EIS of carburised CoCrMo: Evolution of parameters characterising the metal-electrolyte interface

•Carburising increases the polarisation resistance of CoCrMo's passive film.•A drop in impedance was recorded beyond 20 h of immersion.•Time constant dispersion within the passive film was reduced after carburising.•Carburising caused faster film growth kinetics of the passive film at OCP.

Corrosion-wear is of major concern for Cobalt-Chromium-Molybdenum (CoCrMo) hip implants as it leads to premature failure of the prosthesis and treating the CoCrMo surface using low temperature carburising treatment is a plausible investigation to mitigate such problem. This diffusion treatment introduces a supersaturated carbon solid solution, termed S-phase, which hardens the CoCrMo without detriment to the corrosion resistance. There are only basic studies on the electrochemical characteristics of S-phase surface and therefore, from a scientific and industrial point of view, there is a lot of scope to investigate it further. This research uses electrochemical impedance spectroscopy (EIS) to study the metal/electrolyte interface and how it changes after carburising and over a time span of 24 h. The technique reveals that the CoCrMo/Ringer's solution interface contains a highly resistant passive film and an electrical double layer when under equilibrium conditions. Comparison of the impedance results attained when testing untreated and carburised CoCrMo reveals that the treated metal has a passive film which has higher polarisation resistance compared to that of the untreated alloy. This may have been a consequence of the fact that the treated metal's passive film has lower time constant dispersion and faster film growth kinetics compared to the untreated alloy's passive film. In addition, both untreated and carburised interfaces suffer a considerable loss in their polarisation resistance after 20 h of testing which indicates a non-monotonic relationship between passive film impedance and electrochemical potential.