Microstructure and Corrosion Behavior of ZnAl12Mg3Si0.3 Double-Batch Hot-Dip Coatings

Abstract

This article presents the microstructure (SEM) and corrosion behavior of ZnAl12Mg3Si0.3 (ZAMS) coatings obtained by the double hot-dip method on Sebisty steel with increased strength. On the basis of chemical composition studies in micro-areas (EDS) and phase composition studies (XRD), structural components of the coating and corrosion products formed on the coating surface after exposure to the neutral salt spray (NSS) test (EN ISO 9227) were identified. The presence of the Fe(Al,Si,Zn)3 intermetallic phase was found in the Fe-Al intermetallic layer, while in the outer layer, dendrites rich in Al and Zn were identified. In these dendrites, the eutectics of Zn/MgZn2 and precipitates of the MgZn2 phase and Si were located. The NSS test showed better corrosion resistance of ZAMS coatings compared to conventional zinc hot-dip coatings. The increase in corrosion resistance is due to the formation of favorable corrosion products: simonkolleite—Zn5(OH)8Cl2·H2O and hydrozincite—Zn5(OH)6(CO3)2, and the presence of the MgZn2 phase in the coating, which is more anodic than other structural components.

Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment

“…The structural components of these zones, although they show different morphologies, have similar chemical compositions, and EDS analysis allows the identification of Al, Fe, Zn and Si in their micro-area (Table 3, points 6 and 7). These results correspond very well with the content of components in the diffusion layer of ZnAl12Mg3Si0.3 coatings in FeAl3 intermetallics [15]. Si and Zn have high solubility in Fe-Al intermetallics in the solid state [24].…”

“…Si and Zn have high solubility in Fe-Al intermetallics in the solid state [24]. As the authors show in their work [15], the total content in at.% (Al+Si+Zn) is close to 75 at.%, which, with a Fe content close to 25 at.%, gives the atomic ratio (Al+Si+Zn)/Fe corresponding to FeAl 3 intermetallics. The atomic ratio (Al+Si+Zn)/Fe at points 6 and 7 is 3.01 and 2.93, respectively.…”

“…The temperature of each bath was 450 • C. The samples were immersed in each bath for 60 s. Then, the samples were cooled in air. The procedure for producing the coatings was selected based on detailed research conducted in [15]. A conventional galvanizing bath with Al and Bi technological additives was used as the zinc bath.…”

“…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 [5,[16][17][18][19][20][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.…”