Corrosion Behavior Of Modified Medical Magnesium Alloy In Simulated Body Fluid

Compared with traditional medical implanted metal materials,the density of magnesium alloy with higher specific stiffness and strength is closer to human bone,it can be degraded in physiological environment and is suitable for medical implant materials.However,the corrosion resistance of magnesium alloys in physiological environments is poor,which greatly limits its clinical application.Therefore,how to improve the corrosion resistance of magnesium alloy by surface modification technology,and study the corrosion behavior of modified magnesium alloy in simulated body fluid has become the focus of current research.Three modified coatings were prepared using AZ80 magnesium alloy as basic material（BM）.The first is to prepare LSP coating sample by laser shock peening（LSP）technology.The second is to prepare MAO coating sample by microarc oxidation（MAO）method.The third is to perform LSP layer on the surface of the magnesium alloy,and then prepare the LSP/MAO composite coating sample by MAO.Immersion test and stress corrosion test were carried out in simulated body fluid（SBF）for BM,LSP,MAO and LSP/MAO samples.In the immersion test,the surface and cross sectional morphologies of the coatings were characterized by scanning electron microscopy（SEM）,while their chemical composition were obtained by x-ray diffraction（XRD）analysis.The corrosion evolution behavior of the coatings during immersion was compared and analyzed,and a physical model of degradation process was established.The stress corrosion test was used a slow stress strain tensile（SSRT）tester at two strain rates of 1×10^-6S^-1 and 5.3×10^-7S^-1,respectively.The micro-morphology of the fracture was used to analyze the stress corrosion sensitivity of the four specimens.In addition,the SSRT test at 5.3×10^-7S^-1 strain rate was combined with electrochemical synchronization test to analyze the impedance evolution and crack initiation mechanism of the four samples during SSRT.Based on the above experiments and analysis,the following conclusions are obtained:1)Immersion test results showed that both MAO coating and LSP/MAO composite coating can improve the corrosion resistance of magnesium alloy matrix,but the corrosion resistance of LSP/MAO composite coating is better than that of MAO coating at each stages of degradation process.At the later stage of immersion,the MAO coating has nearly failed,but the LSP/MAO composite coating still has a good protective effect on the matrix.2)SEM results showed that the thickness of LSP/MAO composite coating is larger than that of MAO coating at each degradation stages.At 1800 h,the bonding force between MAO coating and matrix becomes worse,and the coating is nearly peeling off,while the bonding force between LSP/MAO composite coating and matrix is still strong.3)XRD results showed that Mg（OH）₂ and Ca₂P₂O₇ corrosion product layers were formed on the surface of the coating during the degradation process.The formation of Ca/P compound also indicated that MAO coating and LSP/MAO composite coating have good biological activity and compatibility.4)Stress corrosion tests results showed that LSP layer,MAO coating and LSP/MAO composite coating can improve the mechanical properties and stress corrosion sensitivity of magnesium alloy.LSP/MAO composite coating has the best resistance to stress corrosion.5)Stress corrosion tests results at different strain rates showed that the stress corrosion susceptibility of matrix and three modified specimens increases with the decrease of strain rates.6)LSP treatment results in the formation of nanocrystalline structure on the surface of magnesium alloys,which improves the corrosion resistance of magnesium alloys to SBF.Grain refinement also causes more uniform deformation and less stress concentration when the material is subjected to external stress,and thus improves the mechanical properties of the material.