## 3 Experimental Results
### Corrosion Behaviour Study  
The results of the polarization curves for each zone are shown in Figure 10 and Table 2.  
According to Figure 10 and Table 3, compared with the parent material and the welding point polarization curve of the same material, the corrosion potential of the welding point was lower than that of the corresponding parent material, which illustrates that the corrosion tendency of the welding point was greater than that of the body material, and the change in corrosion current was also consistent. The corrosion currents of the body material and the welding joint of the HC340 LA steel without coating were \(4.258\times 10^{-4}\) mA\(\cdot\)cm\({}^{-2}\) and \(3.602\times 10^{-3}\) mA\(\cdot\)cm\({}^{-2}\). The corrosion current decreased by 8.46 times, while the corrosion currents of the parent material and the welding joint of the HC340 LAD + ZM with Zn-Al-Mg coating were \(5.934\times 10^{-5}\) mA\(\cdot\)cm\({}^{-2}\) and \(1.323\times 10^{-4}\) mA\(\cdot\)cm\({}^{-2}\). The corrosion current decreased by 2.23 times. This is because the welding heat input increased in the welding process, which led to tissue transformation and the loss of the surface coating of the welding point during the welding process, thus leading to the deterioration of corrosion resistance.  
The corrosion potentials of the HC340 LAD + ZM base material and the welding joint containing zinc aluminum magnesium coating were (\(-1.167\) V) and (\(-1.218\) V), respectively; the corrosion potentials of the base material and the welding joint of HC340 LA without coating were (\(-0.5721\) V) and (\(-0.705\) V), respectively. According to the longitudinal comparison, the HC340 LAD + ZM had a lower corrosion potential at both the  
Figure 10: HC340 LA (**a**) and Zn\(-\)Al\(-\)Mg\(-\)coated HC340 LAD + ZM (**b**) polarization curves.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Region** & **E\({}_{\text{corr}}\)/V** & **I\({}_{\text{corr}}\)/(m**A\(\cdot\)cm\({}^{-2}\))** \\ \hline HC340 LA & Base metal & \(-0.572\) & \(4.258\times 10^{-4}\) \\  & Soldered dot & \(-0.705\) & \(3.602\times 10^{-3}\) \\  & Base metal & \(-1.167\) & \(5.934\times 10^{-5}\) \\  & Soldered dot & \(-1.218\) & \(1.323\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Electrochemical parameters of HC340 LAD + ZM and HC340 LA polarization curves.  
base material and the welding joint, so its corrosion tendency was greater. The corrosion current was just the opposite. The corrosion currents of the parent material of the zinc-aluminum-magnesium-coated plate and the uncoated plate were 5.934 \(\times\) 10\({}^{-5}\) mA\(\cdot\)cm\({}^{-2}\) and 4.258 \(\times\) 10\({}^{-4}\) mA\(\cdot\)cm\({}^{-2}\), respectively. The corrosion current of the zinc-aluminum-magnesium plate was 7.17 times that of the uncoated plate. The corrosion currents of the welding joint of the zinc-aluminum-magnesium-coated plate and the uncoated plate were 1.323 \(\times\) 10\({}^{-4}\) mA\(\cdot\)cm\({}^{-2}\) and 3.602 \(\times\) 10\({}^{-3}\) mA\(\cdot\)cm\({}^{-2}\). The gap in the corrosion current was 27 times greater. The reason why the corrosion potential of the Zn-Al-Mg-coated plate was higher than that of the uncoated plate and the corrosion current was smaller is that the corrosion of the Zn-Al-Mg-coated plate was the first to develop surface corrosion. The coating was mainly zinc, whose corrosion potential is higher (\(-\)1 V), and the uncoated plate was the corrosive matrix ferrite (\(-\)0.4 V). Due to the presence of primary crystal phase Zn; Al; MgZn\({}_{2}\); binary co-crystal tissue Zn/MgZn\({}_{2}\); and ternary co-crystal tissue Zn/MgZn\({}_{2}\)/Al in the zinc-aluminum-magnesium coating--in which MgZn\({}_{2}\) and Mg\({}_{2}\)Zn\({}_{11}\) had higher microhardness, better compactness and better single-phase corrosion resistance--the zinc and aluminum magnesium plates had higher corrosion potential, but a slower corrosion rate [28; 29; 30].