Niobium is one of the valve metals [1] and, when exposed to air or water, it readily forms a chemically inert and mechanically strong oxide layer [2]. Nb is used as a popular alloying element in carbon and steel industry [3] due to its high corrosion resistance. Nb is also used for fabricating super conducting radio frequency cavities [2] used in particle accelerators which demand very smooth surfaces. The passive layer on Nb is stable in most aqueous media. However, it can be attacked by hydrofluoric acid [4, 5] or strong alkali [6]. Nb anodic dissolution in HF shows clear active and passive regions, followed by an increase and a large current plateau signifying transpassive dissolution [5]. The polarization and EIS data of Nb in warm alkali and acidic fluoride media was analyzed in the framework of surface charge approach combined with interfacial reactions [4].

The species that are reported to be formed in aqueous HF solutions are HF, H⁺, F^-, HF₂^- and H₂F₃^- [7] and the dissociation of HF involves the following reactions.

HF ↔ H⁺ + F^-, HF + F^-↔ HF₂^-, along with HF + HF₂^- ↔ H₂F₃^-

The corresponding equilibrium constants are respectively, 6.84 × 10^-4, 5 and 0.58 [7]. In this study, we present the experimental results of anodic dissolution of Nb in different concentrations of HF and propose a kinetic model to match the observed results.

All the experiments were carried out using Nb rotating disc electrode (RDE) in a standard three-electrode cell with Ag/AgCl (3.5 M KCl) as reference and Pt wire as counter electrode. HF concentration was changed between 50 mM and 1000 mM. In order to reduce the solution resistance effects, a supporting electrolyte (1 M Na₂SO₄) was used. The electrode rotational speed was maintained at 900 rpm. Before each experiment, the electrode was mechanically polished with 400, 800 and 1200 grade emery paper followed by ultra-sonication in ethanol and DI water. Potentiodynamic polarization data were acquired by sweeping the potential from open circuit potential (OCP) to 1 V above OCP, at 2 mV/s scan rate.

Figure 1A shows the typical anodic behaviour of Nb measured as described above in different concentrations of HF used. The peak current values shift towards right with increase in HF concentration. The potential at which the peak occurs also shifts to more anodic values at higher HF concentration. The concentration of all the constituent species increases with increase in nominal HF concentration [8]. Therefore, few experiments were carried out adding H₂SO₄and KF to aqueous HF solutions to isolate the effect of different species on the dissolution (Fig 1B). Reaction mechanism analysis was employed to understand the dissolution mechanism of Nb in HF. A four step mechanism involving two dissolution steps, chemical and electrochemical was proposed to explain the results. The model captures all the major characteristics of the polarization curves.

References:

[1] A. De Sá, C. Rangel, P. Skeldon, and G. Thompson, Port. Electrochim. Acta, 24, 305 (2006).

[2] C. D'Alkaine, L. De Souza, and F. Nart, Corros. Sci., 34, 29 (1993).

[3] M. Sowa, K. Greń, A. I. Kukharenko, D. M. Korotin, J. Michalska, L. Szyk-Warszyńska, L. M. Mosiałek, J. Żak, E. Pamuła, E. Z. Kurmaev, S. O. Cholakh, and W. Simka, Mater. Sci. Eng., C, 42, 529 (2014).

[4] M. Bojinov, S. Cattarin, M. Musiani, and B. Tribollet, Electrochim. Acta, 48, 4107 (2003).

[5] S. Cattarin, M. Musiani, and B. Tribollet, J. Electrochem. Soc., 149, B457 (2002).

[6] C. Baruffaldi, R. Bertoncello, S. Cattarin, P. Guerriero, and M. Musiani, J. Electroanal. Chem., 545, 65 (2003).

[7] K. W. Kolasinski, J. Electrochem. Soc., 152, J99 (2005).

[8] S. Ramanathan and I. I Suni, J. Electrochem. Soc., 146 (2) 570 (1999).