The role of grain refinement and film formation potential on the electrochemical behavior of commercial pure titanium in Hank's physiological solution

Abstract

In this study, the electrochemical behavior and semiconducting properties of the passive film formed on the nano-grained commercial pure titanium (CP-Ti), successfully made by seven-cycle accumulative roll bonding process, were compared to those for annealed CP-Ti in Hank's physiological solution at 37°C. Polarization plots and electrochemical impedance spectroscopy measurements revealed that the passive behavior of the nano-grained sample was significantly improved in comparison with that of annealed CP-Ti. Mott–Schottky analysis showed that the passive film formed on the nano-grained CP-Ti had a lower donor density and reduced flat-band potential than annealed sample. Moreover, based on the Mott–Schottky analysis in conjunction with the point defect model, it was shown that with increasing formation potential, the calculated donor density for both annealed and nano-grained samples decreases exponentially and the thickness of the passive film increases linearly along with the formation potential. These observations were consistent with the point defect model predictions, noting that the point defects within the passive film are metal interstitials, oxygen vacancies, or both. From the viewpoint of passive stability, nano-grained CP-Ti appeared to be more suitable for implant applications compared to that of annealed CP-Ti, mainly due to the formation of thicker and less defective passive film.