| 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.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 magnesium alloys. On the basis of this electrolyte, the effects of all kinds of factors on the formation on anodic films,their performances and a new treatment method——two step anodic 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. Eddy-current instrument was used to measure the thickness. 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. Dispensation.A good uniform pore anodic film with good adhesion to the substrate and corrosion resistance can be obtained in the electrolyte solution composed of NaOH45g/L,Na2SiO380g/L,natrium citricum5g/L,additive A40 ml/L.2. Surface topography.The surface of coating disposed uniformly many fused mass, which is volcano-like particles and whose top accompanied with micro-holes, also some cracks were existed on the surface. With the oxidation time extending, the sizes of fused mass increased and holes get uniform and their apertures enlarged. 3. Parameter influence.Besides electrolyte solution, the anodization of magnesium alloys is also affected by some other factors, such as current density, treatment time and post treatment, and so on. The research results show that under the same condition, the anodic films became thicker and thicker with anodizing time prolonging and the surface of the anodic film becomes rougher and rougher, the film pores become bigger, too. When anodization is conducted by various current densities, the anodic films are quickly formed with bigger current density, but the surfaces of the films are rougher than that obtained with smaller current density and the micropores become bigger, too. So the process parameters should be adjusted according to the requirement.4. Technology.In a novel environmently friendly anodizing technique of AZ31 magniseum alloys, the second step current density affects the roughness of the surface. The thickness of the film became bigger when the anodizing time of the both two step prolonged. But the anodizing time of the first step was the main factor. |
