| Magnesium alloys have the have been widely used in many fields because of their excellente properties,but magnesium has a low standard potential?-2.37V?,and it that the surface cannot form a self-protective coating layer on its surface,and the poor corrosion resistance greatly limited the development and application of magnesium alloys.Therefore,it is of great theoretical and practical significance to carry out research on corrosion and protection of magnesium alloys.Surface treatment is the most direct and effective way to improve the corrosion resistance of magnesium alloy.Among them,the plasma electrolytic oxidation?PEO?can form a layer of ceramic coating on the surface of magnesium alloy to improve the comprehensive properties of magnesium alloy.However,there are porous,cracks and other defects in the magnesium alloy PEO coating,which affects the protective effect of the coating layer on the substrate.For this reason,it is proposed that the PEO coating is prepared by adding graphene and Y2O3 particles in the electrolyte and make the particles to diffusely grow in the coating.In this paper,we investigate the effects of Y2O3 particles and graphene addition on the microstructure and properties of AZ91 magnesium alloy PEO coating prepared in silicate based electrolyte.Firstly,Y2O3 particles and graphene composite PEO coating were prepared on the surface of AZ91 magnesium alloy.The thickness of the coating was measured by a coating thickness meter and the hardness test was measured by a microhardness test.The surface and cross section morphology of the coating was observed by scanning electron microscope?SEM?.The X-ray energy dispersive spectroscopy?EDS?,and X-ray photoelectron spectroscopy?XPS?were used to analysize the composition of the coatin,X-ray diffraction?XRD?was used to analysize the phase composition of the coating,the electrochemical impedance spectroscopy?EIS?and potentiodynamic polarization curves were used to characterize the corrosion resistance of the coating.Based on the above analysis and test,the coating formation process model of Y2O3particles and graphene PEO coating layer was established,and also the corrosion mechanism of the coating layer was explored.The main research contents and conclusions were as following:?1?The effect of Y2O3 particle size on the microstructure and properties ofPEO coating on AZ91 magnesium alloy was investigated and compared.It was found that Y2O3 particles with particle size of 40nm and 1?m were all successfully incopored into the PEO coating of AZ91magnesium alloy,and the corrosion resistance and hardness of AZ91 magnesium alloy were obviously improved.The size of Y2O3 particles size have different effects on the performance of AZ91 magnesium alloy PEO coating.The effect of 40nm Y2O3 particles on the PEO coating of AZ91 magnesium alloy is better than that of 1um Y2O3 particles.The hardness of AZ91magnesium alloy PEO coating prepared by adding 1g/L 40nm Y2O3 particles is about 103HV higher than that of AZ91 magnesium alloy substrate,and the corrosion current density is6.76×10-8A/cm2,which significantly improves improved the properties of corrosion resistance.?2?The effects of different concentrations of nano-Y2O3 particles on the microstructure and properties of on PEO coating AZ91 magnesium alloy were studied.The results showed that when3 g/L of nano-Y2O3 particles are added to the electrolyte,the coating has the best effect.The hardness of the PEO coating of AZ91 magnesium alloy added with nano-Y2O3 particles increased by more than 10 times,and the corrosion current density was reduced by 3 orders of magnitude compared with AZ91 magnesium alloy matrix,which significantly improved the properties of corrosion resistance.The main phase of the composite PEO coating was Mg2SiO4,MgO,Mg and Y2O3.Among them,the nano-Y2O3 has a negative potential and moves to the magnesium alloy matrix during the reaction process,the nano-Y2O3 particles enter into the pores and cracks of the coating,which compensates for the structural defects of the coating and improves the corrosion resistance of the magnesium alloy.?3?The effects of different concentrations of graphene addition on PEO coating microstructure and properties of AZ91 magnesium alloy were studied.The results showed that with the increase of the concentration of graphene,the denseness of the coating was better and the hardness of the coating was higher.The better the corrosion resistance.When the concentration of graphene added in the electrolyte is 250 mg/L,the PEO coating of AZ91 magnesium alloy prepared has the best performance.The hardness of the coating is about 16 times that of AZ91magnesium alloy matrix,and the corrosion current density is 1.15×10-77 A/cm2,the corrosion current density of AZ91 magnesium alloy is reduced by 3 orders of magnitude.The addition of graphene to the electrolyte does not change the phase composition of the coating,but instead covers the surface of the coating after the PEO coating is formed,thereby blocking the corrosive medium and forming a layer of protection against the PEO coating. |
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