The Organizational Structure Of Medical Cobalt-based Alloys And Corrosion Resistance

In recent years artificial joints made of cobalt-based alloys are gaining renewed interests due to their excellent wear resistance. In present paper, we investigated the cast CoCrMoC alloys and two kinds of forged cobalt-based alloys: CoCrMo and CoNiCrMo. The chemical compositions of these three alloys are based on standards ASTM F75 and ASTM F562. By means of optical microscopy, X-ray diffraction (XRD) and hardness test, the effects of various heat-treatments on microstructure were studied. Then, we studied systematically, by electrochemical method, the corrosion resistance properties of these alloys with various treatments and microstructures, and the effects of electrolytes (with composition simulating body fluid) on corrosion resistance properties of the alloys.The results showed that with the increase of heat treatment temperature, dendritic microstructure of as-cast CoCrMoC alloys was homogenized gradually. After 1h solution treatment at 1200℃, a homogenized FCC microstructure was obtained. Isothermal aging at 800℃ promoted the FCC → HCP martensitic phase transformation. The amount of HCP phase increased with increasing aging time and the FCC matrix totally transformed to HCP phase after 24 hours. Both forged CoCrMo alloy and forged CoNiCrMo alloy were composed of austenite and completed recrystallization for 0.5h at 1000℃. It was found that austenite and martensite coexisted in both hot rolling and heat treated samples of CoCrMo. The amount of martensite further increased after annealing for forged CoCrMo alloy, while there is no such transformation in forged CoNiCrMo alloy.The typical anodic polarization curve of cobalt-based alloys consists of three stages by the value of current densities. The first stage was the spontaneous passive stage at which the potential was lower than 0.5V. In this stage the current density changed a little, which should be attributed to the surface passive film composed of Cr³⁺oxide or hydroxide. When the potential arrived at about 0.5V, the current densityrose rapidly. This stage is called transpassive stage at which the passive film began to dissolve and that is believed to be caused by further oxidation of Cr3+ to Cr6+. In the third stage with the potential above 0.8V, the slope of polarization curve was much small which is believed to be caused by oxidation of H2O on the alloy surface.The alloys showed typical anodic polarization behavior of cobalt-based alloys in saline solution. A secondary passive region was observed at potential about 0.65V in Hanks' solution and the secondary passive region of forged CoNiCrMo alloy was narrower than the other two alloys. The addition of citrate in saline solution decreased the transpassive potential and heat treatments did not significantly influence the anodic polarization behavior for all three alloys. Furthermore, the Ecorr values of three alloys were measured and exhibited the same sequence in the three electrolytes above: the forged CoCrMo is the highest and forged CoNiCrMo is the lowest.