Electrochemical behavior of cobalt–chromium alloys in a simulated physiological solution

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 a potential of about 0.4 V (SCE). This study examined the electrochemical behavior of MP35N in phosphate‐buffered saline (PBS). Tests were performed on mechanically polished MP35N rod and electropolished MP35N wire using cyclic potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy. In both cases, the surface oxide was shown to undergo two distinct changes as the potential is increased. The first change appears to be associated with solid‐state oxidation reactions involving the conversion of Cr(III) to Cr(VI) and of Co(II) to Co(III) at similar potentials. The second change involves transpassive dissolution that results in the release of chromate and nickel ions. At potentials below where these reactions occur, the impedance spectra showed near‐capacitive behavior, and the data could be fitted by a parallel resistance‐capacitance (as a constant phase element) circuit associated with the passive oxide film. The thickness of the oxide was determined from the capacitance and found to be consistent with surface analytical results reported in the literature. Resistivities obtained from resistance values indicated changes in the film composition as the potential was increased, particularly after the onset of the solid‐state reactions. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res 2010