The Influence of Ni Content on the Stability of Copper—Nickel Alloys in Alkaline Sulphate Solutions

The electrochemical behaviour of copper–nickel alloys with different Ni content (5–65%) in sulphate solutions of pH 12 was investigated. The effects of temperature, immersion time, and concentration of sulphate ions were also studied. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. Potentiodynamic measurements reveal that the increase in nickel content increases the corrosion rate of the alloy in sulphate solution linearly. Nevertheless, an increase in the nickel content along with increase in immersion time improves the stability of the Cu–Ni alloys due to the formation of a stable passive film. An equivalent circuit model for the electrode/electrolyte interface under different conditions was proposed. The experimental impedance data were fitted to theoretical data according to the proposed model. The relevance of the model to the corrosion/passivation phenomena occurring at the electrode/solution interface was discussed.     

Electrochemical and XPS investigations of cobalt in KOH solutions

The electrochemical behaviour of cobalt in KOH solutions of different concentrations was studied. The effects of applied potential, temperature and the presence of aggressive Cl^– ions were investigated. Different electrochemical methods such as open-circuit potential measurements, polarisation techniques and electrochemical impedance spectroscopy (EIS) were used. The electrochemical behaviour of cobalt in naturally aerated KOH solutions is characterized by three different regions according to the alkali concentration. Corrosion behaviour was observed at high concentrations (0.3–1.0 M); passivation at lower concentrations (0.01–0.05 M), and at intermediate concentrations (0.1–0.2 M) corrosion followed by passivation was recorded. The corrosion parameters (i _corr, E _corr, and R _corr) under various conditions were calculated. Equivalent-circuit models for the electrode–electrolyte interface under different conditions were proposed. The experimental impedance data were fitted to theoretical data according to the proposed models. The relevance of the proposed models to the corrosion–passivation phenomena occurring at the electrode–solution interface was discussed. The electrochemical experimental results and discussions were supported by surface analytical techniques.     

Impedance study of the passive film on stainless steel 304 in pH 8 carbonate solution

The effects of applied d.c. potential and polarization time on the passivation of stainless steel 304 (SS304) were investigated in deaerated 1 M NaHCo₃ aqueous solutions at pH 8. Electrochemical impedance spectroscopy was used in conjunction with a rotating disc electrode. The data were analysed by considering an equivalent circuit. The changes in impedance parameters at applied d.c. potential signal changes in the properties of passive films on SS304 and allow to distinguish the parameters at low potential (–0.6 to 0.3 V vs SCE) from a different one at high potential (0.5 to 0.8 V vs SCE). The oxidation reactions were controlled by both charge transfer and mass transfer processes. Diffusional resistance was high for both passive films and was considered to represent the resistance to movement of ions or vacancies through the surface layer of oxide films. It is deduced that the passive film present in the low potential region is partially dissolved at 0.4 V vs SCE and that a new passive film is formed in the higher potential region. The equivalent circuit used to obtain the best fit and the fitting parameters was dependent on the electrode potential and the polarization time. The reproducibility of the impedance spectra at constant potentials demonstrate that the passive film formation is highly irreversible process. No traces of localized corrosion were detected but, for a high potential and long polarization time, the electrode surface coloration to a uniform gold colour confirms the film thickening.     

Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells

The polarization processes occurring at the electrode–electrolyte interfaces of solid oxide fuel cells (SOFC) were investigated by electrochemical impedance spectra measured at single cells under realistic operating conditions. The approach presented is based on distributions of relaxation times which are the basic quantity of interest in electrochemical impedance data analysis. A deconvolution method was developed and implemented that yields these characteristic distribution patterns directly from the impedance spectra. In contrast to nonlinear least squares curve fit of equivalent circuit models, no a priori circuit choice has to be made. Even more importantly, the excellent resolving capacity allows the untangling of the impedance contributions of up to three physically distinct processes within one frequency decade. With the method, processes with the highest polarization losses can be identified and targeted to improve cell performance. Based on the distributions, a general strategy for the identification of the reaction mechanisms is given. The evaluation of the distributions in terms of peak parameters is illustrated by a physical model for oxygen reduction at the SOFC cathode–electrolyte interface. The method is expected to find many applications in electrochemistry beyond the field of solid oxide fuel cell development.     

