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

Abstract

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 HClO 4 containing fluoride with various concentrations ranging from 0 to 0.1 M. The promoting effect of fluoride ͑especially when ͓F − ͔ Ͼ 1 ϫ 10 −3 M͒ 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 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 − ͔ Յ 1 ϫ 10 −3 M, leading to a high surface coverage of ss Ϸ 1, while the chemical dissolution reaction is the rate-determining step at higher fluoride concentrations of ͓F − ͔ Ͼ 1 ϫ 10 −3 M, leading to a decreased surface coverage of ss Ͻ 0.5.Titanium ͑and its alloys͒ is one of the most important metal materials widely used in engineering and technological applications. 1 Titanium-based materials possess excellent corrosion resistance in various electrolyte solutions due to the easy formation of a highly protective oxide film at the titanium ͑and its alloys͒ surface. But some aggressive inorganic anions such as F − and Cl − existing in aqueous environments can attack the passivity of titanium and its alloys. [2][3][4] The effects of solution pH and fluoride concentration on the anticorrosion properties of the surface passive TiO 2 films formed by anodic oxidation of titanium at different passivation potentials were widely investigated by many researchers using various electrochemical and surface analytical techniques. 3,5-10 The active-passive transition kinetics of titanium in electrolyte solutions without 11-15 and with fluoride 9,16 was also studied using various electrochemical methods. However, very few studies were reported on the dissolution mechanism and kinetics of titanium in fluoride-containing solutions at open-circuit potentials ͑OCPs͒. 16 Recently, we investigated the influence of fluoride concentration and the film formation potential on the bulk and surface properties of anodic oxide films ͑as well as their breakdown͒ formed on titanium in fluoride-containing perchloric solutions. 3 The passivity of titanium would be destroyed by the chemical dissolution of the oxide if the fluoride concentration was higher than 1 ϫ 10 −3 M. 3 This work investigates the effect of fluoride on the anodic dissolution kinetics of metal ...

Determination and explanation of the pH-related critical fluoride concentration of pure titanium in acidic solutions using electrochemical methods

“…Determination of the critical fluoride concentration OCP measurements have been used by many researchers to determine the critical fluoride concentration, which was regarded as the maximum fluoride concentration where the OCP increased with time [12][13][14][15]. However, the critical value obtained by OCP measurements was not consistent with that obtained by the potentiodynamic polarization tests for titanium in aerated 0.05 M H 2 SO 4 with fluoride ions [15].…”

“…However, trace amounts of fluoride ions in the condensates of flue gases can deteriorate the corrosion resistance of titanium by interacting with the protective oxide film and forming soluble titanium-fluoride compounds [2,9,10]. But the destructive effect prevails only when the fluoride concentration exceeds a critical value, which has been found to be related with pH [11][12][13][14]. The maximum concentration where the open circuit potential increased with immersion time was determined as the critical fluoride concentration by many researchers [12][13][14].…”

“…But the destructive effect prevails only when the fluoride concentration exceeds a critical value, which has been found to be related with pH [11][12][13][14]. The maximum concentration where the open circuit potential increased with immersion time was determined as the critical fluoride concentration by many researchers [12][13][14]. In our previous work, the theoretical critical fluoride concentration was defined as the concentration where the maximum anodic current density (I m ) was equal to the cathodic current density ((| I c |) Em ) at the critical potential (E m ), by investigating the corrosion behaviors of titanium in fluoride-containing 0.05 M H 2 SO 4 [15].…”

One‐Step Synthesis of CoS‐Doped β‐Co(OH)2@Amorphous MoS2+x Hybrid Catalyst Grown on Nickel Foam for High‐Performance Electrochemical Overall Water Splitting

“…The impedance data were fi t to an equivalent circuit of the corresponding Armstrong and Henderson model [ 48 ] and calibrated to charge transfer resistances. 5 mg of catalysts were dispersed in 1 mL solution (10:9:1 = water/ethanol/5 wt% Nafi on solution), and sonicated at least 30 min to form a homogeneous ink.…”

“…Electrochemical impedance spectroscopy (EIS) was studied with the frequency sweeping from 10 mHz to 500 kHz using a 10 mV AC impedance. The impedance data were fi t to an equivalent circuit of the corresponding Armstrong and Henderson model [ 48 ] and calibrated to charge transfer resistances. All potentials were determined with respect to RHE using the following equation…”