| Magnesium alloy has been widely used in the research of biomedical materials in recent years due to its mechanical properties similar to human bone,good biocompatibility,good degradability and low price.The corrosion rate of magnesium alloy is too fast in human body environment,which is easy to dissolve and cannot meet the requirements of service life.Therefore,surface modification is needed.Calcium phosphate coatings are easy to prepare and have good biocompatibility.They are often used in the modification of biomedical materials.In this paper,the corrosion mechanism of AZ31 magnesium alloy was studied by simulated immersion experiment.Fe-doped calcium hydrogen phosphate,polydopamine and polydopamine/fe-doped calcium hydrogen phosphate composite coatings were prepared on AZ31 magnesium alloy surface by liquid deposition,chemical deposition and hydrothermal method,respectively.The morphology,composition,corrosion resistance and coating formation process of the coatings were investigated.The corrosion behavior,corrosion products,corrosion morphology and corrosion rate of AZ31 magnesium alloy in simulated body fluids were studied by simulated body fluid immersion experiments.In the simulated body fluid environment,the magnesium alloy is dissolved on the surface to form an oxide film,which provides a coating-forming power with hydrogen evolution and induces the deposition of the calcium hydrogen phosphate coating on the surface.The corrosion products are mainly composed of Mg O,a small amount of Mg（OH）2 and Ca HPO4·2H2O,and the corrosion products are distributed on the surface in a dry form with large cracks.The corrosion rate decreased with the increase of immersion time,and the average corrosion rate was 23.77 mg/cm2·day after 9 days of immersion.The iron-doped calcium phosphate coating prepared by liquid deposition and the Fe3+ion was successfully doped into the coating and evenly distributed on the surface of it.The effects of p H and Fe3+concentration of conversion solution on the microstructure and properties of the coating were systematically studied,and the coating prepared at p H 6covered the surface of the magnesium alloy uniformly in the form of sheets,with a maximum thickness of about 70μm and a corrosion potential of-0.9888 V.When the molar percentage of Fe3+is 2%,the maximum thickness of the coating is about 50μm,and the corrosion potential is-0.9735 V which had the best corrosion resistance.The polydopamine coating was prepared as an intermediate coating on the surface of AZ31 magnesium alloy by chemical deposition,and the polydopamine coating was the most complete at p H 10,solution concentration of 1 mmol/L and conversion time of 13 h.Furthermore,an iron-doped calcium diphosphate coating was prepared on the polydopamine intermediate coating by hydrothermal method,and the coating covered the surface of the matrix uniformly in a spherical shape.The polydopamine intermediate coating adsorbed Ca2+in solution by electrostatic force to induce the formation of calcium hydrogen phosphate coating.The thickness of the coating prepared after 13 h of hydrothermal heat was the largest,and the corrosion potential was-1.0768 V,which had the best corrosion resistance. |
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