| Mg alloys have excellent physical and mechanical properties, in particular its high strength:weight ratio makes it an ideal metal for automotive and aerospace applications, where weight reduction is of significant concern. Unfortunately, Mg alloys are highly susceptible to corrosion. This has limited its use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. The simplest way to avoid corrosion is to coat the substrate to prevent contact with the corrosive environment.In this paper, we reported three kinds of technologies to construct appropriate coating. The corrosion behavior of Mg alloys and coated Mg alloys in 3.5% NaCl solution was investigated by electrochemical measurements. The results indicate that the corrosion resistance of coated Mg alloys has been improved in the 3.5% NaCl corrosive medium. The main work and innovation of the thesis are listed as following:1. Mg-Al layered double hydroxide (LDH) as a container, loaded with tungstate anions, were synthesized via the coprecipitation method. The resulting compounds were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), and transmission electron microscopy (TEM), respectively. Constructed epoxy resin doped with LDH-WO42-was used as an anticorrosion coating painted on the Mg alloy. The corrosion inhibitor of tungstate would release during the osmotic process of corrosion medium. The inductively coupled plasma (ICP) measurements demonstrated the anion-exchange nature of the process, involving the release of tungstate anions from layered double hydroxide by the presence of chloride anions. Polarization measurements showed that the filtrate as electrolyte exhibited a lower corrosion current density value due to the presence of WO42-. The electrochemical impedance spectroscopy (EIS) analysis showed that the coating could effectively protect the alloy from corrosion. The function of the layered double hydroxide loaded with tungstate doped in organic coating was discussed. When the physical damage to the coating occurs, the released inhibitor can provide adequate corrosion protection.2. A hydrotalcite/hydromagnesite conversion coating was fabricated on a Mg alloy substrate by a hydrothermal crystallization method, based on an interface reaction between hydrotalcite slurry and Mg alloy. The conversion coating was obtained by synthesis involving separate nucleation and aging steps, where the film was formed in the aging process. The film was uniform and compact. The film was characterized by XRD, SEM, EDS techniques to study the structure, phase purity and morphology respectively. Polarization measurements and electrochemical impedance spectroscopy (EIS) showed that the film could improve the corrosion resistance of Mg alloy. The cross-cutting showed that the adhesion between the film and the substrate was very strong which enhances its potential for practical application.3. A hydrotalcite/hydromagnesite conversion coating with hierarchical structure has been fabricated on a Mg alloy substrate by in situ hydrothermal crystallization method. A MgO layer existing between the hydrotalcite/hydromagnesite film and the substrate was formed prior to the hydrotalcite/hydromagnesite film during the crystallization process. After surface treatment with silane coupling agent, the surface of conversion coating changes from hydrophilic to hydrophobic. Scanning electron microscopy (SEM) revealed that the silylated conversion coating with hierarchical structure maintains the original rough surface of which was composed of numerous micro-scale flakes and beautiful flower-like protrusions. Polarization measurements have shown that the hydrophobic conversion coating exhibited a low corrosion current density decreased about three orders of magnitude compared with that of bare Mg alloy substrate, which means that the hydrophobic conversion coating can effectively protect Mg alloy from corrosion. Electrochemical impedance spectroscopy (EIS) showed that the hydrophobic conversion coating could effectively improve the corrosion resistance of Mg alloy. |
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