## 1 Introduction  
Hot-dip galvanizing is a coating technology used to protect steel from corrosion by forming stable corrosion products on the zinc surface with sacrificial protection owing to the lower electrochemical potential of the latter [1; 2; 3]. Owing to their advantages, galvanized products are applicable in automobile exterior and interior panels, home appliances, highway crash barriers, buildings, ships, solar cell frames, coil coatings, and other areas [4; 5]. The hot-dip coating process [6] can be classified as a batch-type galvanizing process (BGP) and continuous galvanizing process (CGP) according to the manufactured products. In CGP, a steel coil/wire is continuously immersed in a galvanizing bath at a high speed, and the final products are formed after coating. In contrast, BGP is used to coat steel materials that have been processed and welded in their final form before hot dipping. As a result, BGP has the advantage of coating thick plates in small batches at a low initial cost.  
In both processes, surface pretreatment is needed for the steel substrate. In particular, a heat-treatment reduction method is used in CGP, whereas the sample is flux-treated to protect it from oxidation in BGP. Dipping and drying the steel substrate in an aqueous chloride solution promotes the development of the flux layer. After immersion in a molten zinc bath, the protective flux layer reduces the surface tension between the bare steel and melt, thereby improving the wettability of zinc [6]. The most widely used flux for hot-dip galvanization is an aqueous solution of a salt mixture containing ammonium chloride (NH\({}_{4}\)Cl) and zinc chloride (ZnCl\({}_{2}\)) [7; 8].  
Recently, the demand for highly corrosion-resistant coatings to counter harsh environments has increased, thereby rapidly promoting research on alloying zinc coatings [9; 10; 11].  
In particular, Zn-Mg-Al ternary alloy coatings, which have excellent corrosion resistance, have been commercialized, and their industrial demand has rapidly expanded. As existing commercialized Zn-Mg-Al alloy coatings are based on CGP, BGP-based galvanizing technology should be developed for the diversification of products. To produce a Zn-Mg-Al alloy coating by BGP, a flux of optimal composition should be developed to address the coating defect of conventional flux (NH\({}_{4}\)Cl:ZnCl\({}_{2}\)). In particular, conventional flux reacts with the bath, which affects the wettability by deteriorating the molten metal/substrate interface owing to the formation of aluminum chloride, thereby forming bare spots [12; 13]. In addition, the release of NH\({}_{3}\) and H\({}_{2}\) gases by the decomposition of NH\({}_{4}\)Cl by Al adversely affects the coating performance [14; 15]. To prevent the formation of AlCl\({}_{3}\) compounds in Al alloy coatings, rare earth metal (Li, Na, K) and alkaline earth metal (Be, Ca, Sr) chlorides are introduced to form stable metal chlorides [16]. Additional studies to improve the properties of Zn-Al and Zn-Al-Mg alloy coatings have introduced various metal (Co, Pb, Ni, K, Bi, and Sn) chlorides to the flux [12; 17; 18; 19; 20]. In this study, SnCl\({}_{2}\) was added to conventional flux to reduce the formation of aluminum chloride by inducing a reaction between the flux and Sn, and the effect of SnCl\({}_{2}\) addition on the coating properties of hot-dip Zn-Mg-Al-alloy-coated steel sheets was investigated. The flux composition was optimized by comparative evaluation of the microstructure, surface quality, adhesion, corrosion resistance, and thermal properties of the Zn-Mg-Al alloy coating.