Study On The Influence And Mechanism Of Second Phase On The Corrosion Resistance Of Magnesium Alloy Treated By Ultrasonic Vibration

Magnesium and its alloys are widely used in many fields due to their low density,high specific strength,high specific height and excellent casting properties.However,its high chemical activity makes its corrosion resistance poor,which limits its wider and comprehensive application.The microstructure and corrosion resistance of magnesium and its alloys before and after ultrasonic vibration were characterized by Optical Microscope(OM),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),Potentiodynamic Polarization Curve(PDP),Electrochemical Impedance Spectroscopy(EIS)and simulation.The microstructure and corrosion resistance of pure magnesium,Mg-3Al-1Zn and Mg-4Zn alloys before and after ultrasonic vibration were studied.The ?-Mg17Al12 phase changed from a thick and continuous network structure to a fine and uniform short bar/particle.The MgZn2 phase changed from a large network to a strip structure.The results of weight loss,hydrogen evolution and electrochemical tests showed that the alloy treated by ultrasound had a lower corrosion rate in 3.5wt.%NaCl solution.The reduction of corrosion rate was mainly attributed to the reduction of grain refinement.The ?-Mg was preferentially dissolved for both cases of untreated and ultrasonic treated alloys as evident by the micrograph of alloys following immersion test.The result of numerical simulation of micro-galvanic corrosion of Mg-3Al-1Zn alloys showed that the anodic average current density of ultrasonic-treated alloy was about 17.6%lower than that of-the untreated alloy.By combining this work with the simulation method,we find that ultrasonic vibration can refine the particle size and improve the corrosion resistance of alloy.The results of microstructure and corrosion resistance of AZ91-xGd(x=0,0.5,1.0,1.5)alloy treated by ultrasonic vibration show that:The addition of Gd led to the formation of many fine Al2Gd/Al-Mn-Gd particles that consumed Al and reduced the volume fraction of ?-Mg17Al12 phase.Meanwhile,the ?-Mg17Al12 phase morphology in ultrasound-treated AZ91-Gd alloys changed from semi-continuous reticular structure to rod-shaped and granular structure.The ultrasound-treated AZ91-1.0 wt%Gd alloy with fine granular ?-Mg17Al12 phase and Al2Gd/Al-Mn-Gd particles showed better corrosion resistance than other ultrasound-treated alloys.Micro-galvanic corrosion was formed and rapidly extended to the matrix alloy interior along local coarser reticular ?-Mg17Al12 phase,then causing localized serious corrosion.The finer and dispersed rod-shaped ?-Mg17Al12 phase and Al2Gd/Al-Mn-Gd particles led to micro-galvanic corrosion uniformly distribute on the surface of alloys,and formed a uniform corrosion layer with 18 mm thickness.The results of microstructure and corrosion resistance of AE43 alloy before and after ultrasonic vibration indicated that the ?-Mg17Al12 phase changed from a continuous network to a granular.The coarse Mg2Gd phase becomes a fine uniformly distributed particle.The results of immersion experiment,polarization curve and impedance spectrum showed that the corrosion resistance of the alloy after ultrasonic vibration was significantly improved.The corrosion current density decreases and the radius of capacitive loop increases.Impedance spectrum fitting result shows that the film resistance and charge transfer resistance of the alloy are increased after ultrasonic vibration.AFM results implied that the potential difference between the matrix and the second phase decreases obviously after ultrasonic vibration.The above results show that ultrasonic vibration has a positive effect on corrosion resistance of the alloy.