| Magnesium alloys are widely used in industry because of their excellent properties.However,magnesium is easily oxidized in the atmosphere and marine environment,and its own oxide film can not meet its corrosion resistance,its application is greatly restricted.In order to improve the corrosion resistance of magnesium alloys and their application value,pure magnesium is taken as the research object,and the surface modification method of micro-arc oxidation is used to improve the microstructure and properties of pure magnesium,which provides theoretical support and data support for the application of micro-arc oxidation of magnesium alloys.In this paper,99.95%pure magnesium was used as the matrix,and the phase composition,metallographic structure and thickness,surface and cross-section micro-morphology and element type and distribution of micro-arc oxidation coating were analyzed by X-ray diffraction analyzer(XRD),metallographic microscope(OM),scanning electron microscope(SEM)and energy dispersive spectrometer(EDS).The roughness,corrosion resistance and bonding force of micro-arc oxidation film were detected by pocket roughness meter,coating adhesion automatic scratch tester and electrochemical workstation,and then the effects of shot peening and cryogenic treatment on the structure and properties of pure magnesium micro-arc oxidation coatings were studied.The best treatment methods and treatment time were selected.The following conclusions were obtained:(1)By analyzing the microstructure and properties of the micro-arc oxidation coating formed by 99.95%pure magnesium in three electrolytes of silicate,aluminate,silicate and aluminate,it is known that the silicate electrolyte is the best electrolyte,and the coatings are relatively dense.The coatings are mainly composed of MgO and Mg2SiO4 phases.Mg2SiO4 belongs to high temperature crystalline phase,which has high corrosion resistance and can effectively improve the matrix corrosion resistance.The self-corrosive current density of the coatings formed in the silicate electrolyte system is one order of magnitude smaller than that of the matrix,which is 3.21 × 10-4 A/cm2.(2)99.95%pure magnesium was shot-peened,and then micro-arc oxidation experiments were carried out in silicate electrolyte.The results show that shot peening treatment can significantly increase the surface roughness of the sample,and increase the coating thickness in different degrees.The maximum thickness of the coating is 31.43 ?m when shot peening treatment lasts for 2 min.The coating thickness is relatively thick when shot peening treatment lasts for 6 min,which is 29.96 ?m.The number of surface holes is small,the overall aperture is small,there are no obvious micro-holes and micro-cracks,and the cross-section is closely bound to the matrix.Moreover,the bonding force of the coating is positively correlated with the change of thickness.The maximum bonding force of the coating is 47.26 N when shot peening is applied for 3 min.Shot peening treatment can effectively improve the corrosion resistance of micro-arc oxidation coatings.The self-corrosion current densities of the coatings are 5.793×10-7 A/cm2 and 7.463×10-7 A/cm2,respectively,when shot peening treatment lasts for 2 min and 6 min,which are three orders of magnitude lower than the self-corrosion current densities of the matrix.Comprehensive analysis shows that the coating structure and comprehensive properties are the best when shot peening treatment lasts for 6 min.(3)99.95%pure magnesium was subjected to cryogenic treatment,and then micro-arc oxidation experiments were carried out in silicate electrolyte.The results show that cryogenic treatment increases the surface roughness of the coating,but the thickness of the coating increases or decreases.The maximum thickness of the coating is 26.39 ?m when it is cryogenic for 36 hours.The variation of adhesion is negatively correlated with the change of coating thickness.The maximum bonding force of the coating is 18.66 N at 8 hours of cryogenic treatment,and the coating is closely bound to the substrate without obvious micro-pore and micro-crack,but the thickness of the coating is only 14.78 ?m at this time.Cryogenic treatment can also effectively improve the corrosion resistance of micro-arc oxidation coatings.The coating with the smallest self-corrosive current density is 4.644×10-7 A/cm2,which is three orders of magnitude smaller than the self-corrosive current density of the matrix.Secondly,the self-corrosive current density of the sample coatings after cryogenic treatment for 16 hours is 1.315×10-6 A/cm2,which is two orders of magnitude smaller than that of the matrix.Comprehensive analysis shows that the coating structure and comprehensive properties are the best when cryogenic treatment lasts for 16 hours. |
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