Fabrication of Anion-Exchanged Electrodeposited Zn-Al Layered Double Hydroxides (LDHs) Coating on ZX21 Mg Alloy

Abstract

Because of their low density and high specific strength, Mg alloys are considered promising lightweight alloys for various applications. However, due to the high electrochemical activity and poor corrosion resistance, surface modifications of Mg alloys are needed to increase the service life. Hence, an active layered double hydroxides (LDHs) coating was investigated in this study. Owing to the brucite-like layered structure, the weakly bonded interlayer anions can be easily exchanged with other anions in the environment. With the unique anion-exchange mechanism, LDHs coating provides dual protection to the Mg alloy substrate. It can not only act as a physical barrier but also absorb aggressive anions such as Cl- and release corrosion inhibitors to the environment.Compared to the commonly used hydrothermal method to fabricate LDHs coating, which requires high temperature and long process time, electrochemical deposition has the advantages of high deposition rates, simple experimental setup, and highly controllable process parameters. Therefore, in this study, a three-electrode electrochemical setup was used to fabricate Zn-Al LDHs coatings on a ZX21 Mg alloy. Moreover, different anions are intercalated into the interlayer galleries of the prepared Zn-Al LDHs coating through post anion-exchange processes.Through electrochemical analysis in a 3.5 wt% NaCl solution, the prepared anion-exchanged electrodeposited Zn-Al LDHs coating improved the corrosion resistance of ZX21 Mg alloy. Electrochemical impedance spectroscopy (EIS) showed that corrosion impedance increases, while potentiodynamic polarization curves revealed that the corrosion current density (i corr) of the coated sample was an order of magnitude lower than the ZX21 Mg alloy substrate. Besides the improvement of corrosion resistance, the formation mechanism of Zn-Al LDHs coating by electrodeposition was also investigated by electrochemical analysis and in situ optical microscopic (OM) observations. The corrosion behavior and microstructure of Zn-Al LDHs coating and the roles played by different intercalated anions will be discussed based on electrochemical analysis and microstructure characterizations. Figure 1