# Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment
## 3 Results and Discussion
### Corrosion Behaviour Characterization  
Figure 5 shows the SEM images of the corroded Zn-Al Mg-Si coating surface, which was exposed to the sulfur dioxide test with general condensation of moisture for 30 day cycles. The appearance of the coating surface reveals defects in the form of holes (Figure 5a) and cracks (Figure 5b). Two structural components of the coating are also clearly visible: Al-rich dendrites and Zn/ZnMg\({}_{2}\) eutectic. The EDS spectrum from the surface of Al-rich dendrites (Figure 6) shows the presence of Al, but also small amounts of S and O (point A). In the Zn/MgZn  
Figure 4: Results of the sulfur dioxide test with general condensation of moisture: (**a**) development of unit mass change of coatings; (**b**) surface appearance of coatings after the 30 cycles exposure; (**c**) structure of the comparative Zn coating (\(\delta_{1}\)—FeZn\({}_{10}\), \(\zeta\)—FeZn\({}_{13}\), \(\eta\)—solid solution of iron in the zinc). Figure 5: SEM images of corroded Zn-Al-Mg-Si coating surface after 30 cycles of exposure in the sulfur dioxide test with general condensation of moisture: (**a**) holes on the coating surface; (**b**) structural components of the coating—Al dendrites and Zn/MgZn\({}_{2}\) eutectic and corrosion cracks. a high S content. Zn is also concentrated in this area, as well as small amounts of Al, Mg and O (point B). SEM images also indicate the presence of characteristic corrosion products located in interdendritic areas (Figure 6, red square). The micro-structure of these corrosion products shows a very heterogeneous structure (Figure 7). The EDS spectrum in the micro-area of these corrosion products shows high content of Zn, Al and Mg (area C). At the same time, a significant S content is concentrated in it, and the presence of O can also be noticed. The XRD pattern from the coating surface after the corrosion test is shown in Figure 8. A clearly identified corrosion product, zinc hydroxysulfate, is present on the coating surface--Zn4 SO4(OH)6.5H2O. The results of the EDS analysis allow for the conclusion that these are eutectic Zn/ZnMg2 areas. These areas contain mainly Zn, S and O (Figure 6, point B), which are components of zinc hydroxysulfate. Additionally, the location of this corrosion product may be confirmed by the small content of Mg, which is a component of Zn/ZnMg2 eutectic. Figure 6: SEM images and corresponding EDS patterns of the corroded Zn-Al-Mg-Si coating after the sulfur dioxide test with general condensation of moisture. Figure 7: SEM images and corresponding EDS patterns of the LDH area of Zn-Al-Mg-Si coating after the sulfur dioxide test with general condensation of moisture. The XRD pattern clearly confirms the presence of layered double hydroxides (LDHs) in the corrosion products. LDH is a hydroxyl compound with a crystalline layered structure of hydrolatelite. LDHs are hydroxides of two metallic elements with the general formula [M\({}^{2+}\)1-xM\({}^{3+}\)x\(\cdot\)(OH)\({}_{2}\)]\({}^{\mathrm{A+}}\)(A\({}^{\mathrm{n-}}\))\({}_{\mathrm{x/n}}\)\(\cdot\)mH\({}_{2}\)O. M\({}^{2+}\) and M\({}^{3+}\) are divalent and trivalent metal cations, respectively, and the anion A\({}^{\mathrm{n-}}\) is an interlayer anion [26]. The tested Zn-Al-Mg-Si coatings can be a source of divalent cations (Zn\({}^{2+}\) and Mg\({}^{2+}\)), as well as a trivalent cation (Al\({}^{3+}\)). When exposed to corrosive environments, its aggressiveness is determined by the presence of high concentrations of carbonates, chlorides or sulfur dioxide. Therefore, under the influence of corrosive atmospheres, the most probable anions are CO\({}_{3}\)\({}^{2-}\), Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\)[17]. The morphology of corrosion products in the interdendritic area (Figure 7) indicates the location of LDH there. EDS analysis in micro-area C, including high concentrations of Zn, Mg and Al, indicate the presence of divalent cations Zn\({}^{2+}\) and Mg\({}^{2+}\), as well as trivalent cation Al\({}^{3+}\).