Mechanistic Analysis of Zr Dissolution in Acidic Fluoride Media Using Electrochemical Impedance Spectroscopy

Ta and Nb are group V valve metals which resist corrosion. Anodic dissolution of Ta and Nb in acidic fluoride media of varying HF concentration is investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. Polarization curves showed a clear active and passive region in all the solutions employed in this study. At a given HF concentration, Nb anodic polarization currents are larger compared to those of Ta. EIS of Ta and Nb exhibited a low frequency capacitive loop in active region, and a low frequency negative differential resistance in the passive region. The surface of Ta and Nb was characterized using XPS, and it reveals the presence of both sub oxide and pentoxide. The electrochemical results could be explained by a four step mechanism involving sub-oxide and pentoxide intermediates. The analysis shows that direct dissolution of sub-oxide occurs via an electrochemical pathway and is facilitated by HF − 2 species and HF in undissociated form. On the other hand, chemical dissolution of pentoxide, which occurs in parallel, is facilitated by the HF species in undissociated form.

Section: Resultssupporting

confidence: 73%

On the Anodic Dissolution of Tantalum and Niobium in Hydrofluoric Acid

Meethal

Rao

Sapra

et al. 2018

“…On the other hand, k 8-dc is almost two orders of magnitude less than k 7-dc and hence electrochemical dissolution Zr crease in the current in the active region and the continuous decrease in current in the passive region. 40 The present detailed model also captures the slight increase in current increase near 0.5 V vs. OCP in the passive region. The model predictions for the EIS data in both active and passive regions are shown as complex plane plots in Figs.…”

mentioning

confidence: 67%

“…1 was evaluated to model the experimentally observed features in the impedance spectra of anodic dissolution of Zr in HF based solutions. 40 In the active region, the model could predict two capacitive loops in the high and mid frequency regions, but the pseudo-inductive loop at low frequencies was not predicted by the model. 40 It is to be noted that if an impedance spectrum at one dc potential were modeled, then all the features, including the low frequency inductive loop can be predicted by the mechanism given in Eq.…”

mentioning

confidence: 90%

“…It is worth noting that the earlier simpler model, with two adsorbed intermediates, could not capture the low frequency pseudo inductive loops in the active region or the capacitive arc in the passive region described above. 40 The analysis shows that, to successfully model the complete data set, species with the same charge but different chemical composition have to be treated as separate entities.…”

mentioning

confidence: 99%

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Mechanistic Analysis of Anodic Dissolution of Zr in Acidic Fluoride Media

Amrutha

Rao

Ramanathan

2018

Self Cite

Anodic dissolution of Zr in 10 mM HF was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. The surface state of the electrode was analyzed using X-ray photon spectroscopy, and the surface morphology was characterized using atomic force microscopy. EIS data acquired at multiple dc potentials were subjected to mechanistic analysis. Reaction mechanism analysis approach reveals that at least four intermediates are required to describe the observed results. The intermediates are likely to be Zr sub-oxides, oxyfluorides and ZrO 2 . The proposed mechanism successfully predicts the major features observed in polarization and impedance spectra. At low overpotentials, the fractional surface coverage of Zr 3+ species is higher than that of Zr 4+ species, and the electrochemical dissolution rate is higher than the chemical dissolution rate. As the overpotential increases, the surface is covered with Zr 4+ species and chemical dissolution rate becomes comparable to electrochemical dissolution rate. Although the surface is covered with Zr 4+ species at higher overpotentials, significant chemical and electrochemical dissolution processes continue to occur and hence Zr is not protected in acidic fluoride media under anodic conditions. © The Author Zirconium and alloys containing Zr are extensively used in the nuclear power industry as fuel cladding agents, in chemical industries as an alloying component, and in biomedical implants.1-4 The wide acceptance of this metal and its alloys is due to their superior mechanical strength, and high resistance toward corrosion, and H 2 embrittlement.5-8 These properties are due to the formation of a stable, protective passive film of ZrO 2 .9-11 Although Zr exhibits excellent corrosion resistance in most of the harsh environments, it readily dissolves in acidic fluoride media. 6,7,12,13 Earlier scientific reports on Zr dissolution are mostly based on conventional techniques, such as weight loss measurements, 14 measurement of volume of H 2 gas evolved, 7 and steady-state polarization techniques.15 Prono et al. 16 have analyzed the polarization of Zr in 0.12 M NaF + 7 M HNO 3 and proposed a seven-step mechanism comprising three competitive paths to describe the dissolution. This analysis was limited to the modeling of polarization data obtained at a single concentration. In our previous work, Zr anodic dissolution mechanism in the acidic fluoride medium was studied using the mechanistic analysis of current potential polarization data in various HF solutions.17 A mechanism containing two adsorbed intermediates and two dissolution steps, shown in Eq. 1, was proposed. The model successfully predicted the major characteristics of the polarization curves, and the results predicted that the chemical and the electrochemical dissolution steps were affected by the concentration of HF − 2 and HF, respectively. Anodic dissolution of other valve metals such as Ti 18,19 and Nb 20 have been characterized using potentiodynamic polarization and electrochemical imped...

“…Bojinov et al 31 used the surface charge approach to describe the dissolution of Nb in acidic fluoride medium. The anodic dissolution mechanism of Ti, Zr, Nb and Ta in hydrofluoric acid has been reported earlier [32][33][34][35][36][37][38] where a four step mechanism with two adsorbed intermediate species and two dissolution pathways were proposed. In summary, the corrosion rate of Ti and Zr and several alloys of valve metals in conc.…”

mentioning

confidence: 69%

Studies on Group IV and V Valve Metal Corrosion in Acidic Fluoride Media

Rao¹,

Meethal²,

Amrutha³

et al. 2020

Self Cite

Dissolution rates of Ti, Zr, Nb and Ta in acidic fluoride media of different concentrations at various temperatures were measured by weight loss method, and were complemented by electrochemical measurements at room temperatures. A mathematical model to estimate the corrosion rate of these four valve metals based on the overall reaction mechanism for metal dissolution as well as H2 evolution is developed from the weight loss experiments. Valve metal dissolution process occurs through two independent pathways assisted by (HF2)− and equilibrium HF species. Except Ti, the dissolution of other valve metals studied are not significantly influenced by the H2 evolution reaction. Among the four metals evaluated, Nb offers the best corrosion resistance at high temperature and high HF concentration. The corrosion resistance of all four valve metals estimated from linear polarization, electrochemical spectroscopy and Tafel extrapolation in various concentrations of solutions match well. For Ti, dissolution rates calculated from Tafel extrapolation were lower than those based on weight loss experiments, while for Zr, Nb and Ta, they were mostly higher than those based on weight loss experiments.