Improving the Coatability of Zn–Mg–Al Alloy on Steel Substrate by the Surface Pretreatment of SnCl2-Added Zinc Ammonium Chloride

Abstract

The applicability of galvanized products in various industries has increased the demand for highly corrosion-resistant coatings to counter harsh environments. Among these, Zn–Mg–Al ternary alloy coatings have excellent corrosion resistance, resulting in their commercialization and industrial demand. To increase the diversification of their products, the ideal flux composition in these coatings should be optimized. In this study, we investigated the effects of conventional flux (ZnCl2:NH4Cl) in the hot-dip galvanization of Zn–Mg–Al ternary alloy coatings. Additionally, we developed a new flux to improve the coating properties of Zn–Mg–Al ternary alloy coatings on steel sheets. During hot dipping, SnCl2 on the steel substrate decomposed faster than conventional flux, thereby eliminating the AlCl3 residues in the coating and surface defects. The thermogravimetric-differential thermal analysis studies unveiled the mechanisms for improved coatings. The thermodynamic calculations confirmed the spontaneous substitution owing to the presence of SnCl2 in the flux. Therefore, the developed and optimized flux enhanced the adhesion of the alloy coating.