Transpassive dissolution of Ni-Cr alloys in sulphate solutions—comparison between a model alloy and two industrial alloys

The application of a general model for the transpassive dissolution mechanism of binary Ni-based alloys to industrial alloys, Alloy 600 and Alloy C276, containing Ni, Cr, Fe and Mo, in 1 M sulphate solutions at pH 0 and 5 is described. A comparison of the electrochemical behaviour of these two alloys to a binary Ni-15%Cr alloy is also included. The techniques used were ring-disc voltammetry, impedance spectroscopy and resistance measurements. Soluble high-valency products were found to be released in a considerable amount from all the materials. The presence of Mo in Alloy C276 was found to increase the transpassive oxidation rate in comparison to alloys 600 and Ni-15%Cr at pH 0, but the same effect of Mo is not so well pronounced at pH 5. The mechanism of transpassive dissolution was found to be similar on every material at pH 0. At pH 5 the mechanism of the transpassive dissolution on Alloy C276 at high overpotentials is different from that at low overpotentials or from that at pH 0. This change is concluded to be due to the increased effect of adsorbed intermediates at the film/solution interface. The model was found to reproduce the steady state current and the impedance spectra satisfactorily.     

Influence of the electrolyte composition and temperature on the transpassive dissolution of austenitic stainless steels in simulated bleaching solutions

The transpassive corrosion of highly alloyed austenitic stainless steels—UNS N08904, UNS S31254 and UNS S32654—was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.     

Pitting corrosion studies of super austenitic stainless steels in natural sea water using dynamic electrochemical impedance spectroscopy

Potentiodynamic anodic polarisation and dynamic electrochemical impedance spectroscopic (DEIS) measurements were carried out on type 316L stainless steel (SS), alloys 33 and 24 in natural sea water environment in order to assess pitting corrosion resistance. The results revealed that the pitting corrosion resistance was higher in the case of alloys 33 and 24 than 316L SS; due to the higher contents of nitrogen, chromium and molybdenum. DEIS measurements were performed over a wide range of potentials covering the corrosion potential, passive region, breakdown region and dissolution region. It was shown that the impedance measurements in potentiodynamic conditions allow simultaneous investigation of changes in passive layer structure. The impedance spectra of various potential regions were also discussed. The Nyquist plots were fitted using non-linear least-square (NLSS) method for different potential regions.     

Transpassive behaviour of titaniummolybdenum alloys in 1 N H2SO4

The electrochemical behaviour of titaniummolybdenum alloys (10, 20,30 wt.%Mo) in boiling, deaerated 1 N H₂SO₄ was investigated. Attention was focused on the potential region of transpassive dissolution of molybdenum. Potentiostatic polarization curves in conjunction with Auger electron spectroscopy were used to correlate the current-potential relationship in this region with the composition of the oxide films. It was shown that at open-circuit potential, for Ti—20 and 30Mo an accumulation of molybdenum in the oxide film results in alloy passivity. However, in the region of the transpassivation with respect to the molybdenum, the presence of Mo in the oxide led to a higher dissolution rate than that exhibited by pure titanium. In the transpassivation region, the steady state currents increased with increasing potential up to a characteristic anodic potential. At potentials more positive than this point, the anodic currents declined. This decrease in current was correlated with a depletion of Mo from the outer layers of the oxide with the consequent formation of a more protective titaniumoxide film structure.     

Electrochemical impedance spectroscopy of patterned steady states on electrode surfaces

In this work we report modelling of electrochemical impedance spectra for the CO bulk electrooxidation on Pt, an electrochemical system with an S-shaped current-potential curve (S-NDR (negative differential resistance) system). Galvanostatic control and parameter ranges, in which self-organized stationary CO-coverage patterns exist on the electrode surface are considered. The patterns consist of two stationary domains with different CO coverages. The simulations show that at very low frequencies the applied current modulation imposes a domain size modulation which occurs at nearly constant electrode potential. Consequently, the interface impedance modulus tends to zero at very low frequencies and no negative real impedance can be observed in the simulated impedance spectra. For higher modulation frequencies, the faster current modulation affects the domain size expansion and contraction processes, inducing an increase of the interface impedance modulus and a dependence of its phase on the frequency. These results demonstrate that the dynamics of pattern formation affects considerably the linear response of an electrochemical system. Furthermore, they suggest that measurements of impedance spectra can offer valuable information on the dynamics of pattern formation and of the electrochemical processes involved.     

