# Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment
###### Abstract  
Zn-Al-Mg-Si coatings are an excellent alternative to conventional hot-dip galvanizing coatings. Their high corrosion resistance in corrosive environments containing chlorides and CO\({}_{2}\) is well recognized. But sulfur dioxide is also an important stimulator of corrosion in the atmospheric environment. This article presents the results of microstructure (SEM/EDS/XRD) and corrosion behavior tests of Zn-Al-Mg-Si coatings obtained by a double hot-dip method on HSLA steel. The corrosion resistance of the coatings was determined in the sulfur dioxide test with general condensation of moisture (EN ISO 6988). In the corrosion test, Zn-Al-Mg-Si coatings showed twofold smaller weight loss compared to conventional hot-dip zinc coatings. It was found that the corrosion behavior of coatings was influenced by the structural components revealed in the outer layer: Al-rich dendritic and interdendritic areas with Zn/MgZn\({}_{2}\) eutectic, MgZn\({}_{2}\) intermetallic and Si precipitates and their electrochemical nature. The increase in corrosion resistance was caused by the formation of beneficial corrosion products: layered double hydroxides (LDHs) based on divalent Mg\({}^{2+}\) and Zn\({}^{2+}\) cations, trivalent Al\({}^{3+}\) cations and SO\({}_{4}\)\({}^{2-}\) anions, and zinc hydroxysulfate--Zn\({}_{4}\)SO\({}_{4}\)(OH)\({}_{6}\cdot\)SH\({}_{2}\)O. The presence of Si precipitates could cause pitting corrosion of coatings. hot-dip galvanizing; double hot-dip method; Zn-Al-Mg-Si coatings; corrosion resistance; sulfur dioxide test +
Footnote †: journal: Applied Sciences  
method developed by [PERSON]. However, in the batch hot dip process, the use of a Zn-Al-Mg bath has major limitations, which primarily include the need to use a flux and the formation of a periodic layered structure, which leads to excessive coating thickness [9]. Previous research has shown that the use of the double hot-dip method allows for the repeatable production of ZnAl\({}_{5}\) coatings using conventional fluxes [10], and the addition of Si to the bath stabilizes the structure of the coating and eliminates the periodic layer structure [11]. Increasing the Al content in the bath causes a proportional increase in corrosion-resistant coatings. On the other hand, a higher Al content increases the solidification temperature of the bath. [PERSON] et al. [12] showed that the ZnAl\({}_{12}\)Mg\({}_{3}\) alloy solidifies at a temperature of 414.1 \({}^{\circ}\)C. It can, therefore, be concluded that in order to maintain the temperature for the traditional hot-dip galvanizing process, the most technologically advantageous content of the bath is 12 wt.% Al and 3 wt.% Mg.  
Zn-Al-Mg coatings are very popular because they are able to provide steel with much better corrosion resistance than traditional zinc coatings. So far, many studies have been carried out to understand the corrosion mechanisms of Zn-Al-Mg coatings. The research provided important information about corrosion products. Corrosion products have been thoroughly characterized in neutral salt spray tests (NSST) [13; 14; 15], as well as in natural atmospheric environments [16; 17; 18; 19; 20; 5; 16; 5; 21]. Current data suggest that Zn-Al-Mg coatings and the corrosion products formed thereon provide significant improvements in corrosion resistance in chloride-containing environments. [PERSON] et al. [20] report that in the initial stages of atmospheric corrosion, ZnAlMg coatings are covered with layered double hydroxides (LDHs) based on Mg-Al and Zn-Al cations and CO\({}_{3}\)\({}^{2-}\) anions, while on zinc coatings, the corrosion products contain Zn\({}_{5}\)(OH) 8 Cl\({}_{2}\cdot\)H\({}_{2}\)O, zinc hydroxcarbonate and ZnO. Chlorides and CO\({}_{2}\) are the most important corrosion stimulants in atmospheric environment; therefore, there is not much information available on the corrosion behavior of Zn-Al-Mg coatings in CO\({}_{2}\) and chloride-free environments.