| In recent years,magnesium(Mg)and magnesium alloys have received enormous attention for their mechanical properties and degradability in a physiological environment which are similar to the natural bone.Nevertheless,the fast corrosion rate has severely limited their clinical applications.During the process of Mg degradation,large amounts of hydrogen are generated,which will destroy the host tissue.It has been demonstrated that the surface modification is an effective method to enhance the corrosion resistance and control the degradation rate of AZ31 substrateIn this work,in order to enhance the corrosion resistance and mineralization of AZ31 magnesium alloy,a strontium doped apatite coating was successfully synthesized on magnesium alloy substrate via one-step hydrothermal method.The effects of Sr doping content and hydrothermal temperature on the microstructure,composition,electrochemical properties of the coating were investigated,then the corrosion behaviour and mineralization ability was examined by immersion tests.The results showed that when the p H value of solution was 6.3,the temperature was 100?and the reaction time was 3 h,the Sr doped apatite coating consisted of a dense bottom layer and a loose surface layer.Sr doping could effectively reduce the cracks in the dense bottom layer.The potentiodynamic polarization tests in simulated body fluid(SBF)showed that Sr doping could greatly improve the corrosion resistance.When the Sr/Ca molar ratio was 0.10,a crack-free dense coating was obtained and the corrosion current density of the coated magnesium alloy was 2.04?A/cm2,much lower than that of the uncoated counterpart(46.00?A/cm2),suggesting that Sr doped apatite coatings could provide a long-time protection for AZ31magnesium alloy substrate in the immersion process.The long-term corrosion behaviors of the coated samples were performed in vitro immersion tests.Calcium-deficient apatite was rapidly formed on the surface of the sample Sr/Ca 0.10 after immersion for only one day,suggesting its excellent mineralization ability and anti-corrosion ability.During the immersion period,the p H value of samples revealed a little variation in the range of 7.40 to 8.39,the average degradation rate was only 0.17 mg/day?cm2after 33 days'immersion and the impedance about samples soaked for 33 days was 5800 ohm·cm2,higher than that of samples soaked at initial stage,which indicating that the deposited apatite layer could provide a favorable protection for the AZ31 magnesium alloy substrate.Samples with high impedance were prepared at hydrothermal temperature of140?,and p H value of reaction solution and the corrosion behaviour was examined.Results showed that the coating was dense and uniform.The impedance of the coated sample was 500000 ohm·cm2and the corrosion current density was 0.08?A?cm-2,indicating that the coating could effectively improve the corrosion resistance of magnesium alloy in SBF. |
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