Electrochemical Investigation of Titanium Electrode in Aqueous Tetrafluoroborate Solution and in Aqueous Formate Solution

Titanium alloy electrodes are widely employed as both cathodes and anodic substrates. The superior corrosion resistance and low density of titanium makes it a useful material for industrial applications. In oxygen containing environments such as air or water, a thin titania layer spontaneously forms on bare titanium surfaces. Historically, the operative surface states of titanium have been termed: active [essentially metallic], transitional, and passive [comprising a titania over-layer]. In this project, the electrochemical response of a titanium electrode in contact with aqueous tetrafluoroborate solution [or in contact with aqueous formate solution] was monitored to tentatively identify the surface states. Tetrafluoroboric acid is strongly ionizable in water and resultant HBF 4 solutions are able to oxidize various metals. Starting with either HBF 4 or KBF 4 , the electrochemical response of a titanium disk electrode in contact with aqueous tetrafluoroborate solutions [sparged with argon gas] was recorded. Cyclic voltammograms of 0.1 molar HBF 4 on titanium, without additional electrolyte, showed two cathodic features near -0.51 V and -0.95 V versus Ag-AgCl electrode [see figure below]. The more negative feature is posited to correspond to active state titanium with H 2 evolution. Scan reversal near -0.95 V to the positive direction promptly generates a prominent anodic peak near -0.65 V though the post-peak current remains anodic and rather steady out to +2.0 V. The prominent anodic peak may correspond to dissolution of titanium metal. Interestingly, the shapes of the cyclic voltammograms of aqueous KBF 4 solutions were experimentally determined to be pH dependent. By contrast, formic acid is weakly ionizable in aqueous solutions. Cyclic voltammograms of 3.5 molar HCO 2 H solutions on titanium showed a cathodic feature near -0.87 V that may correspond to active state titanium and, upon positive direction scanning, an anodic feature that starts near -0.25 V and plateaus after +0.2 V. The anodic plateau is posited to correspond to passive state titanium. Cycling between about +0.55 V and -1.0 V led to a detectable negative potential shift in the H 2 evolution reaction [possibly due to titania formation during positive scanning]. At the similar hydrogen ion condition of pH 1, cyclic voltammograms of aqueous HCO 2 H and HBF 4 were different: the prominent anodic peak was not observed for formic acid. Figure 1