The Research On An Environment-friendly Process For Magnesium Alloy AZ31 Anodizing

As the lightest structure material in engineering, magnesium alloys are widely used in automotive, electronic and aeronautical industries. Magnesium alloy AZ31 has good ductibility and medium intensity, the price is lower as compared with other magnesium alloys, it is the most widely used anamorphic magnesium alloy. However, their applications are severely limited by their low corrosion resistance.Anodic oxidation processes are simple and the cost is lower in contrast to other surface treatment methods. The anodic coatings with the porous structure can protect and decorate the metal. They are a good base of other organic coatings, anodic oxidation process is an effective and common surface treatment method for magnesium and its alloys.In this dissertation, magnesium alloy AZ31 was applied as the experimental material. A new environmental protection type process has been developed after lots of experiments.The anodic electrolyte contains none of chromate and other poisonous ingredient to the environment and people, but it could significantly improve corrosion resistance of magnesium alloys. On the basis of this electrolyte, the effects of all kinds of factors on the formation of anodic films and their performances have been researched so as to establish the optimum process of anodic oxidation.Constant current anodization was introduced to oxidize the magnesium alloy samples. Potentiodynamic polarization measurement was used to evaluate the corrosion resistance of anodic films rapidly. Horizontal metallographic microscopy (HMM) was used to measure the thickness and to observe cross-section morphology. Morphologies of coatings, the element compositions of the anodizing coating and the crystal structure was charactered by SEM, EDX and X-ray diffraction(XRD). Through investigating the anodic films formed in various anodizing electrolyte solutions, the conclusions are drawn as follows:(1) A good uniform pore anodic film with good adhesion to the substrate and corrosion resistance can be obtained in the electrolyte solution composed of KOH 50 g·dm^-3,Na₂SiO₃ 60 g·dm^-3,Na₂B₄O₇ 40 g·dm^-3.(2) More and more attention is paid on phytic acid in the field of metal protection because of its unique structure and chemical character.The effect of various concentration of phytic acid in the anodic electrolyte solution has been investigated. The results shows that a better anodic film can be obtained in the electrolyte main...