# Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment
## 3 Results and Discussion  
### Top Surface Microstructure of Coatings  
Figure 1 shows SEM images of the top surface of the coatings, while the chemical composition characteristics of the micro-areas of the coating are presented in Table 2. Structural components with a characteristic dendritic structure can be observed on the coating surface (Figure 1a). The morphology of the interendritic areas includes components with a lamellar eutectic structure and areas with a compact and coherent structure, which may be characteristic of internetals (Figure 1b). EDS analysis indicates that the dendrite region is rich in Al (Figure 1c, point 1), while the intermetallics regions contain mainly Zn and Mg (Figure 1c, point 2). The Zn concentration in the dendrites indicates that it is most likely a solution of Zn in Al (\(\beta\)). The atomic fraction of Zn to Mg in the intermetallic region is close to 2, which may correspond to the MgZn\({}_{2}\) phase. Bright lamellars in the eutectic (Figure 1d, point 3) contain a Zn-rich phase, which, due to the Al content, is a solid solution of Al in Zn(\(\alpha\)). Dark lamellars contain mainly Zn, but also significant amounts of Mg (Figure 1d, point 4). The most probable phase is MgZn\({}_{2}\) intermetallic, although the chemical composition cannot clearly confirm this due to the very fine structure of the eutectic. The local occurrence of characteristic, approximately equiaxed precipitates was also observed on the coating surface, the chemical composition of which indicates that they are Si precipitates (Figure 1d, point 5).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & & & & **Content [wt.\%]** & & \\ \hline  & & **C** & **Si** & **Mn** & **S** & **P** & **Fe and Others** \\ \hline Steel & HSLA & 0.06 & 0.20 & 0.80 & 0.003 & 0.009 & rest \\ \hline  & & **Al** & **Fe** & **Si** & **Mg** & **Bi** & **Zn and others** \\ \hline \multirow{2}{*}{Bath} & Zn-Al-Mg-Si & 11.86 & 0.024 & 0.32 & 3.15 & 0.0001 & rest \\  & Zn & 0.0059 & 0.031 & 0.001 & 0.002 & 0.061 & rest \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of HCLA steel and research baths.  
Figure 2 shows the XRD spectra of top surface of the Zn-Al-Mg-Si coating. This study shows that aluminum, zinc and MgZn\({}_{2}\) intermetallic are present on the coating surface. The Al peaks confirm the existence of the Al(\(\beta\)) solution as Al-rich dendrites. The MgZn\({}_{2}\) peaks confirm that the Mg- and Zn-rich phases that appeared in the interdendritic region and the dark lamellars are composed of MgZn\({}_{2}\) intermetallics. The peaks for Zn confirm the presence of the Zn(\(\alpha\)) solution as bright lamellars. The light and dark lamellars, therefore, form the Zn/MgZn\({}_{2}\) eutectic. Only two weak peaks for Si were identified in the XRD pattern. However, the corresponding EDS analysis can sufficiently confirm the occurrence of Si precipitates on the coating surface.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Point No.** & **Mg (at.\%)** & **Al (at.\%)** & **Si (at.\%)** & **Zn (at.\%)** & **Phase** \\ \hline
1 & - & 80.9 & - & 19.1 & Al-rich phase \\
2 & 32.8 & - & - & 67.2 & MgZn\({}_{2}\) intermetallic \\
3 & 0.6 & 9.1 & - & 92.8 & Zn-rich phase \\
4 & 9.4 & 7.2 & - & 83.4 & MgZn\({}_{2}\) intermetallic \\
5 & - & 1.1 & 98.1 & - & Si \\ \hline \end{tabular}
\end{table}
Table 2: Results EDS analysis of the top surface of Zn-Al-Mg-Si coatings and the corresponding phase (analysis points as shown in Figure 1).  
Figure 1: SEM images of top surface of Zn-Al-Mg-Si coatings: (**a,b**) structural components on the coating top surface; (**c**) EDS analysis points in dendrites and intermetallics; (**d**) EDS analysis points in eutectics and Si precipitation.