Initial Corrosion Mechanism and Protection Performance of Hot-Based Zn-Al-Mg Alloy Coating Steel Cut Edge

The low-aluminum and low-magnesium Zn1.5Al1.1Mg hot-based Zn-Al-Mg alloy coated plate was employed as research object, the initial corrosion mechanism of the cut edges of low aluminum, low magnesium Zn-Al-Mg alloy coated plates was clarified by SEM, EDS, XRD and other methods. Moreover, through SEM and XRD method, the types and morphology of corrosion products at the cut edges of the specimens after immersion and cyclic corrosion experiments were analyzed for evaluating the protective performance of Zn-Al-Mg alloy coatings on their cut edges. Results showed that the cut edge of the Zn-Al-Mg alloy coating was composed of three microstructures: pure Zn phase, Zn-MgZn2 binary eutectic and Zn-MgZn2-Al ternary eutectic. In general, the ternary eutectic and binary eutectic containing MgZn2 phase served as corrosion initiation points during the initial corrosion stage of Zn-Al-Mg alloy cut edges, and then extended to the entire coating. During the long-term corrosion process of cutting edges in Zn-Al-Mg alloy coatings, the Zn element played a major protective role. Specifically, free Zn2+ accumulated at the cutting edge of the coating by forming basic zinc carbonate [Zn5(OH)6(CO3)2] and basic zinc chloride [Zn5(OH)8Cl2·H2O], hindering the longitudinal expansion of the corrosive medium. Nevertheless, when these precipitates transformed into porous ZnO, the protective capabilities of the coating failed, and the cut edge was damaged.