The effect of static applied potential on the 24-hour impedance behavior of commercially pure titanium in simulated biological conditions

Potential step impedance analysis was utilized to evaluate the electrochemical impedance of commercially pure titanium (cpTi) samples that were polarized to static potentials (range from 21000 mV to +1000 mV vs. Ag/AgCl) and immersed in physiologically relevant electrolytes [phosphate buffered saline (PBS) and cell culture medium with 10% fetal bovine serum (AMEM + FBS)] for 24 hrs. The cpTi impedance outcomes were a complex function of voltage, solution constituents, and immersion time. In the 0 mV to +1000 mV range, oxide growth was observed over 24 hr immersion in both solutions based on decreasing current density (∼10^-6 A/cm² to ∼10^-8 A/cm ²) and increasing R_p (200 kΩ cm² to ∼10 MΩ cm²). Below 0 mV, the 24 hr R_p decreased with negative potential to ∼15 kX cm2. After 24 hr immersion, oxide dissolution and/or adsorption of organic species caused the capacitance to increase at -1000 mV (AMEM + FBS & PBS) and at -600 mV (AMEM + FBS only). Twenty-four hours of immersion in AMEM + FBS at -1000 mV and -600 mV produced a surface coloration that is likely due to alteration of oxide valance state and/or doping level. This work shows that Ti surface oxide and its electrochemical behavior can be altered dramatically under sustained cathodic potentials.