Electrochemistry of Refractory Metals: Hf, Mo, Cr

Adhoum, Nafaâ; Arurault, Laurent; Bouteillon, J.; Cotarta, A.; Gabriel, Jiřı́; Poignet, Jean-Claude

doi:10.1007/978-94-015-9135-5_6

Cited by 4 publications

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“…is controlled by the rate of charge transfer. 69 The disagreement between our data and studies 28,29 probably can be explained by different concentration of fluoride ions in the melt. In the case of an irreversible electrochemical reduction process, the electrode reaction is characterized by the transfer coefficient (α) multiplied by the number of electrons (n α ) in the step determining the rate of the electrode process.…”

Section: Resultscontrasting

confidence: 87%

“…Thus, fluoride additions led to the formation a more stable HfF 6 2− fluoride complexes. 30 The authors 28,29 using a semi-integral techniques found that electrode reaction 2 is reversible over the temperature range from 973 to 1123 K. At the same time in these studies, the significant shift of peak potentials to the negative region with increasing sweep rate was observed. Thus, the conclusion on the reversibility of the process (2) is debatable.…”

mentioning

confidence: 75%

“…The electrodeposition of hafnium coatings was chosen because they successfully used for protection niobium from oxidation at temperatures 773-973 K. 19,20 The electrochemical behavior of hafnium in alkali halide melts has been the subject of studies. The analysis of results showed that hafnium cathodic reduction in alkali chloride melts containing HfCl 4 involves two successive two-electrons stages In studies 28,29 was determined that the electroreduction of Hf (IV) chloride complexes in an equimolar mixture NaCl-KCl occurs in a single step with the formation of metallic hafnium at temperatures 973 K and 1123 K. The electron transfer process was found a quasireversible at 973 K and 1073 K and reversible at 1123 K. The investigation of the influence of the second coordination sphere influence on electrochemistry of hafnium showed that in the CsCl melt, the reduction of Hf (IV) to metal is one irreversible stage. 34 The influence of fluoride ions on the reduction of hafnium tetrachloride in an equimolar NaCl-KCl mixture was investigated in Refs.…”

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confidence: 99%

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Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

Kuznetsov¹

2020

J. Electrochem. Soc.

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The electroreduction of HfF 6 2− complexes to Hf on a molybdenum electrode in the NaCl-KCl-K 2 HfF 6 -NaF (5 wt%) melt in the temperature range 973-1123 K was studied by cyclic voltammetry and impedance spectroscopy. It was found that discharge of Hf (IV) complexes to Hf occurs in one step and is irreversible. The diffusion coefficients of Hf(IV) fluoride complexes were determined by cyclic voltammetry. The impedance spectroscopy was used for the determination of the standard rate constants of charge transfer. Electrodeposition of hafnium coatings on a molybdenum substrate was performed in the NaCl-KCl-K 2 HfF 6 (10 wt%)-NaF (5 wt%) melt at the cathodic current density 5-100 mA cm −2 and temperatures 973, 1023 and 1073 K. The influence of the current density and temperature on the roughness of hafnium coatings was studied. It was determined that hafnium coatings with significantly higher purity than that of the hafnium anode were obtained confirming that metal impurities were removed during electrodeposition. The oxidation kinetics data were obtained on a sample with hafnium and a sample of pure molybdenum, which were tested in a water/oxygen mixture at 773 K. The hafnium coating improves the corrosion resistance of coated plates in comparison with that of pure molybdenum.

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Section: Resultscontrasting

confidence: 87%

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confidence: 75%

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confidence: 99%

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Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

Kuznetsov¹

2020

J. Electrochem. Soc.

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“…This became apparent only in the joint analysis of the anode and cathode-electrolyte electrode systems. 1 However, the conclusion 1 about the mirror image of cathodic and anodic processes, in our opinion, is valid only for melts of the same anionic composition. This conclusion is not always true for melts of mixedligand composition, for example, chloride-fluoride melts, as we will show.…”

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confidence: 88%

On the Ratio of Cathodic and Anodic Processes during Electroreduction-Electrooxidation of Hafnium in Molten Salts

Kuznetsov¹

2022

J. Electrochem. Soc.

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Investigation of hafnium cathodic and anodic processes in chloride melts containing HfCl4 revealed that two-hafnium electroreduction processes are corresponded two electrooxidation processes. The only difference between these processes was the presence of salt passivation under certain dissolution conditions. In chloride-fluoride melts containing K2HfF6 the number of cathodic and anodic processes can be different due to the amount of hafnium ions passing into the melt during anodic dissolution exceeding the amount of fluorine anions necessary to bind them into fluoride complexes. Based on the experimental data it was found that the conclusion about the mirror reflection of cathodic and anodic processes is invalid in the case of a heteroligand composition of melts. It was determined that the dissolution in the NaCl-KCl-K2HfF6 (10 wt%) melt at the anodic current density of >0.02 А∙cm-2 was accompanied by the formation of Hf(IV) fluoride complexes and Hf(II) chloride complexes. It was shown that the appearance of hafnium complexes with different valence during anodic process in the NaCl-KCl-K2HfF6 (10 wt%) melt is a highly undesirable phenomenon, which leads to the formation of metallic film on the surface of the melt decreasing the current efficiency for coating deposition.

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“…The available results were mainly acquired in molten chlorides or chloro-fluorides media. In molten chlorides, Poinso et al [13,14] and Adhoum et al [15] did not agree on the on Hf(IV) ion reduction mechanism in NaCl-KCl mixture. Spink et al [16] in CsCl, showed that Hf(IV) ions reduction is a one step process exchanging 4 electrons leading to the formation of Hf metal.…”

Section: Introductionmentioning

confidence: 94%

Generalized method for determining fluoroacidity by electrochemical diffusion coefficient measurement (application to HfF4)

Kergoat

Gibilaro

Massot

et al. 2015

Electrochimica Acta

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A B S T R A C TA universal method for fluoroacidity evaluation was developed and based on a mass transport approach: it simply consists in measuring the diffusion coefficient of an electroactive species in various molten media. The reduction behaviour of Hf(IV) ions was investigated in molten fluorides and diffusion coefficients of Hf(IV) ions were measured. Results showed that diffusion coefficients decrease with fluoroacidity, due to the effect of solvent viscosity (which is linked to bridged fluorines). A global approach of mass transport in solution was then proposed, taking into account both solute and solvent. The Schmidt number (Sc) defined as the ratio between solvent viscosity and solute diffusivity was calculated in order to take into account these two parameters. Results showed that Sc increases with fluoroacidity, in a much more sensitive way than D. This universal method can extended to all electroactive species and to all bath fluoroacidity.

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Electrochemistry of Refractory Metals: Hf, Mo, Cr

Cited by 4 publications

References 0 publications

Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

On the Ratio of Cathodic and Anodic Processes during Electroreduction-Electrooxidation of Hafnium in Molten Salts

Generalized method for determining fluoroacidity by electrochemical diffusion coefficient measurement (application to HfF4)

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