| In recent years,with the development of deformation technology of Mg alloy at room temperature,micro profiles such as fine wire,fine pipe,etc.,have been successfully produced through the technology of cumulative multi pass cold drawing combined with intermediate annealing.These materials have great potential in the field of biomedical materials,such as biodegradable staples,sutures,cardiovascular stents and fiber-reinforced orthopedic fixation composites.Although Mg alloy,as a degradable material,makes use of the corrosion characteristics of Mg,it also puts forward higher requirements for the basic research of Mg corrosion.First of all,the fast corrosion rate is still an obstacle in the field of medical materials.The problem to match the corrosion rate of Mg alloy with the application requirements is still to be solved.Secondly,Mg and its alloys are very prone to local corrosion,which can lead to premature or sudden failure of devices.In view of the above problems,the microstructure evolution of pure Mg,Z2(Mg-2wt.%Zn)and Z4(Mg-4wt.%Zn)during hot extrusion,cold drawing and heat treatment was studied.Immersion corrosion tests were carried out in 5wt.%NaCl solution for different processed materials,and the corrosion laws of pure Mg and Mg-Zn alloys were systematically revealed.The typical fiber texture<11(?)0>and<10(?)0>//ED formed in extruded pure Mg and Mg-Zn wires.However,the micro crystallographic orientation distribution of pure Mg was not uniform.The grains in the marginal area was more orderly distributed:not only<11(?)0>and<10(?)0>parallel to ED,but also the basal plane was parallel to ND(normal direction).However,most of the basal planes in the central area were perpendicular to ND.At the same time,some of the grains had the same orientation as the grains in the marginal area.Compared with the marginal area,most of the grains rotated along ED in the central area,contributing to the crystallographic orientation gradient.With the cold drawing strain of 7%?21%,the crystallographic orientation gradient of pure Mg still existed.The results of electrochemical tests on the cylindrical surface of the wire showed that,for Mg and Mg-Zn alloys,the corrosion potential and corrosion current of the extruded wire were reduced after cold drawing.For Z2 wire with different processing conditions,the surface corrosion rates calculated by hydrogen evolution method and electrochemical method were basically the same.However,the value of corrosion rate measured by weight loss method was obviously higher than the above two methods.It is indicated that weight loss method is not suitable for measuring the corrosion rate of Mg fine wires in NaCl solution.The results of hydrogen evolution test showed that the corrosion rate is lower when the corrosion tends to spread evenly on the surface.However,when there was serious localized corrosion,the specimen usually showed a faster corrosion rate.For pure Mg,the average corrosion rate of specimens with localized corrosion was about 4 times higher than that of uniformly corroded specimens.On this basis,this paper focuses on the short-term corrosion behavior of Mg alloy,in order to reveal the development mechanism of localized corrosion.SEM+EBSD method is useful to observe the relationship between the location of corrosion initiation and microstructure simultaneously.The results were very consistent with the SKP potential distribution results,indicating that this method is reasonable and reliable for the study of local corrosion of Mg alloy.Based on the theory of multiphase micro couple,the local corrosion micro couple model of magnesium alloy was improved and established.In this model,the microstructure with different corrosion tendency in magnesium alloy was regarded as microelectrode,and the adjacent microelectrodes were regarded as corrosion micro couple.The types of corrosion couple and the properties of microelectrode are the thermodynamic basis of the development of local corrosion.In this paper,the adj acent grains with different orientations were regarded as micro couples,and the properties were calculated by the first principle calculation.The surface energy of {0001} plane in Mg was the lowest(0.48J/m~2),indicating that the basal plane was most stable.The corrosion rate of the cylinder plane was 15?17 times higher than the basal plane.The ratio of {10(?)0} to {11(?)0} was 1.15,indicating the corrosion rate of the two planes were very close.The corrosion rate of hot extruded wire decreased first and then increased with the increase of Zn content.The order of corrosion potential of the extruded samples was Mg,Z4,Z2.According to the microstructure and corrosion behavior of the longitudinal section of the wire,the corrosion performance of Mg-Zn alloys was not only determined by the content of Zn,but also closely related to the microstructure.The dynamic recrystallization of Z2 and Z4 alloys was inhomogeneous during hot extrusion,forming fine-grained strip structure along the extrusion direction.There was obvious potential difference between the strip structure and the matrix.The strip structure and the matrix formed micro couple,resulting in localized corrosion in Mg-Zn alloys.The central area of the extruded pure Mg specimen was more prone to localized corrosion,while the corrosion of the marginal area was relatively uniform.This is because in the marginal area,the crystallographic orientation of grains was more orderly distributed.Thus,the driving force of local corrosion provided by the micro couple was small.In the central area,the surface was composed of grains with various orientations,thus the driving force of corrosion provided by the micro couple was larger.Cold drawing had little effect on the hydrogen evolution corrosion rate of Z2 and Z4 wires.There was no obvious improvement of local corrosion either.The main reason is that after cold drawing deformation,the strip structure still existed.There was obvious potential difference between strip and non-strip structure,providing driving force of local corrosion development in Z2 and Z4.After annealing at high temperature,the crystallographic orientation gradient disappeared.Thus,the driving force of local corrosion decreased,contributing to a lower corrosion rate.For cold drawn Z2 wire,complete recrystallization occurred through proper annealing treatment.This eliminated the strip structure,resulting in a lower corrosion rate of Z2 wire.However,annealing process had no significant effect on the corrosion rate of cold drawn Z4 wire,because the strip structure defects in Z4 were more difficult to be eliminated by heat treatment. |
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