| Because of its specific advantages such as low density, easy machining and excellent mechanical property, magnesium alloy is believed to be highly practicable in airline industry, aerospace industry, war industry, transportation industry, biomedical research, electronic industry and so on. However, the poor corrosion resistance of magnesium alloy hinders its further applications for it can be eroded easily by oxygen, water and salt in the open air. Therefore, improving the corrosion resistance of magnesium alloy has become an important research topic in this field.This paper outlines the characteristics, application, corrosion and protection of magnesium alloy. Based on the development tendency of protection technologies for magnesium alloy and the precious work of our laboratory, environmental microarc oxidation (MAO) technology of AZ91D magnesium alloy, the working mechanism of benzoate additive and the corrosion behavior of MAO film in simulated body fluid were studied in this thesis. The followings are the main points:1. An environmental friendly alkaline borate electrolyte using benzoate (sodium benzoate (NaBz), or potassium acid phathalate (KAP) additive was developed. The effects of NaBz and KAP on the MAO process and the properties of the MAO film were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy (EIS), respectively. The results showed that in the presence of benzoate, violent sparking and gas release were restrained, and a moderate MAO condition was obtained. The quality of the MAO film was greatly improved. The size of the pores was reduced, the micro-cracks on the MAO film were eliminated, and the MAO film with compact structure and smoothing appearance was produced. Electrochemical testing results showed the obtained MAO film was excellent in corrosion resistance.2. The working mechanism of benzoate additive was approached. The effect of benzoate in the alkaline borate electrolyte on the anodic process and MAO process was investigated. The adsorption behavior of benzoate on the surface of AZ91D magnesium alloy was studied by EIS. The oxidation process of AZ91D magnesium alloy was studied by current density transient curves and phenomenon observation. The results indicated that the adsorption of benzoate on the surface of AZ91D magnesium alloy was spontaneous and followed the Temkin adsorption rule. Benzoate affected the anodic process of magnesium alloy. In the presence of benzoate. a kind of oxidation film with denser and uniformer structure was prepared and it had higher electrical resistivity. Due to the denser, smoother structure and the higher electrical resistivity, the current density in MAO stage was reduced and the sparking became fine and homogeneous. Gas evolution was also restrained. At this moderate MAO condition, the channels of sparking and gas became smaller, ablation phenomenon was eliminated and the oxidation film with dense structure and high corrosion resistance was formed.3. The corrosion resistance of the MAO treated magnesium alloy in the simulated body fluid (SBF) was studied. AZ91D magnesium alloy was treated by MAO in alkaline borate electrolyte with benzoate additive. Its corrosion resistance was investigated by SEM, XRD, EDS, Tafel, EIS and immersion test. The results showed that the prepared MAO film did not contain Cr6+, F", Mn2+ and other harmful chemicals and was safe as medical implant material. The MAO treated magnesium alloy was stable in SBF. Immersion test showed the weight loss, the average corrosion rate and the pH variation of the MAO treated magnesium alloy were far lower than that of the untreated ones, suggesting better durability and biocompatibility.In short, environmental friendly MAO process, the working mechanism of additive and the corrosion resistance of the MAO treated magnesium alloy in SBF were studied in this thesis. These probing works may provide new insights for the development of new MAO process, working mechanism of additive and application of magnesium alloy for medical purpose.
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