## 2 Materials and Methods  
A hot-dip Zn-Mg-Al-alloy-coated steel sheet was manufactured using a batch-type vertical immersion hot-dip galvanization simulator. Commercial low-carbon cold-rolled steel sheets with a thickness of 0.8 mm were used as the substrate, and the composition is tabulated in Table 1. The steel was cut to 150 mm \(\times\) 30 mm, degreased (10% NaOH at 60 \({}^{\circ}\)C for 10 min), and pickled (10% HCl at room temperature for 10 min) with rinsing in deionized water between steps. The samples were flux-treated at 60 \({}^{\circ}\)C for 1 min and dried in an oven at 120 \({}^{\circ}\)C for 5 min before immersion in a molten bath of Zn-1 wt.% Mg-1 wt.% Al alloy at 480 \({}^{\circ}\)C for 5 min. Different flux compositions obtained by adding 10-40 wt.% tin(II) chloride monohydrate (SnCl\({}_{2}\cdot\)H\({}_{2}\)O) to the NF solution (Table 2) were investigated.  
The cross-sectional microstructure analysis of the alloy coating was performed using a sample cut to 20 mm \(\times\) 10 mm, cold-mounted, and polished with emery paper (gri size 220-2500) and a 1 \(\upmu\)m diamond paste. A light optical microscope (LOM, Leica DM 2500, Heidelberg, Germany) and field-emission scanning electron microscope (FE-SEM, Hitachi SU6600, Tokyo, Japan) were used to examine the microstructure. The solidification of the Zn-Al-Mg alloy was thermodynamically analyzed using the FactSage software package.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**C** & **Mn** & **S** & **P** & **Si** & **Al** & **N** & **Fe** \\ \hline
0.140 & 0.300 & 0.010 & 0.015 & 0.025 & 0.030 & 0.004 & Bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Composition of the commercial steel used in this study (wt.%).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \multirow{2}{*}{**Flux**} & \multicolumn{3}{c}{**Flux Composition (wt.\%)**} \\ \cline{3-5}
**Designation** & & & **NH\({}_{4}\)Cl** & **ZnCl\({}_{2}\)** & **SnCl\({}_{2}\cdot\)H\({}_{2}\)O** \\ \hline NF & Normal Flux & 75.0 & 25.0 & - \\ \hline SF10 & NF + 10\% SnCl\({}_{2}\) & 67.5 & 22.5 & 10.0 \\ \hline SF20 & NF + 20\% SnCl\({}_{2}\) & 60.0 & 20.0 & 20.0 \\ \hline SF30 & NF + 30\% SnCl\({}_{2}\) & 52.5 & 17.5 & 30.0 \\ \hline SF40 & NF + 40\% SnCl\({}_{2}\) & 45.0 & 15.0 & 40.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Flux compositions and sample designation.  
The alloy phase formation behavior and residual chlorides with different flux compositions were analyzed using an electron probe microanalyzer (EPMA, JEOL JXA-8500F, Tokyo, Japan). The crystallographic phase formation in the alloy coating was analyzed by X-ray diffraction (XRD, Rigaku RINT-2000, Tokyo, Japan) with a Cu-K\(\alpha\) (1.54 A) source. The surface properties were evaluated by measuring the surface roughness using a high-resolution optical three-dimensional surface profiler (Nanoscope NS-3500, Tokyo, Japan). The bending test was performed in accordance with ASTM E290 to assess coating adhesion. Scotch tape was attached to the bent surface and removed for observation using image analysis software (Leica application suite, version 4, Imaging Software For Metallographic Applications; Leica Microsystems GmbH, Wetzlar, Germany, 2013). Thermal analysis was performed using a thermogravimetric-differential thermal analyzer (TG-DTA, DTG-60, SHIMADZU, Kyoto, Japan) to measure the decomposition behavior of the flux with and without SnCl\({}_{2}\)-H\({}_{2}\)O. The decomposition behavior of the chlorides in the flux according to the temperature change was analyzed by heating the chlorides in an alumina crucible to 800 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min in an Ar atmosphere.