# Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment
## 3 Results and Discussion
### Corrosion Behaviour Characterization
However, the high S concentration and O content indicate that the SO\({}_{4}\)\({}^{2-}\) anion has been incorporated into the LDH structure. As previous long-term field exposure studies of ZnAlMg coatings show [17; 18], it is possible that SO\({}_{4}\)\({}^{2-}\) anions may exist in the structure of corrosion products, but they have not been clearly confirmed. However, it is known that LDH compounds easily exchange both cations and anions [26], which, due to the complex composition of the atmosphere in these studies [17; 18], may change the identity of interlayer ions. The presence of two different divalent cations Zn\({}^{2+}\) and Mg\({}^{2+}\) may indicate the coexistence in the corrosion products of the tested coatings of two compounds, Zn-Al SO\({}_{4}\) LDH and Mg-Al SO\({}_{4}\) LDH, in which SO\({}_{4}\)\({}^{2-}\) anions are incorporated into the layered structure. The coexistence of two LHD compounds based on Zn-Al and Mg-Al cations in the corrosion products of ZnAlMg coatings was demonstrated by the authors [19; 20; 21], although, due to the more complex composition of the environment and the research time, these compounds were based on CO\({}_{3}\)\({}^{2-}\) anions, which are the most stable, but can simultaneously cause the exchange and release of other anions [26]. In LDH structures, both divalent and trivalent cations (M\({}^{2+}\) and M\({}^{3+}\)) can be replaced by other metal ions of the same valency and similar ionic radius [27]. Therefore, it cannot be ruled out that in the corrosion products formed on the surface of the tested coatings, Zn\({}^{2+}\) and Mg\({}^{2+}\) cations occupy their positions interchangeably, forming Mg/Zn-Al SO\({}_{4}\) LHD ternaries. However, the precise identification of LDHs is very complicated due to the diversity of their stoichiometry. Additionally, the identification of LDHs in corrosion products by XRD may be difficult due to the variability of crystallographic parameters related to differences in the ratios between various cations and anions caused by the  
Figure 8: XRD spectra of corrosion products on the Zn-Al-Mg-Si coating surface after the sulfur dioxide test with general condensation of moisture. ability of LDH structures to easily undergo ion exchange [16]. The mechanism of LDH formation in the corrosion products of ZnAlMg coatings has not yet been clearly explained. The structure and morphology of the phases of the tested Zn-Al-Mg-Si coatings are very complex. Structural components detected on the surface of coatings have different corrosion potential values. The corrosion potential of Al in the passive state in aqueous solutions is higher than in the active state [28]. It can be assumed that Al-rich dendrites are in a passive state. This is confirmed by the presence of oxygen in the EDS analysis (Figure 6, point A). In an aqueous solution in 0.6 M NaCl, the corrosion potential E\({}_{\rm corr}\) vs. SCE (Saturated Calomel Electrode) of Al has a value of \(-849\) [mV], while E\({}_{\rm corr}\) vs. SCE for Zn and MgZn\({}_{2}\) is \(-1028\) and \(-1095\) [mV], respectively [29]. Therefore, the interdendritic areas of the coating containing Zn/MgZn\({}_{2}\) eutectic and MgZn\({}_{2}\) intermetallic precipitates will act as an anode in relation to the more noble Al-rich dendrites. Under these conditions, the dissolution of MgZn\({}_{2}\) intermetallic and Zn interdendritics leads to the enrichment of these areas by Mg\({}^{2+}\) and Zn\({}^{2+}\) cations. SO\({}_{2}\) reacts with water or moisture to form sulfuric acid [30]. In acidic solutions, Al dissolves, forming Al\({}^{3+}\) ions [31]. Reduction of oxygen at the cathode (Al dendrites) may lead to alkalization of the corrosive environment in the immediate vicinity of Al-rich dendrites. Under such conditions, the formation of a passive Al(OH)\({}_{3}\) layer is possible [31].