Ti and Ti alloys are well known for their excellent chemical stability and biocompatibility. Ti metal shows high corrosion resistance due to the presence of protective native oxide film. Although TiO₂ layer is stable and protective in action, it can be dissolved in HF [1-3].

Anodic dissolution of Ti in acidic fluoride medium shows three distinct regions, viz. active, passive and transpassive [1,2]. The anodic current increases with potential in the active region and it decreases with potential in the passive region. The current increases slowly with potential in the transpassive region. Transpassive dissolution has been extensively studied [1,3]. Using potentiodynamic polarization and electrochemical impedance spectroscopic data, the anodic dissolution of Ti in 0.1 M HF in the active and passive regime has been investigated recently and a four step mechanism has been proposed [2].

HF is a weak acid and is known to form various species in solution, as described in the equation below.

HF ↔ H⁺+F^-

HF+F^-↔HF₂^-

HF+HF₂^-↔H₂F₃^-

The equilibrium constants for the above reactions are K1 = 6.84 × 10^-4, K2 = 5 and K3 =0.58 respectively [4]. Although a four step mechanism has been proposed to explain the anodic dissolution of Ti in HF[2], the influence of HF concentration has not been incorporated in the mechanism yet. It is important to quantify each species present in solution and their influence on individual steps of the reaction.

In this work, the effect of varying the concentration of HF on Ti dissolution was investigated. A rotating disk Ti electrode was used in a three electrode cell with Ag/AgCl (in 3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was varied from 25 mM to 1000 mM.  In all the experiments, 1 M Na₂SO₄ was used as the supporting electrolyte_. The electrode was rotated at 900 rpm. Potentiodynamic polarization experiments were conducted at a scan rate of 20 mV/s, starting at -250 mV below open circuit potential and ending in the transpassive regime on the anodic side. A few experiments were also conducted at a scan rate of 2 mV/s, in solutions of 50 mM, 500 mM and 1000 mM HF concentrations, and the current response was the same at both 2 mV/s and 20 mV/s scan rates. The results are presented in Fig. A.

The anodic polarization curves (Fig. A) show that the active and passive regimes are clearly visible in all the HF concentrations studied here. The inset shows the results at lower concentrations, expanded for clarity. The peak current value increases with an increase in HF concentration. The location of peak potential also shifts to more anodic values at higher HF concentration. The potential and logarithm of the absolute value of current in the cathodic and anodic regime (active region only) are presented in Fig. B. The cathodic and anodic branches of potentiodynamic polarization were extrapolated to estimate the corrosion current.

The estimated corrosion current values and the peak current values near active-passive transition were correlated with nominal HF concentration and as well as with the concentration of individual species occurring at equilibrium. In order to identify the effect of individual species unambiguously, a few experiments were also run by adding H₂SO₄ or KF to the experimental solutions (results not shown here). The corresponding changes in the peak current values and corrosion current estimates were also analysed. The results indicate the corrosion current and peak current can be expressed as proportional to [H⁺]^α and [F^-]^β , where α is 0.72 and β is 0.92. This suggests that undissociated HF is likely to play a major role in Ti dissolution.

 References:

1. Kong, D-S., Langmuir. 24 (2008) 5324- 5331.
2. Fasmin, F., Praveen, B.V.S., Ramanathan, S., J. Electrochem. Soc. 162 (9) (2015) H640-H610.
3. Acevedo-Pena, P., Gonzalez, I., J. Electrochem. Soc. 159 (3) (2012) C101-C108
4. Messnaoui B., J. Solution Electrochemistry 37 (2008) 715-726