Preparation And Corrosion Resistance Of Strontium Phosphate Conversion Coating On AZ91D Magnesium Alloy

Magnesium alloy is used in the repair of human bone because of its advantages of non-toxicity,biodegradability and biomechanical compatibility.In addition,magnesium alloy can completely eliminate the need for re-surgical removal of implants after fracture healing,but its clinical application is limited because of its poor corrosion resistance.Chemical conversion coating treatment has the advantages of low-energy and easy operation,which can effectively improve the corrosion resistance of magnesium alloy and reduce the corrosion rate.In this paper,a biocompatible strontium phosphate conversion coating was prepared to improve its corrosion resistance.Strontium phosphate conversion coating was prepared on the surface of AZ91D magnesium alloy.Scanning electron microscope observation showed that the coating was composed of irregular massive crystals and flake crystals.The cross-sectional morphology showed that the thickness of strontium phosphate conversion coating was5?7?m.Through EDS energy spectrum analysis,the coating is mainly composed of O,P,Sr and a small amount of Mg elements,and has a bilayer film structure,and the main phase of the film is Sr HPO₄ and a small amount of Mg HPO₄and Sr₃(PO₄)₂ by XRD and XPS analysis.The corrosion resistance of strontium phosphate conversion coating was verified by immersion test,hydrogen evolution test and electrochemical test.According to the test of open circuit potential and the change of hydrogen evolution rate in the process of strontium phosphate conversion coating formation,it was reasonable to divide the coating formation process into three stages,and the coating formation mechanism of strontium phosphate conversion coating is proposed.The effects of reaction temperature and p H on the morphology,thickness and corrosion resistance of the coating were discussed.The experimental results indicated that the conversion coating formed at 65°C and p H3.5 is compact and uniform and has the best corrosion resistance in Hank's solution.The corrosion behavior of strontium phosphate conversion coating prepared at different temperatures in Hank's solution was studied by AC impedance test,and it was analyzed by SEM and EDS that the strontium phosphate coating layer was not dissolved after soaking in Hank's solution for 7 days,and there were a lot of P,Sr and a small amount of Mg and Ca elements on the surface,and the corrosion products were calcium phosphate and carbonate.The effect of pretreatment process on the corrosion resistance of strontium phosphate conversion coating on AZ91D magnesium alloy was studied.The results indicated that acid pickling alone is more beneficial to improve the corrosion resistance of strontium phosphate coating than acid pickling+alkali washing and acid pickling+alkali washing+acid pickling,and the surface roughness of magnesium alloy substrate before mechanical grinding is different.The micro-morphology and corrosion resistance of strontium phosphate conversion coating are very different.The results indicated that the strontium phosphate conversion coating with surface roughness of about 0.282?m has the best corrosion resistance.The strontium phosphate conversion coating was silanized with silane coupling agent.Scanning electron microscope showed that the pore defects of the coating were closed after silanization treatment,and the thickness of silane composite coating is about 10?m.EDS elemental analysis showed that the surface of the coating was Si,O,P,and there were a large number of Si-O-Si bonds on the surface.Through potentiodynamic polarization curve,AC impedance test and immersion test,it was verified that the corrosion resistance of the coating was obviously improved after silanization treatment,and the corrosion resistance of strontium phosphate conversion coating was improved.