Investigation Into the Repassivation Kinetics of CoCrMo for Applications in a Simulated Biological Environment

The re-passivation kinetics and composition of the passive film of CoCrMo alloys in simulated body fluids have been investigated, with key emphasis being to assess the effect that proteins have on these features. The kinetics were analyzed using potentiostatic polarization, applying a second order exponential decay to the current transients obtained, which consists of two phases: coverage and thickening. Repassivation occurred quickest in a phosphate environment with presence of bovine serum albumin (BSA) hindering the process as it inhibits access of the oxidant. By using X-ray photoelectron spectroscopy (XPS) the composition of the re-passivated layer was studied. As expected, the film is mainly composed of chromium (III) oxide with small amounts of cobalt (II) oxide and molybdenum oxides (IV-VI). When exposed to BSA the percentage of molybdenum in the passive film decreases. This is shown to be due to the protein having a high affinity for the element causing it to be lost to solution when the metal was exposed to corrosion.