Titanium and cobalt-based alloys are widely used as implant materials and typically exhibit excellent biocompatibility due in part to their passive-state corrosion resistance. However, when an orthopaedic device involves the potential for relative sliding at the surface of these active-passive alloys, as at fretting-prone interfaces between mating modular components, fracture of the passivating surface oxide, deformation of the nearsurface with the generation of particulate wear debris, and concomitant active-state corrosion may result. In this study a parametric approach was adopted to elucidate mechanistic information on the effect of the constituents of the aqueous corrosion environment and mixed-metal coupling on the fretting corrosion behavior of Ti-6-4 and Co-Cr-Mo. Similar-metal fretting experiments were performed to establish an understanding of the fretting corrosion behavior of each alloy without the possibility of confounding galvanic effects. The analysis methods included measurements of the electrochemical potential and current, dissolution product concentrations, friction coefficients, and fretted surface areas in addition to qualitative characterizations of the fretted surface morphology and solid corrosion product compositions. As this work was part of a continuing effort in the study of titanium biocompatibility, the focus throughout was on the fretting-enhanced corrosion of Ti-6-4.

Serum, albumin, and EDTA each reduced the fretting corrosion damage to Ti-6-4 similar-metal couples. The mechanisms underlying their effect included boundary lubrication of the fretted surfaces, complexation of the dissolution products, and, for serum and albumin, inhibition of the cathodic reactions. Co-Cr-Mo similar-metal fretting in the pH 7.4 model solutions was also effected by serum additions to the base electrolyte solution, leading to inhibition of the fretting-enhanced anodic reactions. A hydrated CrPO₄ solid corrosion product was formed when fretting at pH 7.4.

Significant galvanic effects during Co-Cr-Mo/Ti-6-4 mixed-metal fretting were revealed by the anodic polarization of the fretted Ti-6-4 surfaces. Fretting-enhanced Ti dissolution was not observed with mixed-metal fretting in the pH 7.4 model solutions. It was proposed that the fretted Co-Cr-Mo surface acts as both an abrader and as a cathode during fretting against Ti-6-4.