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A model is presented which describes the effect of mechanical and materials parameters on the wear-assisted corrosion rate of passive metals under sliding wear conditions. The model is based on a consideration of contact between the sliding surfaces at multiple asperities and it takes into account the passivation behavior of the metal. Wear experiments were carried out in a reciprocating pin-on-plate tribometer permitting the control of mechanical and electrochemical conditions. An alumina pin was rubbed on nickel, chromium, stainless steel, and titanium alloy plates, in sulfuric acid or sodium sulfate solution. The relative importance of mechanical and electrochemical metal removal was evaluated while applying an anodic potential. Additional experiments were performed under cathodic polarization. The results show that the proposed model can describe correctly the effect on dissolution rate of different mechanical parameters such as applied normal force, stroke length, frequency, and sliding speed. Qualitative agreement was observed with the predicted effect of the materials parameters hardness and passivation charge, but uncertainties concerning the real value of passivation charge and, in some cases, wear of the alumina pin limit the predictive capability of the model when comparing different materials. The experimental results obtained in this study demonstrate that to understand the mutual interactions between mechanical and electrochemical parameters affecting wear-accelerated corrosion it is necessary to look at the tribocorrosion system as a whole.

a b s t r a c tA good biocompatibility, excellent mechanical properties and high corrosion resistance characterize CoCrMo alloys. Therefore they are widely used for artificial joints in biomedical implants. However, the degradation of the implants during service life leads to the release into the body of toxic ions and wear particles. This continuous degradation is of concern for long-term stability of the implants. Published literature has highlighted the relevance of lubrication as well as metallurgical and contact mechanical factors on the degradation of CoCrMo implant alloys. Recent experimental investigations have proposed tribocorrosion, i.e., the interplay of mechanical wear and corrosion by the body fluids, as one of the crucial degradation mechanism of implants. Tribocorrosion is sub-discipline of tribology and corrosion that recently made significant progresses in mechanistic understanding and modelling. The present work aims at evaluating published results on the degradation of CoCrMo alloys using existing tribocorrosion concepts. Results show that wear accelerated corrosion due to mechanical removal of the passive film during sliding is a major contribution to the overall degradation. Further, a transition from low (10 À 6 N/mm 3 m) to high (10 À 4 N/mm 3 m) wear coefficients was found at a threshold electrode potential close to 0.2 V SHE These findings clearly show that electrochemical phenomena play a key role on the tribological behaviour of biomedical CoCrMo alloy implants.

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The corrosion behavior of CoCrMo alloy in simulated body fluids has been analyzed by electrochemical techniques and surface analysis. Interaction of albumin and phosphates present in the body fluids on the passive film of the alloy was also investigated. Electrochemical techniques such as potentiodynamic and potentiostatic polarization and electrochemical impedance spectroscopy were employed. Further, ex situ X-ray photoelectron spectroscopy and Auger electron spectroscopy analysis of the passive films were carried out. The study reveals that phosphates and proteins present in simulated body fluid play a significant role in the electrochemical properties of the metal/oxide/electrolyte interface. Surface analysis showed that both species competitively adsorb on the alloy surface. For a given passive potential, the impedance behavior of passive CoCrMo was found to depend on the way passive conditions were established. A simple model has been developed assuming a multilayer structure of the surface including an outer layer where albumin and phosphate ions adsorb, the passive film ͑inner layer͒, and the metal. This model is consistent with the obtained electrochemical and surface analysis results.

Tribocorrosion of Al-Si-Cu-Mg alloys was investigated in 0.05 M NaCl and 0.1 M NaNO3 solutions under severe sliding and controlled electrochemical conditions. A simple galvanic coupling model was developed to analyze and quantitatively predict the evolution potential of the open circuit potential during tribocorrosion. According to this model and the obtained results, galvanic coupling was established in the NaNO3 solution within the wear track between passive and mechanically depassivated areas. In the NaCl solution, galvanic coupling was established between the whole depassivated wear track and the surrounding area. This difference was attributed to different mechanical properties of the passive surfaces.