Electrochemical behaviour of cobalt in aqueous solutions of different pH

A systematic study of the corrosion and passivation behaviour of cobalt in aqueous solutions of different pH was carried out. Open circuit potential measurements, polarization experiments and electrochemical spectroscopic (EIS) investigations were employed. The experimental results show that the metal surface is always covered by a native passive film which consists of CoO. The formation of the oxide film obeys a two-electron charge transfer process. The dissolution of the barrier film is controlled by the pH of the solution. In neutral and basic solutions the barrier film is stable. In these media a barrier film thickening with the formation of secondary layer is considered. In acidic solutions, the passive film is unstable and dissolves via a pure chemical process. The mechanism of the corrosion and passivation processes taking place at the electrode/electrolyte interface in the different solutions is discussed. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to confirm the electrochemical measurements and the suggested mechanisms.     

The passivity of AISI 316L stainless steel in 0.05 M H2SO4

The passivity of AISI 316L stainless steel (AISI 316L) in 0.05 M H₂SO₄, in the steady-state condition, has been explored using various electrochemical techniques, including potentiostatic polarization, electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis. Based on the Mott–Schottky analysis in conjunction with the point defect model (PDM), it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. The thickness of the barrier layer was increased linearly with the film formation potential. These observations were consistent with the predictions of the PDM, noting that the point defects within the barrier layer of the passive film are metal interstitials, oxygen vacancies, or both. Also no evidence for p-type behavior was obtained, indicating that cation vacancies do not have any significant population density in the passive film.     

Electrochemical corrosion behavior of magnesium and titanium alloys in simulated body fluid

The electrochemical behavior of AZ91D and Ti–6Al–4V alloys was investigated in simulated body fluid (SBF) at 37 °C. The aim of the present study was to evaluate their corrosion performance through the analysis of corrosion resistance variation with time, using electrochemical impedance spectroscopy (EIS) tests and corrosion current density using potentiodynamic polarization measurements. Very low current density was obtained for Ti–6Al–4V alloy compared to that of AZ91D alloy, indicating a typical passive behavior for Ti alloy. EIS results exhibited high corrosion resistance indicating a highly stable film on titanium alloy compared to magnesium alloy in SBF.     

Electrochemical impedance spectroscopy for studying passive layers on steel rebars immersed in alkaline solutions simulating concrete pores

Present paper deals with the use of the electrochemical impedance spectroscopy to identify different processes in the passive layer growth over steel rebar surface immersed in an alkaline media simulating the concrete pore solution. Two cases have been considered: a passive layer spontaneously grown in a high alkaline media and a passive layer assisted by the application of an anodic potential in the same media. The application of electric equivalent circuits allows distinguishing between the different mechanisms occurring in this passive layer when grows in different conditions. An electric equivalent circuit with two RC loops connected in parallel is often used for fitting the EIS diagrams obtained for spontaneous growth of passive layers in the alkaline solution simulating the concrete pores. However, when the passive layer is formed under anodic polarization, a Warburg element must be introduced in the equivalent circuit. According to the Point Defect Model (PDM), this Warburg element is allocated to the transport of oxygen vacancies through the passive layer, which concentration changes with the potential.     

Hydrophobic interfacing layers for improvement of corrosion protection by polymeric coatings

Water sorption of coating materials is the main cause of coating deterioration, adhesion loss and substrate corrosion. By introducing alkanethiol self-assembled monolayers (SAMs), a hydrophobic interfacing layer between coating and substrate metal can be constructed. The effect of the hydrophobic SAMs interfacing layer on the corrosion protection of epoxy coatings was evaluated using electrochemical techniques including Tafel polarization, electrochemical impedance spectroscopy and impedance–time transition measurement. It was found that the SAMs interfacing layer improved the corrosion protection of the coating significantly. The improvement was attributed to the strong interaction between SAMs and the metal substrate, the compact structure and low water affinity of the SAMs interfacing layer, which prevent water absorbed by the coating from reaching the coating–metal interface and spreading along the interface.     

