Degradation Behavior Of Magnesium And Its Alloy In Simulated Physiological Stress Environment

At present,most of the in vitro degradation experiments of magnesium and its alloys are static immersion experiments,however,the physiological environment of the human body will apply low-frequency dynamic loading on the serviced magnesium alloy devices.The existence of static stress and dynamic stress will influence the degradation behavior of magnesium alloy implants,then,the degradation rate and degradation mechanism of magnesium alloy will be affected.AZ31B magnesium alloy was mainly researched in this paper,and an electrochemical test apparatus for simulating physiological stress environment was constructed.The degradation behavior of magnesium alloy under unstressed condition,static tensile loading,static compressive loading and cycled dynamic loading were studied.The effects of loading magnitude,frequency,form and immersion time on the degradation behavior and degradation rate of magnesium alloy were studied systematically.It is found that:The second phase particles are distributed along the axial direction during the extrusion process of the wrought AZ31B magnesium alloy,which results in the existence of zonal corrosion traces along the axial direction of the specimen surface;the surface of the specimen substrate lacks membrane protection in the early stage of corrosion,and the corrosion rate is faster;With the prolongation of immersion time,insoluble corrosion products and Ca-P phase deposit on the surface of the specimen,which prevents the diffusion of corrosive media and electron transport,and slows down the corrosion rate of magnesium alloy substrate.The polarization resistance of pure magnesium increases firstly and then decreased during the degradation process,while the polarization resistance of AZ31B magnesium alloy increases gradually and tends to be steady.Compared with the AZ31B magnesium alloy under unstressed condition,the static tensile load and the compressive load accelerate the corrosion of the magnesium alloy,and the corrosion rate increases under high stress magnitude and tensile load.The acceleration effect of the applied load on the corroded alloy was obvious in the first 24hours,and gradually become flat after 24 hours.Due to the negative difference effect of magnesium alloy,the corrosion current density calculated by the weight loss method is higher than the corrosion current density obtained by the linear polarization method.In the initial stage?within 24h?,the anodic corrosion current density of the magnesium alloy and the applied stress obey the linear relationship of7)7)7)7)4)4)_{?8?8?8?8?8?8?8?8?^well;with the prolongation of immersion time?24¹20h?,massive corrosion product generates and deposition product accumulates on the surface of the specimen,which lead to the experimental data gradually deviates from the theoretical model.The corrosion rate of magnesium alloy is more sensitive to high stress magnitude and tensile load.The erosive ions in solution promote the nucleation of corrosion pits on the surface of magnesium alloy,the applied static stress increases the surface free energy and reduces the solid binding energy of magnesium alloy,thus the reaction activation energy of magnesium alloy corrosion is reduced.Meanwhile,the applied loading will cause stress concentration on the bottom of pits which promotes the growth and connection of the pit.Compared with the static load,the cyclic tensile and compressive load will further accelerate the corrosion of the magnesium alloy,and acceleration is more obvious under high stress amplitude and cycle frequency of the dynamic load.The effect of cyclic alternating load and corrosive environment on the degradation behavior of magnesium alloys is a significant interaction instead of a simple superposition.,the surface of the magnesium alloy will repeat the process of elastic expansion of rod-slip of film-fracture of film-dissolution of film-passivation of substrate.The higher the frequency of the alternating load,the more times the process occurs in a certain period,and then the erosive ions attack the substrate of the magnesium alloy matrix,therefore,the magnesium alloy is more susceptible to corrosion damage.Under the dynamic alternating load,the corrosion current density and frequency match a linear relationship of7?7?7?7?4?4?0¹?1}The thermal activation energy decreases with the increase of the stress amplitude and the frequency of dynamic load,and then the corrosion reaction rate is accelerated.