## 3 Results
### Thermal Decomposition of the NF and SF30 Samples  
The thermal decomposition behavior of the NF and SF30 fluxes was studied using TG-DTA, as shown in Figure 6. As both fluxes are hygroscopic, moisture evaporation lowered their weight in the temperature range 100-150 \({}^{\circ}\)C (R1), as confirmed by the endothermic peak of the DTA curve. A fast weight loss occurred in the NF flow at 310-400 \({}^{\circ}\)C, which is the ammonium chloride decomposition temperature range (R3). Weight reduction slowed after ammonium chloride decomposition in the coating bath at 400-500 \({}^{\circ}\)C and was further slowed above 500 \({}^{\circ}\)C. Finally, the flux decomposition was completed at 600 \({}^{\circ}\)C. Meanwhile, for the SF30 flux, an additional decomposition behavior of tin chloride was observed at 340-460 \({}^{\circ}\)C, which is the decomposition temperature for ammonium chloride (R4). The endothermic peak of the DTA curve overlapped, and the deconvolution of the peak was achieved, as shown in Figure 6c. Unlike NF, the weight reduction was continuous in the coating bath temperature range 400-500 \({}^{\circ}\)C due to the decomposition of SnCl\({}_{2}\), which continued until the end of the flux decomposition. Therefore, SF30 decomposed at a lower temperature than NF. From the experimental results, the SnCl\({}_{2}\) addition to the flux promoted the decomposition of the flux in contact with the molten metal, thereby improving the wettability of the molten metal and suppressing the residual Al/Cl compound formation in the coating layer. Figure 3: EPMA phase maps of the (**a**) NF and (**b**) SF30 samples of the elements marked at the bottom of the map. Figure 4: (**a**) Phase diagram (Zrn: 85%–100%, Mg and Al: 0%–15%) and (**b**) solidification behaviors of the Zn–1 wt% Mg–1 wt.% Al composition (Liquid \(\rightarrow\) Solid). Figure 5: XRD of the SF30 sample with the Zn, MgZn\({}_{2}\), and Al phases. Figure 6: (**a**) TGA and (**b**) DTA curves showing the thermal decomposition behavior of the fluxes for high-quality coatings with improved adhesion. (**c**) Deconvolution of the SF30 DTA curve. For the NF sample, Al present in the Zn-1 Mg-1 Al molten alloy reacted with the flux to form the Al/Cl compound (t0, Figure 7a) [14]. \[6\mathrm{NH_{4}Cl}+2\mathrm{Al}\to 2\mathrm{AlCl_{3}}+6\mathrm{NH_{3}}+3 \mathrm{H_{2}} \tag{1}\]  
\[3\mathrm{ZnCl_{2}}+2\mathrm{Al}\to 2\mathrm{AlCl_{3}}+3\mathrm{Zn} \tag{2}\]  
The formation of AlCl\({}_{3}\) depletes the flux (t1, Figure 7a), incorporates it into the coating layer, and fosters the coating defects (t2, Figure 7a) (i.e., deterioration of the coating characteristics), as shown in the EPMA results (Figure 3a). Meanwhile, in the SF samples, SnCl\({}_{2}\) was reduced to Sn by reacting with Zn, according to the following reactions:  
\[\mathrm{SnCl_{2}}+\mathrm{Zn}\to\mathrm{ZnCl_{2}}+\mathrm{Sn} \tag{3}\]  
\[3\mathrm{Sn}+2\mathrm{AlCl_{3}}\to 3\mathrm{SnCl_{2}}+2\mathrm{Al} \tag{4}\]  
The reduced Sn underwent a substitution reaction with AlCl\({}_{3}\) to form SnCl\({}_{2}\) (t1, Figure 7b). The thermodynamic properties for reaction (4) were calculated. The change in enthalpy (\(\Delta\)H) was \(-346.28\) kJ/mol (exothermic), and the change in entropy (\(\Delta\)S) was \(7.74\) J/mol\(\cdot\)K (i.e., increased entropy). Therefore, the Gibbs free energy change (\(\Delta\)G) at 753 K can be calculated as \(-352.1\) kJ/mol, which indicates the spontaneous Sn substitution reaction at the pot temperature. Consequently, the AlCl\({}_{3}\) compound, which caused defects, was removed.