| Magnesium alloy has the advantages of good casting performance,high specific strength,low density,shock resistance,etc.,and has a wide range of application prospects in aerospace,aviation,military and other fields.The poor corrosion resistance of magnesium alloy is the main reason why it cannot be applied on a large scale.Therefore,the surface coating treatment of magnesium alloy can effectively improve the corrosion resistance.The chemical properties of magnesium alloy are very active,and it is difficult for a single protective coating to meet the requirements of magnesium alloy in harsh working conditions.Therefore,composite surface treatment is the future development trend.In the process of service,the surface coating of magnesium alloy will crack or be damaged due to external forces,and the service performance of magnesium alloy will rapidly decline.Currently,protective coatings applicable to magnesium alloy almost have no self-healing ability.Therefore,it is of great significance to study magnesium alloy composite coatings with excellent corrosion resistance and self-healing ability.In this paper,a porous ceramic layer with micro and nano pores was prepared on the surface of magnesium alloy AZ91D by in-situ reactive plasma electrolytic oxidation to improve the adhesion to the substrate.Based on this,a plasma electrolytic oxidation composite coating was prepared using interlayer bimetallic hydroxides and chitosan microspheres modified waterborne epoxy resin.The microstructure,corrosion resistance,and self-healing properties of the plasma electrolytic oxidation composite coating were studied.The microstructure and self-healing morphology of chitosan microspheres,interlayer bimetallic hydroxides,and PEO composite coatings were analyzed using scanning microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,and Fourier transform infrared spectroscopy（FT-IR）.The corrosion resistance and self-healing performance of interlayer bimetallic hydroxides,and PEO composite coatings were characterized using an electrochemical workstation.The conclusions are as follows:The LDHs coating was prepared by hydrothermal method on the surface of magnesium alloy.The LDHs coating was intercalated with nitric acid,stannate and phosphate by ion exchange.The self-corrosion current density of interlayer bimetallic hydroxide coating prepared on magnesium alloy surface is 1～2 orders of magnitude lower than AZ91D substrate,and the corrosion resistance is effectively improved.The LDHs coating modified by stannate and phosphoric acid was soaked for 72h.The phosphoric acid and stannate released by the LDHs coating combined with dissolved magnesium ions to form Mg3（PO4）2 and Mg Sn O3 precipitated and adsorbed on the defect site,forming a film layer with the regenerated LDHs and Mg（OH）2 at the defect site and slowing down the corrosion rate of the magnesium alloy.Interlayer bimetallic hydroxide coating（PEO composite LDHs）was prepared based on plasma electrolytic oxidation coating（PEO composite LDHS）.Compared with single PEO coating,the self-corrosion current density of composite coating is reduced by 1.49～7.29 times,and compared with single LDHs coating,the self-corrosion current density of composite coating is reduced by 2.27～8.11 times,showing better corrosion resistance.The composite coating of stannate and phosphoric acid intercalation has good self-repair ability.The stannate and phosphoric acid are combined with magnesium ions produced at the defect to form a precipitate attached to the defect.Under the joint action of Mg（OH）2and LDHs,the film layer is formed at the defect to delay the corrosion of magnesium alloy.Chitosan microspheres were prepared by emulsion crosslinking method,and the corrosion inhibitor octahydroxyquinoline（8-HQ）was loaded by vacuum impregnation with the loading rate of 11.3%.Chitosan microspheres have a certain PH response,and the release rate is faster under alkaline and acidic conditions.In aqueous solutions with PH=4,7 and 10,the release rate of the solution for 4 hours is 47%,48%and 71%,respectively,which can achieve the effect of sustained release.Compared with the coating without chitosan microspheres modified,the self-corrosion current of the composite water-based epoxy resin coating modified by plasma electrolysis oxidation with chitosan microspheres is nearly doubled,and the polarization resistance is also slightly improved,showing better corrosion resistance.After the prefabrication of defects,the 8-HQ released in the soaked chitosan microspheres combines with Mg2+to form chelate Mg Q2 and adsorbs on the defects.And gradually form a film with Mg（OH）2,delaying the corrosion rate at the defect and reducing the corrosion tendency. |
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