Preparation And Corrosion Resistance Of Polyvinylidene Fluoride Sealing Layers On Micro-arc Oxidation Coatings Of AZ91D Magnesium Alloy

Micro-arc oxidation technology can improve the corrosion resistance of magnesium alloys.However,the oxidation films have a large amount of pores and cracks during the micro-arc oxidation process,causing corrosion resistance to decrease.The corrosive media can contact the magnesium alloy substrate along these channels.So it is necessary to perform sealing treatment on the micro-arc oxide film.The corrosion resistance of polyvinylidene fluoride is great.It can resist the attack of most strong acids,halogens and strong oxidizing substances.In order to improve the corrosion resistance of AZ91D magnesium alloy with micro-arc oxidation film further,the surface of ceramic film was sealed by polyvinylidene fluoride.Scanning electron microscopy and X-ray diffraction were used for observing the microstructure of micro-arc oxidation coating under different sealing process parameters.The contact angle test,drip corrosion test,immersion corrosion tests and polarization curve were used to study the corrosion resistance of sealing layer.The microstructure of micro-arc oxidation coating with sealing treatment by polyvinylidene fluoride were investigated.The results show that:The polyvinylidene fluoride can effectively seal the pores and cracks of the micro-arc oxidation coating,and the surface of sealing layer is dense and flat.Through the experimental comparison found that the sealing effect was best with solution concentration 12g/L,sealing time9min,oxidation voltage is 350V and N₂ low temperature plasma treatment before sealing.The main phase composition of sealing layer are MgO,MgF_2,Mg₂Zr₅O₁₂2 and PVDF,and the phase composition of the film layer under different sealing process parameters is unchanged.The corrosion resistance of micro-arc oxidation coating with sealing treatment by polyvinylidene fluoride were investigated.The results show that:Compared with the unsealed sample,the hydrophobicity of the sample after the polyvinylidene fluoride sealing treatment is better than that of the micro-arc oxidation ceramic film,and water contact angles of samples increase.The color change time of the sample after the sealing treatment has a doubling of growth,and the corrosion rate of the immersion test in the5%NaCl solution reduces,indicating that the sealing treatment significantly improved the corrosion resistance of the micro-arc oxidation layer.The results of polarization curve test show that the corrosion potential of the ceramic coating after sealing treatment increase,the corrosion current density of the sample decreases about two orders of magnitude and the polarization resistance increases about two orders of magnitude,so the corrosion resistance of micro-arc oxidation coating was significantly improved.