The dissolution of niobium electrode in anodic regime in hydrofluoric acid was studied using electrochemical techniques. In the active region, complex plane plots of the impedance spectra exhibited mainly two capacitive loops. In the passive region, the spectra exhibited a mid-frequency pseudo-inductive loop and a low-frequency negative resistance. X-ray photoelectron spectroscopic analysis showed that the surface contains Nb2+ and Nb5+. The impedance data were analysed using equivalent circuit with Maxwell elements as well as mechanistic modelling. A four-step mechanism with a chemical and electrochemical dissolution step is proposed to explain the results. The results show that Nb with an oxidation state of 2 is likely to be an intermediates species. The simulations predict that at low over potentials, the surface is covered mainly with bare metal, whereas at high over potentials, the surface is entirely covered by passivating Nb5+ film. © 2017, Springer-Verlag Berlin Heidelberg.

Srinivasan Ramanathan

Department Of Chemical Engineering

dissolution

Equivalent circuits

Hydrofluoric acid

Reaction intermediates

Spectroscopic analysis

X ray photoelectron spectroscopy

Anodic dissolution

ELECTROCHEMICAL DISSOLUTION

Electrochemical impedance

Electrochemical techniques

Impedance spectrum

MECHANISTIC ANALYSIS

Spectroscopic studies

X-ray photoelectrons

Niobium

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Solid State Electrochemistry

Titanium (Ti) is a lustrous transition metal which possesses excellent corrosion resistance in several aggressive environments but is attacked by acidic fluoride media. In this work, the addition of fluoride ion to nitric acid on the corrosion behavior of Ti and air-oxidized Ti was studied. Air oxidation of Ti at 800 °C for 5 h resulted in the formation of an intact rutile TiO2 layer with a thickness of about 35 μm. The corrosion resistance was investigated from the polarization resistance (RP) values which were obtained from linear polarization and electrochemical impedance spectroscopy (EIS) measurements. A decrease in the RP value of Ti with the addition of fluoride ion in nitric acid was observed, and this indicated the formation of an unstable layer, which resulted in accelerated dissolution. The several order increase in the RP value of air-oxidized Ti in nitric acid containing fluoride ions pointed the improved resistance to corrosion. The diffusivity of species through the air-oxidized TiO2 layer was estimated from the electrochemical equivalent circuit (EEC) analysis of EIS data. Even though the diffusivity increased by several orders with the addition of fluoride ions, air-oxidized Ti provided better protection against nitric acid containing fluoride ions than Ti. The corrosion rates of Ti and air-oxidized Ti in boiling nitric acid containing fluoride ions were also estimated from weight loss experiments. Even in boiling fluorinated nitric acid, air-oxidized Ti provided better corrosion protection, with corrosion rates of about 1000 times less than that of Ti. © 2019, The Author(s).

Fulltext

Transactions of the Indian Institute of Metals

Studies on Corrosion of Titanium and Air-Oxidized Titanium in Fluorinated Nitric Acid

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. HF2- is identified as the contributing species for the chemical dissolution and remaining HF as the contributing species for the electrochemical dissolution.

ECS Transactions

Effect of fluoride concentration on niobium anodic dissolution

The kinetics of anodic dissolution of Zr in hydrofluoric acid (HF) was investigated using potentiodynamic polarization experiments. At lower potentials, an active region with a rapid increase in current with the potential was observed and at higher potentials, a large passivation current plateau was observed. The current decreased only slightly with potential in the passive region. Scanning electron micrographs confirmed that the passivation is incomplete in the region where current exhibits a plateau and x-ray photoelectron spectroscopic analysis showed that oxides and oxyfluorides are present on the surface. A four-step mechanism with two adsorbed intermediate species was evaluated and the model captures the essential characteristics of the polarization plots. The results suggest that HF2− species participates in the chemical dissolution step and actual HF participates in the electrochemical dissolution step. © 2016, Springer-Verlag Berlin Heidelberg.

Kinetics of anodic dissolution of Zr in acidic fluoride media

We investigated the anodic dissolution of polycrystalline titanium rotating disc electrode in 0.1 M hydrofluoric acid in active and passive regions using potentiodynamic polarization and electrochemical impedance spectroscopy. In the active region, complex plane plots of the impedance spectra exhibited three capacitive loops indicting the presence of at least two adsorbed intermediates. In the passive region, they exhibited a negative resistance and a low frequency inductive loop. The high frequency loops exhibit constant phase element behavior indicating that a heterogeneous 2D film with partial surface coverage is present in the active and passive regions. The impedance data was fit to a reaction model and a four step mechanism with two adsorbed intermediate species is proposed to explain the observed trends in the active and passive regions. The change in the surface coverage of the adsorbed intermediate species, with the overpotential is estimated. This model describes the dissolution of Ti via two parallel paths, viz. a chemical step and an electrochemical step. The onset of passivation ensues when the rate of TiO<inf>2</inf> film formation is more than that of its dissolution. Transpassive dissolution becomes dominant when the electrochemical dissolution from the bare surface becomes negligible. © The Author(s) 2015. Published by ECS.

Journal of the Electrochemical Society

A kinetic model for the anodic dissolution of Ti in HF in the active and passive regions

Journal of Fluorine Chemistry

Nano-CoF 3 prepared by direct fluorination with F 2 gas: Application as electrode material in Li-ion battery

ACS Combinatorial Science

Compositionally Dependent Nonlinear Optical Bandgap Behavior of Mixed Anodic Oxides in Niobium–Titanium System

Electrochemistry Communications

Direct observation of metal dissolution during anodization of niobium

Electrochemical impedance spectroscopic studies on niobium anodic dissolution in HF

Journal	Data powered by TypesetJournal of Solid State Electrochemistry
Publisher	Data powered by TypesetSpringer New York LLC
ISSN	14328488
Open Access	No