Estimation of kinetic parameters of the passive state of carbon steel in mildly alkaline solutions from electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data

The unambiguous interpretation of electrochemical impedance spectra of complex systems such as passive metals and alloys in terms of an unique kinetic model is often hampered by the large number of adjustable modeling parameters. In this paper, a combination of in situ electrochemical data and ex situ surface analytical information is employed to validate the estimates of kinetic and transport parameters of the passive state of carbon steel. For the purpose, electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data for the oxidation of carbon steel in mildly alkaline solutions are quantitatively compared with the predictions of the Mixed-Conduction Model for oxide films that represent the passive oxide as an intermediate phase between magnetite and maghemite. Estimates of the kinetic rate constants at the film interfaces, as well as the diffusion coefficients and field strength in the film are obtained and their relevance for the corrosion mechanism of carbon steel is discussed.     

Estimation of kinetic and transport parameters by quantitative evaluation of EIS and XPS data

The relatively large number of adjustable parameters often precludes the unambiguous interpretation of electrochemical impedance spectra in terms of a unique kinetic model. In the present paper, the possibilities offered by a combination between in situ electrochemical impedance spectroscopic data and ex situ surface analytical information to improve the credibility of the estimates of the kinetic and transport parameters are discussed. Two electrode systems in which passive oxide films are formed—stainless steel in simulated pressurised water reactor coolant and tungsten in sulphate-fluoride solutions—are used as representative examples to demonstrate the different approaches taken to analyse the experimental data in terms of the Mixed-Conduction Model. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring by quantitative comparison of the model equations to electrochemical impedance and X-ray photoelectron spectroscopic data are described and the significance of the obtained parameters for the kinetics of the overall process of metal and alloy dissolution in the passive state is discussed.     

A theoretical approach of impedance spectroscopy during the passivation of steel in alkaline media

A theoretical impedance function is deduced for a proposed mechanism of passive film formation of steel in contact with alkaline aqueous media involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite and Fe(III)-oxides. The reduction reaction of dissolved oxygen is considered as the only cathodic reaction compensating the anodic current induced by the formation of iron oxides at open circuit potential. The iron dissolution takes place through a chemical dissolution of ferric oxide. A two-layered passive film with 3D structure is considered. A satisfactory agreement between the digital simulations on the basis of the theoretical impedance function and experimental spectra validates the proposed model.     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

Influence of molybdenum on the anodic dissolution of iron in acidic solutions

The anodic dissolution of pure iron and binary iron-molybdenum alloys (10 and 20 wt% Mo) in molar H₂SO₄ and HCl solutions was studied by d.c. polarization and a.c. impedance techniques. Molybdenum additive influences both the steady state polarization curves and the impedance spectra in the two media, the impact being stronger for the Fe-20 wt % Mo alloy. In general, two-slope polarization curves are obtained for the alloys; impedance spectra exhibit two time constants in addition to the charge transfer one implying the existence of two reaction intermediates. Differences between spectra measured in H₂SO₄ and HCl are discussed. A kinetic model advanced on the basis of literature data for pure iron and molybdenum was able to reproduce quantitatively both the steady state and a.c. impedance results for both alloys in the two media. Kinetic parameters for the anodic dissolution of Fe-Mo alloys are thereby determined.     

Structural and dielectric properties of CuO nanoparticles

The DC conduction and dielectric behaviour of copper oxide nanoparticles prepared by sol-gel method and sintered at 950 °C were studied in the temperature range of 200–526 K. The formation of single phase monoclinic CuO was confirmed by x-ray diffraction. Chemical composition of the CuO ceramic was investigated with X-ray photoelectron spectroscopy (XPS) technique. Although XRD analysis shows the formation of single phase CuO, XPS spectra revealed the presence of Cu³⁺ and Cu²⁺. Deviation from linearity ln (σ_DC) vs. 1/T plot at ~390 K was observed, which indicates that DC conduction in the CuO pellet is dominated by two different conduction mechanisms. The results obtained on AC conductivity indicate that AC conduction mechanism could be well explained by the multihopping model at low frequencies, while high frequency AC conductivity data can be described by small polaron tunnelling model. The dielectric relaxation mechanism in the CuO pellet was studied by impedance spectroscopy. It was found that while dielectric constant is an increasing function of temperature, it decreases with increasing frequency. The obtained impedance spectra indicated that the grain boundary effects and intergranular activities play a crucial role on the dielectric relaxation processes.     

