Electrochemical Anodic Dissolution Kinetics of Titanium in Fluoride-Containing Perchloric Acid Solutions at Open-Circuit Potentials

The influence of fluoride on the anodic dissolution kinetics of titanium was studied by several electrochemical techniques at steady-state open-circuit potentials in 1.0 M HClO4 containing fluoride with various concentrations ranging from 0 to 0.1 M. The promoting effect of fluoride (especially when [F-]>1x10(-3) M) on the anodic dissolution behavior of titanium was characterized and discussed by taking into account the changes in the estimated electrochemical and kinetic parameters. The faradaic impedance for the anodic dissolution of titanium was analyzed both by fitting based on an equivalent electrical circuit model and by theoretical derivation based on a two-step mechanism involving one adsorbed intermediate species. By correlating the faradaic impedance expression derived from the dissolution mechanism with that deduced from the equivalent electrical circuit model, some important kinetic parameters (such as the apparent rate constants K-1 and K-2 and the surface coverage theta(ss)) could be estimated from the equivalent circuit elemental parameters (such as R-ct, R-a, and C-a). The charge-transfer reaction was the rate-determining step at lower fluoride concentrations of [F-]< 1x10(-3) M, leading to a high surface coverage of theta(ss)approximate to 1, while the chemical dissolution reaction is the rate-determining step at higher fluoride concentrations of [F-]>1x10(-3) M, leading to a decreased surface coverage of theta(ss)< 0.5.