A cobalt-based alloy, MP35N, is currently used as a conductor material for thin wires in pacemaker leads. In this study, electrochemical impedance spectroscopy and cyclic polarization measurements of the alloy in a phosphate-buffered saline solution with 100 mM H2O2 indicated a high corrosion resistance and no passivity breakdown. Scanning electron microscopy and energy-dispersive spectroscopy analyses showed the existence of micrometer-sized TiN particles in the microstructure. Volta potential mapping by scanning Kelvin probe force microscopy revealed that the Volta potential of the TiN particles wits several tens of millivolts higher than that of the alloy matrix. By in situ atomic force microscopy measurements in the solution, small pitlike holes were observed in the vicinity of TiN particles and in other areas after 2 weeks of exposure. However, deposition of corrosion products within the pits was observed in the following days, and the pitlike holes were sealed upon further exposure. The results indicated a negligible risk for localized corrosion of this alloy despite the TiN particles present in the microstructure. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3174383] All rights reserved.