Oxidative dissolution and anion-assisted solubilisation in the transpassive state of nickel-chromium alloys

The transpassive state of pure Ni, Cr and two Ni-Cr alloys (10 and 20 wt.% Cr) in 14.8 M H₃PO₄ is studied by voltammetric, rotating ring-disc electrode and impedance spectroscopic measurements. The results indicate that the rate of transpassive dissolution of the alloys in a major part of the transpassive range is higher than the rate of this process for both pure metals. A kinetic model retaining the essential features of the corresponding models for the pure metals proposed in the literature has been found to reproduce both the steady-state current versus potential curves and the ac impedance spectra in the transpassive range. The parameters of the model are determined by a fitting procedure and the relevance of their values for the relative importance of two possible pathways in the transpassive state—oxidative dissolution and anion-assisted solubilisation—are discussed.     

Review study of electrochemical impedance spectroscopy and equivalent electrical circuits of conducting polymers on carbon surfaces

Electrochemical impedance spectroscopy (EIS) is an experimental method for characterizing electrochemical systems. This method measures the impedance of the concerned electrochemical system over a range of frequencies, and therefore the frequency response of the system is determined, including the energy storage and dissipation properties. The aim of this article is to review articles focusing on electrochemical impedance spectroscopic studies and equivalent electrical circuits of conducting polymers, such as polypyrrole, polycarbazole, polyaniline, polythiophene and their derivatives, on carbon surfaces. First, the conducting polymers are introduced. Second, the electrochemical impedance spectroscopic method is explained. Third, the results of EIS applications using equivalent electrical circuits for conducting polymers taken from the literature are reviewed.     

Effects of surface structure on the electrochemical properties of Nimetal complex oxide film electrode

Effects of surface structure on the electrochemical properties of Ni—metal complex oxide film electrodes prepared by radio frequency magnetron sputtering method and by thermal decomposition method have been investigated. Rotating disc electrode technique was applied to evaluate the electron transfer rate of the redox system [Fe(CN)₆]^3?/[Fe(CN)₆]^4? in 0.1 M NaOH solution. Dynamic impedance method to detect the impedance change of electrode/electrolyte interface and X-ray photoelectron spectroscopy were also used. The NiMo complex oxide film electrode prepared by rf magnetron sputtering method was found to have good stability, good reproducibility and effective electrochemical properties in comparison with the oxide film electrodes prepared by thermal decomposition method. Then, of the electrodes prepared by thermal decomposition method, the NiMo complex oxide film and the NiRe complex oxide film were shown to have good electrochemical properties. It was found by XPS analysis that both Mo ion at a higher oxidation state and oxygen influenced from this Mo ion, which showed clear occurrence for the NiMo oxide film prepared by rf sputtering method and no occurrence by thermal decomposition method, played a very important role in the reaction.     

The effect of Ni in the electrochemical properties of oxide layers grown on stainless steels

This study analyses the influence of Ni in the electrochemical behaviour of three different stainless steels in alkaline medium. The studied steels have increasing Ni content: AISI 430, AISI 304L and AISI 316. The obtained results are compared with those of a nickel base alloy (Ni > 42%, w/w) and a pure Ni electrode. Electrochemical impedance spectroscopy and cyclic voltammetry have been the main tools used to study the growth and evolution of the passive layers formed on those materials in alkaline medium. XPS and SEM have been employed for chemical and morphological characterization of the developed passive films.The presence of Ni promotes the formation of thinner and more protecting passive films. This fact noticed in the XPS analysis is reflected in the cyclic voltammograms by an important decrease of the magnetite formation peak current as well as that corresponding to Cr³⁺/Cr⁶⁺ oxidation. The low frequency limit of complex plane impedance plots also increases with the Ni content. In order to better characterise the resistivity of the electrochemically formed films, a more detailed impedance analysis in the high frequency range (1 kHz-10 MHz) has been performed. The analysis of the registered spectra indicates that Ni modifies the conductivity of the oxide layers, promoting the formation of more resistive oxide films.     

