| Magnesium alloy has excellent properties, such as low density, high specific strength and specific rigidity, good damping performance, good electromagnetic shielding performance etc. Therefore, magnesium alloy is applied to aviation﹑ automotive and electronic industry increasingly. But corrosion resistance of magnesium alloy is very poor, which has badly limited its application. Usual surface treatment methods, for example, chemical oxidation and anodic oxidation, are able to protect magnesium alloy, but its corrosion resistance and external appearance etc. can't meet the needs of the application development. In recent years, People have developed some new technology, microarc oxidation can meet above demands.Microarc oxidation is a new material surface treatment technology, which developed on the basis of anodic oxidation. Microarc oxidation is also entitled microplasma oxidation or anodic spark deposition. Al,Mg,Ti or their alloys are placed in electrolyte, using electrochemistry means to produce spark discharge in the microholes of the material surface, and ceramic coating can be formed on the effect of thermochemistry, plasma chemistry and electrochemistry. AZ91D magnesium alloy was treated in different electrolytes. The optimum formula of microarc oxidation electrolyte was fixed on after lots of screening experiment and one of orthogonal tests, which consisted of three ingredients. The electrolyte is innocuous and doesn't pollute the environment. In the optimum electrolyte, the film is slippery after microarc oxidation. Polarization curve﹑polarization resistence and EIS experiment demenstrated that the corrosion resistance of magnesium alloy AZ91D was improved, compared with untreated AZ91D. Then the effect of treatmentment time﹑current density and post treatment on the film performance were studied, the optimum processing parameters were found. When a new ingredient was added into above-mentioned optimum electrolyte, the corrosion resistance of AZ91D sample after microarc oxidation was further enhanced. So the electrolyte composition was improved on. Using metallurgical microscope and SEM, the surface and cross-section of samples were observed. The composition and structure of film were studied by means of EDAX and XRD. The microarc oxidation film formed on the substrate of AZ91D was even and compact, but there were some holes in film. The thickness of film was over ten micron, film and substrate combined close. The film was mainly composed of and MgAl2O4(spinel) and MgO(periclase,syn). It was found that, as a result of instantaneous high temperature of electric spark, Al2O3 and MgO were sintered to form MgAl2O4(spinel).
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