Fretting Crevice Corrosion of Metallic Biomaterials: Instrument Development and Materials Analysis

Mechanically assisted corrosion (fretting corrosion, tribocorrosion etc.,) of metallic biomaterials is a primary concern for numerous implant applications, particularly in the performance of highly-loaded medical devices. While the basic underlying concepts of fretting corrosion or tribocorrosion and fretting crevice corrosion are well known, there remains a need to develop an integrated systematic method for the analysis of fretting corrosion involving metal-on-metal contacts. In this work, a newly developed fretting corrosion test system and a fretting corrosion model was used to explore the effects of the fundamental mechanical (load, frequency) and electrochemical (potential) factors on the fretting corrosion performance of Ti6Al4V/ Ti6Al4V, CoCrMo/ Ti6Al4V, and CoCrMo/CoCrMo material couples. Tests were performed by either varying the potential and frequency at fixed, load, displacement and solution conditions, or varying the applied load at fixed potential, motion and solution conditions. This work also characterized the impedance behavior of different material couples at different potentials and/or contact loads, both prior to and post fretting using electrochemical impedance spectroscopy methods and a crevice impedance model. The results of this study showed that fretting corrosion is affected by material couples, potential, frequency, normal load and the motion conditions at the interface. In particular, fretting currents and coefficient of friction (COF) varied with potential and load and were higher for Ti6Al4V/Ti6Al4V couple. Increase in applied fretting frequency linearly increased the fretting current densities in the regions where the passive film is stable. Information on the mechanical energy dissipated at the interface, the sticking behavior, and the load dependence of the inter-asperity distance calculated using the model elucidated the influence of mechanical factors on the experimental results. The results also showed that there is a strong correlation between the mechanical energy dissipated at the interface and the fretting currents generated. The impedance data showed that impedances of different material interfaces were affected by the crevice, potential, extent of corrosion damage and changes in solution chemistry. In all the experiments performed in this study the fretting corrosion behavior of CoCrMo/Ti6Al4V and CoCrMo/CoCrMo was similar, and dominated by the CoCrMo surface. The model established a clear link between mechanical and electrochemical aspects of fretting corrosion and provided an excellent basis to explain many of the observed behaviors of these interfaces