Passive films and their stability are getting increased attention in the field of electrochemistry due to a number of applications, [1, 2]. Valve metals such as Ti, Zr, Nb and Ta form insulating or semiconducting oxide films under anodically polarization conditions [2]. Zirconium and its alloys are widely used in nuclear and chemical industries [3] due to its high corrosion resistance even in aggressive media. This resistance is due to the formation of a stable, protective passive ZrO₂ film [4].  Although ZrO₂ is passive, it can be attacked by halide ions. Previously, Zr corrosion in the presence of halide ions such as Cl^-, F^-, I^-, Br^- has been investigated [4]. In particular, the anodic polarization of Zirconium in fluorinated concentrated nitric acid medium was studied and a seven step model with three intermediate species has been proposed to explain the results [5].However, a later study proposed that the mechanism of Zr dissolution in HF is altered by HNO₃  and that the nitric acid is simultaneously reduced along with Zr dissolution [6]. Hence these results may not be directly applicable to nitric acid free systems. 

HF being a weak acid dissociates to a limited extent, and can form multiple species as given below in the three equilibrium reactions.

HF ↔ H⁺+F^-

HF+F^-↔HF₂^-

HF+HF₂^-↔H₂F₃^-

The equilibrium constants respectively are K1 = 6.84 × 10^-4, K2 = 5, and K3 = 0.58 [7].

In this work, the effect of varying the concentration of HF on Zr dissolution was investigated. A rotating disk Zr electrode was used in a conventional three electrode cell with Ag/AgCl (in 3.5 M KCl) and Pt wire as reference and counter electrode respectively. HF concentration was varied from 5 mM to 200 mM. In all the experiments, 0.1 M Na₂SO₄was used as the supporting electrolyte. The electrode was rotated at 900 rpm to minimize the mass transport limitations. Potentiodynamic polarization experiments were conducted starting at 250 mV below open circuit potential and ending at 2500 mV above OCP on the anodic side. The scan rate was maintained at 2 mV/s in solutions with HF concentration of 50 mM or less. At 100 mM and 200 mM HF, the scan rates were  increased to 20 mV/s since a black film formed on the electrode at lower scan rates and caused a sudden change in current values. 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 potential the current in the cathodic and anodic regime (restricted to active region) are presented in Fig. B, with the current in logarithmic scale. The corrosion currents were estimated by extrapolating the cathodic and anodic branches.

Fig. A shows that the current increases in the active region, attains a peak value and there is a slight decrease in the current at higher potentials.  Thepeak current value increases with an increase in HF concentration. At higher concentration, the peak potential location shifts to more positive values at higher HF concentrations. In each solution, the remaining amount of HF, H⁺, F^-, HF₂^- were calculated using the equilibrium constants and individual correlation to the corrosion current and peak current were evaluated. The corrosion and peak current values were correlated with the concentration of individual species occurring at equilibrium. A few experiments were also conducted in the presence of H₂SO₄ or KF to help identify the species involved in the dissolution. The corresponding changes in the peak current values and corrosion current estimates were also analysed. The results indicate the peak current can be expressed as proportional to [H⁺]^α and [F^-]^βwhere α is less than 1 and β is greater than 1, indicating that undissociated HF plays an important role in the reaction mechanism.

References:

1. Schultze, J.W., Lohrengel, M. M.,  Electrochimica Acta. 45 (2000) 2499-2513.
2. Olsson, C-O. A., Verge, M.-G., Landolt, D., J. Electrochem. Soc.151 (2004) B652-B660.
3. Ai, J., Chen, Y., Urquidi-Macdonald, M., Macdonald, D. D., J. Nuclear Materials, 379 (2008) 162-168.
4. Sutter, E. M. M., Hlawka, F., Cornet, A., Corrosion Science. 46 (1990) 537-544.
5. Prono, A., Jaszay, T., Caprani, A., Frayret, J. P., J. Applied Electrochemistry, 25 (1995) 1031-1037.
6. R. Klein, Corrosion, 53 (1997) 327-332
7. Messnaoui B., J. Solution Electrochemistry 37 (2008) 715-726.