Structure and Corrosion Resistance of Coatings Obtained by the Batch Double Hot Dip Method in Eutectoid ZnAl Bath with the Addition of Mg and Si

Abstract

The article presents the results of the research determining the synergistic effect of Mg and Si additives in the ZnAl23 bath on the microstructure (SEM), growth kinetics, and corrosion resistance of coatings obtained by the batch double hot dip method. On the basis of the research on the chemical composition in micro-regions (EDS) and the research on the phase composition (XRD), the structural components of the coatings with a content of 3 and 6 wt.% Mg and 0.2–0.4 wt.% Si were identified. The corrosion resistance of the coatings was compared with that of the reference coating obtained in the ZnAl23Si0.4 bath in the neutral salt spray test (EN ISO 9227), and the test with sulfur dioxide in a humid atmosphere (EN ISO 6988). The parameters of electrochemical corrosion of the coatings were determined. This allowed for the conclusion that the addition of Mg and Si to the ZnAl23 bath improved the microstructural uniformity of the coating. In the area of the diffusion layer, the presence of the FeAl3 intermetallic phase was found, while in the outer layer the dendrite of the Al-rich solution with interdendritic spaces, where it locates areas of Zn-rich solution, eutectic Zn/MgZn2 or phase separation MgZn2 and Mg2Si are located. Additions of Mg and Si stabilize the kinetics of the coating’s growth, and its course is parabolic. The conducted corrosion tests showed better corrosion resistance of the coatings obtained in the bath with Mg addition compared to the reference coating obtained in the ZnAl23Si bath. The increase in corrosion resistance results from the presence of a more anodic MgZn2 phase, which, however, may be weakened by the presence of Mg2Si phase precipitates.