Effects Of Cyclic Tensile Stress And Plastic Strain On The Degradation Behavior Of Pure Magnesium In Simulated Body Fluids

Compared with other implanted metal materials,magnesium and its alloys have great advantages due to their low Young's modulus,good biocompatibility and degradability.Therefore,magnesium and its alloys have great development prospects in the field of degradable implant materials.However,since the equilibrium potential of magnesium is too low,it is very easy to fail due to corrosion,and magnesium is subjected to stress after being implanted in the body.Therefore,it is necessary to study its corrosion performance in body fluids and mechanical environment,This can provide reference and guidance for its clinical application by exploring its corrosion mechanism.In this paper,the distilled pure magnesium was used to produce as-extruded magnesium by vacuum melting and extrusion process.The microstructure,mechanical properties and corrosion resistance of extruded magnesium were studied by optical microscopy,scanning electron microscopy,electronic universal testing machine,electrochemical workstation and cyclic stress corrosion test system.The effects on degradation behavior of Mg in simulated body fluids resulted by cyclic tensile stress and plastic strain were systematically studied and the possible corrosion mechanisms were explored.The results of stress corrosion test show that the cyclic tensile stress could effect the corrosion performance of pure magnesium.On the one hand,the polarization resistance of pure magnesium would increase slower after loading stress,and it takes longer time to reach the stable value compared with the samples without stress.On the other hand,the stable value of the polarization resistance of pure magnesium after loading stress is smaller than the case where no stress is loaded.Before samples fracture,the stable values of the polarization resistance of pure magnesium were approximately equal when loading different cyclic tensile stresses.It imply that the magnitude of stress couldn't effect the polarization resistance of pure magnesium.It is because that the stress could accelerate the facture of corrosion product film,and the transform of the corrosion product film from a single layer structure to a double layer or a multilayer structure.Although this transform could reduce the protection of pure magnesium matrix by the corrosion product film,it will increase the resistance of the corrosion product film to stress.Loading stress could increase the self-corrosion current density of pure magnesium in the SBF solution and accelerate the corrosion of pure magnesium.When loading 35%cyclic tensile stress,the self-corrosion current density of pure magnesium of being loaded a 35%cyclic tensile stress is three times of the pure magnesium without a cyclic tensile stress at the immersion process.Plastic strain could reduce the corrosion resistance of pure magnesium,different surfaces suffered different effects.Plastic strain will seriously deteriorate the corrosion resistance along the strain direction,forming a strip-shaped corrosion groove on the surface of the sample,and the greater the plastic strain generated,the worse the corrosion resistance along the strain direction;however,it has little effect on the corrosion resistance in the direction of perpendicular to strain.The dislocation and twins generated during plastic strain maybe the factors of effecting the corrosion resistance of pure magnesium.On the one hand,these defects could store a certain stress resulting in stress corrosion.On the other hand,these dislocations and twinning may reduce the corrosion potential in situ and causing galvanic corrosion.The specific corrosion mechanism needs further investigation.