Corrosion Performance Of ZK60 Magnesium Alloys With Different Mixed Crystal Grain Structures And Their Surface Protection

ZK60 magnesium alloys present a significant perspective as high-strength metal materials in the field of biomedical and aerospace due to low density,high load-bearing capacity and good biocompatibility.However,the high strength and high ductility cannot be combined of ZK60 magnesium alloys limit its development.The mixed crystal structure magnesium alloy contains the structure of fine grains wrapping coarse grains（bimodal structure）,so it can maintain high strength and high ductility.However,there are few studies on the corrosion behavior of ZK60 magnesium alloy with mixed crystal structure.Therefore,corrosion behaviors of four different crystal-structured ZK60 magnesium alloys were studied to reveal the correlation between the corrosion and mixed crystal structure in this work.Meanwhile,the phosphate chemical conversion coating process on ZK60 magnesium alloy was studied to provide a reference for ZK60magnesium alloys and different microstructure magnesium alloys surface chemical conversion coating in this work.Investigating and comparing the corrosion behaviors of three different mixed crystal structure samples and rolled ZK60 magnesium alloy in 0.1 mol/L Na Cl solution,it demonstrated that the microstructure has a strong influence on the corrosion.According to different heat treatment time,the corrosion resistance order of the four samples is 1 h>4 h>8 h>0 h.Moreover,the corrosion of the heat-treated specimens all started with pitting corrosion and gradually developed into filiform corrosion.The unheat-treated specimens due to poor corrosion resistance,in the initial immersion stage has produced filiform corrosion.Meanwhile,corrosion pits of the mixed crystal structure specimens will first occur in the coarse grain area and the intersection of the coarse and fine grain areas,and then the pitting pits will gradually increase in size.With the increase in immersion time the fine grain area will appear around the fine grain black corrosion,and eventually develop into filiform corrosion spread to the entire surface of the specimen.A response surface methodology was used to investigate the conversion conditions and conversion fluid concentration of the calcium phosphate conversion coating process respectively,and the optimum process for the conversion coating was determined.It demonstrated that the coating provides good protection against the substrate,with a reduction in corrosion current density from 25.91μA·cm^-2to 0.38μA·cm^-2.Moreover,a slight difference in corrosion behavior between coating and substrate.Corrosion of the ZK60 magnesium alloy proceeds in a filamentary manner at the beginning of the immersion,and then the corrosion filaments cover the entire surface of the specimen causing deeper damage to the alloy.The corrosion of the conversion coating starts with pitting,filamentary corrosion occurs in the middle of the immersion period.In the later part of the immersion period filamentary corrosion and pitting attack the specimen simultaneously.Investigating and comparing the corrosion behaviors of the phosphate conversion coatings on the three mixed crystal structure samples,it revealed that the different microstructures have great influence on the performance of conversion coatings.The reason was attributed to the shorter heat treatment time the more fine grains and more resistant to corrosion.The higher number of active sites on the surface of the specimen to facilitate coating formation,and its higher corrosion resistance indicates a slower surface reaction during the transformation process,therefore,a dense coating layer can be obtained.