Insight into the Electrochemical and Semiconducting Properties of Native Oxide Films on Ti Metal and Its Ti–6Al–4V Alloy in Borate Buffer Solutions

Titanium and titanium alloys are intriguing materials having widespread industrial and medical applications due to their excellent corrosion resistance and biocompatibility. These entice our interest to scrutinize the electrochemical and semiconducting properties of the naturally grown surficial oxide films on Ti metal and its Ti–6Al–4V alloy in borate buffer solutions of pH 6, 7 and 8 at ambient temperature. The work was carried out using open circuit potential measurements (OCP), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), as well as Mott–Schottky (MS) analysis. For both tested electrodes, the development of OCP in the noble direction with time reveals a continuous growth for their native passive films. PDP results disclose that passive performance of Ti metal increases with increasing solution pH as indicative from the positive shift in the corrosion potential (Ecorr), significant increase in the polarization resistance (Rp) and decrease in corrosion current density (icorr) values. However, for its Ti–6Al–4V alloy, a contrary trend is perceived. The experimental EIS data are fitted to a suitable electrical equivalent circuit model and a number of important parameters including roughness factor (α), oxide film resistance (Rf) and its thickness (df) are all estimated and discussed. The charge carrier donor concentration (Nd) and flat band potential (Efb) estimated from MS analysis for the two samples were also correlated with the obtained electrochemical results. Values for the parameters α, Rf and df are all found to be higher for the metal than its alloy. Meanwhile, raising the solution pH leads these parameters to be boosted for Ti metal and conversely getting lower for its alloy. This concludes a trend going opposite way depending on the tested Ti material that is in agreement with the MS analysis revealing a higher Nd value for the alloy as compared to its metal. Such trend is owing to formation of a more defective passive oxide film on the Ti–6Al–4V alloy as compared to the one grown on its base metal.