Influence of the forming electrolyte on the electrical properties of tantalum and niobium oxide films: an EIS comparative study

The electrochemical oxidation of Ta and Nb and the dielectric behaviour of the oxide films thus formed were investigated in the following electrolytes: H2SO4, HNO3, H3PO4 and NaOH. Characterization of the films was carried out by means of potentiodynamic current–potential profiles (in the range 0–8V) and electrochemical impedance spectra (in the range 0.1Hz–100kHz). The a.c. response of the oxide films was modelled as a single layer structure on the basis of an equivalent circuit with constant phase elements (CPE). The dependence of the oxide resistance and oxide capacitance with potential is also reported.     

Characterization of boron-doped diamond electrodes by electrochemical impedance spectroscopy

Charge transfer on boron doped diamond (BDD) electrodes was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The diamond films of 5 μm thickness and boron content between 200 ppm and 3000 ppm were prepared by the hot filament CVD technique on niobium substrate and mounted in a Teflon holder as rotating disk electrodes. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂ SO ₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆]. Significant deviation in the redox behaviour of BDD and active Pt electrodes was indicated by a shift of the peak potentials in the cyclic voltammograms with increasing sweep rate and lower limiting diffusion current densities under rotating disk conditions. In the impedance spectra an additional capacitive element appeared at high frequencies. The potential and rotation dependence of the impedance spectra can be described quantitatively in terms of a model based on diffusion controlled charge transfer on partially blocked electrode surfaces. Direct evidence for the non-homogeneous current distribution on the diamond surface was obtained by SECM measurements.     

Electrochemical and spectroscopic characterizations of patinas formed on an archaeological bronze coin

A bronze coin datable of the post-Roman empire and discovered in Morocco was studied. The in-depth structure and the composition of both patina and substrate alloy were determined by SEM observations, EDS (elemental dispersion analysis using X-ray) cartographies and Raman spectroscopy on a cross-section. The composition of the patina and that of the substrate alloy were found to be highly heterogeneous. Some patina particles were also scraped off from coin surface. Their electrochemical properties were characterized by cyclic voltammetry and electrochemical impedance spectroscopy with a cavity microelectrode. Cyclic voltammetry revealed the presence of three cathodic current peaks. In situ Raman microspectroscopy on the microelectrode allowed the attribution of two of them to Cu(0)|Cu(I) and Cu(I)|Cu(II). The third one was ascribed to the presence of lead oxide. Moreover, it was observed that tin dioxide initially present in the patina gradually disappeared independently of the potential cycling. This phenomenon was explained by the dissolution of this species through a chemical process. The impedance spectra collected with the cavity microelectrode were simulated suitably with an oxidation-reduction reaction coupled with a hemispherical diffusion model of dissolved oxygen.     

Impedance spectroscopy study of anodic growth of thick zirconium oxide films in H2SO4, Na2SO4 and NaOH solutions

Anodic zirconium oxide films were grown potentiodynamically at a constant sweep rate up to the breakdown potential on rod electrodes made of 99.8% metallic zirconium. Different media of different pH were tested, namely 0.5 M H₂SO₄ (pH 0.3), 0.1 M Na₂SO₄ (pH 9) and 0.1 M NaOH (pH 13). By electrochemical impedance spectroscopy and scanning electron microscopy the oxide film thickness was monitored during the voltage scan. The behaviour was found to be different in the presence and absence of sulphate anions. In the presence of SO₄²⁻, the films were dense but breakdown occurred at 300–340 nm. In NaOH, two relaxations appeared above 50 V and were ascribed to a bi-layered coating structure and the maximum layer thickness was 720 nm before breakdown.     

Influence of ultrasound irradiation on the adsorption of bovine serum albumin on copper

The effect of irradiation by power ultrasound on the adsorption of proteins on copper has been investigated, using bovine serum albumin (BSA) as a model protein in pH 7 phosphate buffer solution. Open circuit potential measurements, cyclic voltammetry and electrochemical impedance spectroscopy were used to characterise the copper/solution interface. Electrochemical impedance measurements at potentials close to the open circuit potential showed that pulsed ultrasound irradiation removes the naturally formed copper oxide films in phosphate buffer solution, and that their re-formation can lead to an oxide film with different electrical characteristics. Adsorption of BSA blocks the surface, decreasing or increasing the interfacial resistance, depending on the applied potential and the oxide characteristics, as well as changing the interfacial capacitance. This study augurs well for application of the combination of electrochemical impedance plus ultrasound to other systems. Special issue dedicated to Prof. Tony Wragg.