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...
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.
The mechanism of dissolution of Zr in 10 mM HF was investigated using potentiodynamic polarization and electrochemical impedance spectra acquired at various dc potentials. The surface morphological changes in the active and passive regions were studied using atomic force microscopy. The EIS spectra acquired at various dc potentials were modeled using electrical equivalent circuit fitting. A four step mechanism which consists two intermediates and a chemical and electrochemical dissolution steps was used to explain the polarization and EIS results. The model predicts the active-passive transitions in the polarization curve and qualitatively matches the patterns of EIS spectra observed in active and passive regions. The changes of the surface coverage and the dissolution rates with over potential were estimated.
Anodic behaviour of niobium in HF was investigated using potentiodynamic polarization technique. The effect of concentration was studied by varying the nominal HF concentration and few experiments were carried out by adding H2SO4 and KF to HF solutions, to determine the effect of various species on Nb dissolution. In the potentiodynamic polarization curves, distinct active and passive regions were seen at all the concentrations studied. A kinetic model involving a chemical and an electrochemical dissolution step in parallel was proposed to model the observed results. The model captures all the major characteristics of the polarization curves.2 HF is identified as the contributing species for the chemical dissolution and remaining HF as the contributing species for the electrochemical dissolution.
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