B. G. Pound, Exponent, Menlo Park, CA
Several cobalt-chromium alloys such as MP35N are used for biomedical implants. The electrochemical behavior of these alloys in the passive range differs from that of other biomedical alloys. In particular, their cyclic potentiodynamic polarization curves exhibit an increase in current at potentials of about 0.4 V (SCE). This study examined the electrochemical behavior of MP35N in phosphate-buffered saline. Tests were performed on electro polished MP35N implants using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. The impedance data were analyzed using equivalent circuit models to evaluate the capacitive and resistive components of the surface oxide at different potentials. The cyclic polarization results together with the impedance data were used to characterize changes in the electrochemical behavior with the increase in current.
Summary: The electrochemical behavior of MP35N was examined in phosphate-buffered saline. An increase in current was observed in the cyclic potentiodynamic polarization curve at approximately 0.4 V (SCE). A combination of cyclic potentiodynamic polarization and electrochemical impedance spectroscopy was employed to show that the initial current increase is related to a solid state oxidation reaction. The potential-pH diagram for cobalt at 37 degC was used together with surface analytical data in the literature to establish that the oxidation involves the conversion of CoO to Co3O4. Thus, cobalt-chromium alloys as a group represent a case where an increase in current during cyclic polarization should not be assumed to reflect either localized corrosion to release metal ions or simply the oxidation of water to form oxygen. Analysis of the impedance data showed that the thickness and resistivity of the oxide varied with potential in a manner indicating that the oxide becomes increasingly defective as it grows.
