# Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment
## 3 Results and Discussion
### Corrosion Behaviour Characterization
[PERSON] and [PERSON] [32] proposed a mechanism of Al dissolution in which there are two fast stages--aluminum ionization and the reaction of Al(OH)\({}_{3}\) formation--combined with a slow stage of dissolution of the passive layer as a result of the interaction of OH\({}^{-}\) ions. The instability of the Al(OH)\({}_{3}\) passive layer is also confirmed by the Pourbaix diagram for aluminum, according to which the passive layer dissolves in solutions close to neutral [33]. [PERSON] et al. [34] claim, however, that the passive layer dissolves as a result of the increase in pH at the surface during cathodic polarization. The presence of an Al(OH)\({}_{3}\) layer on the surface of the tested coatings can be confirmed by the XRD pattern (Figure 8). Although, the presence of only two weak peaks of this phase may indicate its small share and low durability. Al-rich dendrites can, therefore, provide Al\({}^{3+}\) cations both as a result of direct ionization of Al as well as the dissolution of passive layers. However, it should be noted that sulfur compounds are an important factor in the corrosion of aluminum. In their presence, passive layers of oxide or hydroxide are formed on the Al surface [35]. EDS analysis of the surface of Al-rich dendrites (Figure 6, point A) also indicates the presence of S in this area. It can, therefore, be assumed that SO\({}_{4}\)\({}^{2-}\) ions are involved in the formation of corrosion products on the surface of Al-rich dendrites. In industrial atmospheres rich in sulfur compounds, it is most often aluminum sulfate hydrate. However, its structure is amorphous, hence the XRD pattern is not able to confirm the presence of this phase in the corrosion products on the coating surface. It has been proven that the dissolution of Mg from MgZn\({}_{2}\) intermetallic is preferential to the dissolution of zinc [20,36]. [PERSON] et al. [37] found that MgZn\({}_{2}\) intermetallic platelets in eutectic also preferentially dissolve into Zn platelets. Therefore, it should be expected that Mg\({}^{2+}\) cations will be released earlier and their concentration at the coating/corrosion environment interface in the initial phase of the corrosion process will be higher than the concentration of Zn\({}^{2+}\) cations. Therefore, the initial formation of the LDH structure will be based on Mg-Al cations. However, due to the high capacity for ion exchange in LDH structures, further exchange of Mg\({}^{2+}\) cations and their replacement with Zn\({}^{2+}\) cations should be expected, leading to the formation of an LDH structure based on Zn-Al cations. However, at the current stage of research, it cannot be confirmed whether this is the coexistence of two compounds Zn-Al SO\({}_{4}\) LDH and Mg-Al SO\({}_{4}\) LDH, or rather the formation of the ternary Mg/Zn-Al SO\({}_{4}\) LHD. The presence of Mg in LDH structures is beneficial because it improves corrosion resistance. Corrosion studies of Zn-Al-Mg alloys indicate that LDH structures containing Mg, both binary Mg-Al LDH and ternary Mg/Zn-Al LDH, constitute a better barrier to oxygen diffusion [38]. Also, [PERSON] and [PERSON] [39] claim that the thermodynamic stability of LDH is greater than that of the corresponding divalent hydroxides, and the solubility constant of MgAl--LDH is five orders of magnitude greater than that of ZnAl--LDH. However, the XRD pattern (Figure 8) detects peaks for Al and Zn. Detection of structural components of the coating through the layer of corrosion products may indicate a small thickness of this layer. An important structural component of the tested Zn-Al-Mg-Si coatings for their corrosion behavior is the presence of Si precipitates in the outer layer. Figure 9 shows a cross-section of the coating after corrosion in place of the holes visible on the surface (Figure 5). EDS analysis indicates a high concentration of Si in this area (Figure 9, point D), which confirms the location of Si precipitates in this place. Intensive dissolution of the matrix is observed around Si precipitates.