Study On Passive Behavior And Corrosion Mechanism Of Biomedical CoCrMo Alloy Manufactured Through Additive Manufacturing

CoCrMo alloy manufactured through laser beam powder bed fusion(PBF-LB)technique has achieved more attention in producing implant with complex shape.But during its service in human body environment,failures caused by corrosion sometimes happen and affect its biocompatibility.Study on its microstructure and corrosion behavior is important for further regulation of manufacture process and microstructure,in order to achieve a longer service life.In this paper,the passive behavior and corrosion mechanism of a PBF-LB CoCrMo alloy in 0.9%NaCl solution was studied through different methods including microstructure observation,electrochemical test,atomic force microscope(AFM),nanoindentation test,X-ray photoelectron spectroscopy(XPS)and tribo-corrosion test.Firstly,the scan strategy was optimized in order to obtain less anisotropy in electrochemical behavior,the effect of molten pool boundary(MPB)on passive behavior was analyzed.Secondly,the micro-galvanic effect between nanoscale Laves precipitate and y-Co matrix was studied,the beneficial effect of nanoscale Laves phase on passive behavior was analyzed.Thirdly,the effect of simulated inflammation and fretting condition on corrosion behavior was discussed,the effect of critical H2O2 concentration on passive film composition and roughness was clarified,volume loss caused by mechanical and corrosion factors during tribocorrosion process was analyzed separately.Finally,optimization of mechanical and electrochemical behavior through post production heat treatment method was also explored.The main conclusions are as follows:A CoCrMo alloy with relative density above 99%was manufactured through PBF-LB technique at the volumetric energy density of 129 J/mm3.The matrix of PBF-LB CoCrMo alloy consists mainly of y-Co phase with FCC structure,and littleε-Co phase with HCP structure is observed.Nanoscale Co3Mo2Si intermetallic precipitates(also called Laves phase)uniformly distribute on the matrix.Typical passive behavior of PBF-LB CoCrMo alloy is observed in 0.9%NaCl solution,corrosion of MPB happens preferentially at trans-passive potentials.Compared with the zigzag scan strategy with a rotation of 67° between successive layers(also called scan strategy 1,namely "SS1"),the scan strategy with five different rotation angles(also called scan strategy 2,namely "SS2")results in less anisotropy of microstructure and electrochemical behavior,the anisotropy of passive current density and polarized resistance of samples produced through SS2 are only 19%and 28%,respectively.The nanoscale Laves phase in PBF-LB CoCrMo alloy exhibits a Volta potential of 14 mV more positive than the matrix,this kind of micro-galvanic effect facilitates the formation of uniform passive film,and the polarized resistance of PBF-LB CoCrMo alloy increases linearly with immersion time.After immersion of 36 hours,polarized resistance of PBF-LB CoCrMo alloy is larger than that of traditional wrought high carbon CoCrMo alloy in one or two orders of magnitude.The local segregation of Cr and Mo elements caused by carbides in wrought alloy is detrimental to the formation of uniform passive film.What’s more,the nano hardness of Laves phase in PBF-LB CoCrMo alloy is 16 GPa,about 1.6 times of the matrix,resulting in an increase of micro-hardness and strength of the PBF-LB alloy.The H2O2 concentration in inflammation condition has dual effects on corrosion behavior of PBF-LB CoCrMo alloy.After the addition of H2O2 in 0.9%NaCl solutions,reduction of H2O2,OH*and HO2*plays an important role.A critical concentration of H2O2(about 6 mM)exists,below which the corrosion potential shifts positively as the concentration increases,the oxidation of Cr and Co elements both happen,facilitating the passive film formation;and above which,oxidation of Co element becomes the main anodic reaction.Oxide and hydroxide of Co element accounts for 74%of the composition of passive film formed in solutions with 30 mM H2O2 added,the passive film becomes less protective due to the change in composition and roughness.During the tribo-corrosion tests,the open circuit potential of PBF-LB CoCrMo alloy shifts from-0.4 VSCE to-0.7 VSCE after the start of rubbing.The potentiodynamic polarization curve during rubbing exhibits a larger current density at passive potential region than that measured before the start of rubbing.During the growth of passive film on the electrode in 1～5 s,passivation charge density related to faraday capacitance is about 0.1～1.5 mC.During the tribo-corrosion tests,the mechanical volume loss is proportional to applied normal load and doesn’t change much at different potentials.At anodic potential,the increase of the volume loss is mainly caused by corrosion.After aging treatment of the PBF-LB CoCrMo alloy at 750℃,lamellar ε-Co phase forms according to a Shoji-Nishiyama orientation relationship with γ-Co matrix and nanoscale Laves phase remains uniformly on the matrix,the strength,hardness and corrosion resistance increase at the same time.After solution treatment of the PBF-LB CoCrMo alloy at 1200℃,the fine columnar sub-grains ofγ-Co matrix in the as printed alloy are replaced by larger equiaxed grains with largeangle grain boundaries,the corrosion resistance decreases and pitting happens at matrix around the precipitate,the tensile strength and micro-hardness decreases,but the ductility increases and obvious strain induced martensitic transformation phenomenon is observed around micro-cracks.