| Magnesium alloy is one of the most light weight metal structure materials. Their potential applied fields include automobile, aerospace and consuming electronics, etc. However, the extensive use of magnesium alloys is limited due to poor corrosion resistance. Micro-arc oxidation (MAO) technique is one of the most favorable method in the surface treatment of magnesium alloys in recent years. It is a surface treatment technique developed from the common anodic oxidation. By use of this technique, a ceramic coating can be in situ formed on Al, Mg, Ti and their alloys. This enhances the corrosion resistance and abrasion resistance of the magnesium alloy. Unfortunately, there exist micropores and microcracks in the MAO coating. Usually, the two types of electrolyte solution apply to the micro-arc oxidation of magnesium alloy. One of the electrolyte contains Cr(VI) compounds, the other of the electrolyte contains phosphate and/or fluoride. Both of them contains the poisonous ingredients, such as chromate, phosphate, fluoride which do harm to environment and human beings. Therefore, it is essential that environment-friendly electrolyte system was developed and the effects of all kinds of process parameters on the formation and properties of anodic films were investigated. To further enhance the corrosion resistance of magnesium alloys and improve the structure of MAO coating, the composite coatings were prepared on magnesium alloy. The relationship between structure and performance of the composite coatings, and the binding mechanism between the sol-gel layer and MAO layer were studied.In this paper, single factor and orthogonal layout experiment were applied. The effects of main film-forming agent, auxiliary film-forming agent, additives and their concentration on MAO process and film properties were discussed in the system. An environment-friendly MAO electrolyte system was developed as following,30g/dm3 NaAl2O4, 20g/dm3 NaOH,3g/dm3 montmorillonite, 2g/dm3 gum arabic. And the function of single electrolyte in MAO process was analyzed. The effects of process parameters on surface morphologies and corrosion resistance of MAO coating were systematically investigated using the Taguchi experimental analysis method of four factors with three levels. The optimized parameters were 180V voltage,30min treatment time,50Hz frequency,30%duty cycle. The morphology, components, structure and corrosion resistance of MAO coating were examined by means of SEM, EDS, XRD, potentiodynamic polarization and AC impedance. The results showed that the MAO coating was a uniform distribution of porous structure. The components of aluminium and oxygen were higher, and the microstructure of the coating consisted of MgO?MgAl2O4and Al12Mg17 phases. Compared with the untreated magnesium alloy, the MAO coating exhibited higher corrosion resistance.Based on micro-arc oxidation, a process of MAO combined with a sol-gel method was used to prepare composite coatings on magnesium alloy AZ91D. The composite coatings were formed using multi immersion technique to plus a sol-gel layer on the MAO coating. SEM, EDS, XRD, DSC-TGA, FT-IR, potentiodynamic polarization, AC impedance and cyclic anodic polarization curve were used to evaluate the morphologies, components, structure and corrosion resistance of composite coatings. The results showed that the optimum sol preparation process were Si:Zr molar ratio of 2:1, water bath time was 30min, temperature was 50?, pH value was 3. The optimum sol-gel film deposition conditions were obtained as following, dipping times was 3, drying temperature was controlled about 80?, drying time was 1h, curing temperature and curing time were 150?and 0.5h, respectively. The structure of composite coatings was microcrystalline and glass. The composite coatings were more compact and had good adhesion. Few tiny cracks were visible on the surface layer. The corrosion resistance of the composite coatings was enhanced significantly.The morphology, composition and corrosion resistance of oxide film at different growth stages were studied. Gaussian03 analysis software firstly used to calculate the bond length, charge and binding energy of adsorption system. The bound state of the coatings was analyzed in molecular level. The results showed that the structure characteristics and the corrosion resistance of oxide film at different growth stages had a corresponding relationship. Sol-gel layer of the MAO coating surface in the adsorption was more stable. For the preparation of high-performance anti-corrosion coating on AZ91D magnesium alloy provided a theoretical basis.It was researched the corrosion development process of the MAO coating and the composite coatings in various corrosive media by using polarization curves and AC impedance techniques. Then different equivalent circuits were put forward to detailed describe the corrosion behavior of the MAO coating and composite coatings in different corrosive media and different corrosion stages. The results showed that the MAO coating and composite coatings showed some similarity corrosion electrochemical behavior and some different characteristics in 3.5-wt.%NaCl,3.5-wt.%Na2SO4 and simulated sea water solution. Corrosion control process would change in different corrosive media and different corrosion stages.Finally, colored composite coatings were successfully prepared on the magnesium alloy. The structure, morphologies, corrosion resistance and corrosion mechanism of colored composite coatings were analyzed by SEM, EDS, FT-IR, polarization curves and cyclic anodic polarization curve. The results showed that the color composite coatings had smooth surface and better appearance. In particular, add CuSO4·5H2O colored composite coatings had the best corrosion resistance. |
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