HCl). Using the sessile drop method, the influence of different fluxes on the wettability between the liquid Zn-Al-Mg alloy and the steel substrate was elucidated. The results showed that: when the flux composition is ZnCl 2 -NH 4 Cl-KCl-BiCl 3 -HCl, a uniform and dense salt film with a protective effect can be formed on the steel substrate, which prevents the oxidation of the steel substrate and removes the harmful reaction products during hot-dipping. By reducing the solid–liquid interface energy, increasing the work of adhesion between the liquid Zn-6Al-3Mg alloy and steel substrate, and shortening the interface reaction time, the strongest wetting effect between the liquid Zn-6Al-3Mg alloy and the steel substrate was achieved. The coating surface quality was the highest after using the F5 flux. Finally, the mechanism of the assistant plating is discussed.