Preparation And Properties Of Az91d Magnesium Alloy Micro-arc Oxidation Ceramic Coating

Magnesium alloys have an extensive application foreground because of their low density, high specific strength, high specific stiffness, good shock absorption and good electro-magnetic shielding etc. However magnesium alloys have a poor corrosion resistance that hinders their development. In order to improve the anti-corrosion properties, the surface must be modified. In this paper, AZ91D magnesium alloys had been selected for the study and a pulse power was used. The alloy was treated on the surface by micro-arc oxidation(MAO) process, and ceramic coating was prepared and the performance was studied in details.We initially selected some chemicals for the electrolyte solution which were suitable for MAO on AZ91D Magnesium alloy surface. The result showed that a good MAO film can be performed on AZ91D alloy surface in the aluminate system or silicate system. In aluminate system, NaAlO₂ is the main substance and NaOH is auxiliary into the film. Montmorillonite, EDTA and Arab resin play the role of performance improvement in aluminate system. Na₂SiO₃ is the main substance and NaOH is the auxiliary into the film, and montmorillonite, EDTA, Arab resin play the role of performance improvement in silicate system.The composition of the aluminate system was optimized in the thesis. The optimized main components were: NaAlO₂ was 30g/L, NaOH was 20g/L. On this basis, though a single factor experiment and an orthogonal experiment, the additives of aluminate system on the impact of MAO film were obtained as follows: Montmorillonite was 3g/L, 5%Arab resin was 50mL/L. By a single factor experiment and an orthogonal experiment, the conditions of MAO process were obtained as follows: Oxidation time was 30min, the termination voltage was 180V, pulse period was 20ms and occupy-empty ratio was 50%.The optimized composition of the silicate system was studied. The main components of silicate system were selected: Na₂SiO₃ was 30g/L, NaOH was 20g/L. The additives in the solution of MAO were studied by an orthogonal experiment. The optimized composition of additives are obtained as follows: Montmorillonite was 4g/L, EDTA was 2g/ L and 5% Arab resin was 50mL/L. The conditions of MAO process in silicate solution were obtained as follows: Oxidation time was 30min, the termination voltage was 200V, pulse period was 30ms and occupy-empty ratio was 40%.It was analyzed that morphology, composition, phase composition and corrosion resistance properties of the MAO ceramic layer that was formed in the optimal experimental conditions by SEM, EDS, XRD, electrochemical corrosion and so on. SEM results showed the surface of ceramic coating was relatively smooth and dense, but still existed some small micro-porous. Vertical cross-section of ceramic membrane showed it was composed of an outer loosed layer and an inner compact layer. Owing to the metallurgical bonding, the inner layer had good interfacial connection with the substrate. EDS results showed that the components contained in electrolyte were involved in the MAO film by MAO process. By changing the electrolyte composition, concentration and other parameters, we can adjust the phase composition, which is conducive to the further modification of the film. XRD results showed that in aluminate system, MAO coating are mainly composed of Mg, MgO, MgAl₂O₄, Al₁₂Mg₁₇ and so on. While in silicate systems, MAO coating are mainly composed of MgO, Mg₂SiO₄, SiO₂ and so on. According to results of the electrochemical experiments and the corrosion immersion test, we found the corrosion resistance of MAO film was significantly increased compared with the substance of magnesium alloy. The corrosion resistance of MAO film on AZ91D magnesium alloy surface formed in aluminate solution was better than in silicate solution.