Corrosion of thin film cobalt-based magnetic recording media was studied with surface analysis and electrochemical techniques. Corrosion currents of bare cobalt-chromium alloys are inversely proportional to the integral of chromium oxide concentration taken over the near-surface region of the film. For overcoated alloys, the corrosion process involves water adsorption in porous overcoat, preferential cobalt dissolution from the metal alloy, its migration through the overcoat and cobalt hydroxide and/or oxide formation on the overcoat surface. Cobalt concentrations appearing on overcoat surfaces after environmental exposures correlate well with corrosion currents. © 1988, The Electrochemical Society, Inc. All rights reserved.