| For magnesium alloy,it is urgent to improve its corrosion resistance,and the main way to improve the corrosion resistance of magnesium alloy is to use surface treatment.Due to the electrochemical characteristics of magnesium alloys,traditional single modification methods can not meet the application requirements of magnesium alloys.Therefore,in order to further improve the corrosion resistance of magnesium alloy,it is very important to select the correct composite coating system.In view of a variety of surface treatment processes,micro-arc oxidation(MAO)modification technology is low-cost,environmentally friendly and has excellent performance.However,its surface has holes and cracks,and its corrosion resistance still has a huge space for improvement.Therefore,the follow-up treatment of MAO coating on magnesium alloy is very important.In this study,the micro-arc oxidation coating on magnesium alloy was prepared by hydrothermal treatment and inorganic salt sealing technology.The micromorphology,microstructure and phase composition of coating samples prepared under different conditions were tested and analyzed.Meanwhile,the bonding force,self-healing performance and corrosion resistance of the coating were evaluated by bonding force test,electrochemical performance test and full immersion corrosion test.The MAO/PA/Ce composite coating with long-term corrosion resistance and self-healing ability was successfully prepared on AZ31B magnesium alloy by MAO and hydrothermal method.Firstly,phytic acid was used to modify the MAO coating,which can enhance the binding strength between Ce layer and MAO layer,and the time of phytic acid modification was determined.The influence of different hydrothermal time and temperature on the composite coating was explored,and the optimized hydrothermal temperature was 110?and the optimized hydrothermal treatment time was 1 h.The outer layer of MAO/PA/Ce composite coating prepared under optimized hydrothermal conditions is composed of cerium spherical particles,and the adhesion between the coating and the matrix reaches 4B grade,which solves the problem of poor adhesion of cerium conversion film.Compared with the matrix,the results of electrochemical test showed that the corrosion resistance of magnesium alloy was significantly improved by MAO/PA/Ce composite coating,and the self-corrosion current density decreased by two orders of magnitude.The self-healing performance of MAO/PA/Ce composite coating was verified by long-term immersion EIS test and scratch repair test.When the coating was damaged,the self-healing coating could release cerium ions into the damaged area,and the membrane impedance of the sample could still reach 1.09×10~4?·cm~2 after immersion for 14 days.The Mg(OH)2/Mg3(PO4)2 composite coating on MAO treated AZ31B magnesium alloy was prepared by one-step hydrothermal method.In situ growth of Mg(OH)2/Mg3(PO4)2 on MAO coating was achieved by using a single Na3PO4 solution as hydrothermal solution.The influence of different hydrothermal solutions and hydrothermal conditions on MAO/Mg(OH)2/Mg3(PO4)2 coating was investigated.The experiment proved that only in Na3PO4 solution,the composite coating with good density could be prepared under the optimized hydrothermal conditions.Finally,the hydrothermal treatment temperature was 110?and the hydrothermal treatment time was 18 h.The Mg(OH)2/Mg3(PO4)2 composite coating can completely seal the micropores and microcracks in the MAO coating by the in-situ Mg(OH)2 layer and a large number of spherical Mg3(PO4)2 particles,and shows the highest 5B rating in the coating adhesion scratch test.The self-corrosion current density of MAO/Mg(OH)2/Mg3(PO4)2 samples decreased by three orders of magnitude compared with the magnesium alloy matrix,and the impedance value of magnesium alloy increased from 2.96×10~2?·cm~2 to 2.48×10~6?·cm~2. |
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