Research On Micro-arc Oxidation Protection Of AZ31 Magnesium Alloy Sheet

As one of the lightweight materials,magnesium alloy with its low density,high specific strength,and excellent electromagnetic shielding properties are widely used in the automotive and electronics industry.Magnesium alloy sheet is an important development direction of the magnesium alloy industry in the future,but the poor corrosion resistance of magnesium alloy is vulnerable to corrosion in the atmosphere,which limits its application scope.The corrosion problem of magnesium alloy needs to be solved urgently.The use of micro-arc oxidation technology can be in situ on the surface of the magnesium alloy plate to grow a tightly bonded micro-arc oxidation(MAO)coating with the substrate,thereby improving the corrosion resistance of the substrate magnesium alloy sheet,but due to the high voltage generated during the micro-arc oxidation process is prone to cause the magnesium alloy sheet to occur through the ablation phenomenon,the correct and reasonable regulation of the process parameters is the key to the preparation of the micro-arc oxidation coating.In this paper,orthogonal experiments were carried out to investigate the effects of multi-factor interaction,termination voltage and current density on the structure and corrosion resistance of the micro-arc oxidized coating layer of the AZ31magnesium alloy sheet with a thickness of 0.6mm.A preliminary study was carried out to investigate the effect of coating thickness on the thinning of the foil.The results of the study are as follows:The results of the orthogonal experiments show that frequency,negative duty cycle and oxidation time not only act individually on the coating structure but also the interaction between the three influences the structure of the coating and its corrosion resistance.The extreme difference analysis revealed that the oxidation time had the most significant effect on the coating thickness,but the negative duty cycle also determined the coating thickness to some extent,while frequency had the least effect.In HNO₃ media,the thicker the coating,the better the corrosion resistance,where the oxidation time is four times more decisive than the negative duty cycle.On the contrary,the negative duty cycle and the interaction between the three have a greater influence on the corrosion resistance of the coating in NaCl media.This is not conducive to corrosion resistance in a Cl^-containing corrosive environment.The results show that the porosity and thickness of the coating increases and then decreases with the increase of the termination voltage,and at a termination voltage of450 V,the porosity of the coating is 9.3%and the coating is relatively dense,which shows the best corrosion resistance in NaCl.When the current density is 5A/dm²,the porosity of the coating is lowest and dense,and the corrosion resistance of the coating is best at this time.After micro-arc oxidation,the corrosion resistance of the magnesium alloy sheet is significantly improved,the mechanical properties of the base sheet are not affected after coating,and the micro-hardness of the base magnesium alloy sheet is increased.The thin and dense film layer is not only closely bonded to the base but also has good wear resistance.After micro-arc oxidation of the magnesium alloy foil with a thickness of 0.1 mm,the foil thickness was found to be reduced by approximately half of the film thickness.