Passivity and passivity breakdown of 304 stainless steel in alkaline sodium sulphate solutions

The passivity and passivity breakdown of 304 stainless steel were investigated in 0.25 M Na₂SO₄solutions of pH 10. The effect of applied potential and the presence of Cl^– ions in the electrolyte were also studied. Different electrochemical methods such as open circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results showed that the steel electrode passivates under open circuit conditions and also under potentiostatic control. The rate of passive film thickening under open circuit conditions follows a simple logarithmic law. Addition of Cl^– ion shifts the polarization curves in the active direction and above a critical chloride concentration, [Cl^– ] 0.15 M, pitting corrosion occurs and the pitting potential, E _pit, decreases linearly with the logarithm of [Cl^–]. The addition of sulphate ions to the chloride-containing solutions was found to inhibit the pitting process, and at [SO^2- ₄] 0.25 M, a complete immunity to pitting corrosion was recorded. The impedance measurements provided support for film thickening and film breakdown reactions. An equivalent circuit model which consists of a pure resistor, R , in series with a parallel combination of a pure resistor, R _p, and a constant phase element, Q, was proposed to describe the electrode/electrolyte interface. The passive film thickness was found to increase with applied potential up to a critical value of 0.3 V. At higher voltages, breakdown of the passive film occured.     

Inhibition of CuNi10Fe corrosion in seawater by sodium-diethyl-dithiocarbamate: an electrochemical and analytical study

The efficiency of sodium-diethyl-dithiocarbamate (NaDDTC) as corrosion inhibitor for CuNi10Fe alloy was studied in natural seawater by means of immersion tests, polarization measurements, electrochemical impedance spectroscopy (EIS) and Auger electron spectroscopy (AES). After immersion of the alloy for 24 h in various concentrations of NaDDTC, layers were formed which act as a physical barrier to the corrosion attack. The polarization curves obtained with electrodes in seawater indicate retardation of both the anodic and cathodic processes. From EIS measurements equivalent circuits, which illustrate the behaviour of the reaction product film, were proposed and individual circuit elements at the open circuit potential and anodic polarization were defined. The results obtained for inhibitor efficiency from weight losses, corrosion currents and from impedance measurements are in fair agreement. The inhibition film does not lose efficiency even at anodic polarization, although the AES measurements indicate a change in the organic molecule structure. The hypothesis proposed is that a possible reconstruction of the organic molecule takes place, with release of CS₂.     

Electrochemical behavior of aluminum bronze in sulfate-chloride media

The electrochemical behavior, especially the corrosion and passivation, of a Cu–Al bronze was investigated. Conventional electrochemical techniques including open-circuit potential, anodic polarization, cyclic voltammetry and electrochemical impedance spectroscopy were used. It was found that the addition of chloride ion up to 0.15 M in 0.5 M Na₂SO₄ solution decreases the corrosion rate due to the formation of CuCl, whereas at higher concentration of the chloride ion, the corrosion rate increases due to the formation of the soluble CuCl₂⁻. The activation energy was found to be 10 kJ mol⁻¹. This value indicates that the corrosion process is under diffusion control. The impedance measurements showed that the passive film can be represented by a duplex layer, a relatively thick porous outer film on top of a thin compact layer. An equivalent circuit was used to explain and analyze the impedance data. The model includes another R-C combination and Warburg impedance in addition to the simple Randles cell to account for the spontaneously formed passive film and the diffusion phenomena.     

Electrochemical behaviour of heat-treated AlZnMg alloys in chloride solutions containing sulphate

The electrochemical corrosion and passivation of Al5Zn1.7Mg0.23Cu0.053Nb alloys, submitted to different heat treatments (cold-rolled, annealed, quenched and aged, and quenched in two steps and aged), in sulphate-containing chloride solutions, has been studied by means of cyclic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The cyclic polarization curves showed that sulphate addition to the chloride solution produced a poor reproducible shift of the breakdown potential to more positive potentials. The repassivation potentials, much more reproducible, and practically separating the passive from the pitting potential region, were slightly displaced in the negative direction with that addition. When the alloys were potentiodynamically polarized in the passive potential region, sulphate was incorporated in the oxide film, thus precluding chloride ingress. In addition, Zn depletion was favoured, whereas Mg losses were avoided. Different equivalent circuits corresponding to different alloys and potentials in the passive and pitting regions were employed to account for the electrochemical processes taking place in each condition. This work shows that sulphate makes these alloys more sensitive to corrosion, increasing the fracture properties of the surface layer and favouring the pitting attack over greater areas than chloride alone.     

Nature of the passive film on nickel

The passive film formed anodically on nickel in borate buffer solution in both the passive and transpassive regions is found to be p-type in electronic character, corresponding to a preponderance of metal vacancies (over oxygen vacancies and nickel interstitials) in the barrier layer. However, at high anodic potentials, some n-type character was detected by Mott-Schottky analysis, which is probably due to the presence of free charge carriers (electrons) from the evolution of oxygen and/or the oxidative ejection of Ni³⁺ at the barrier layer/outer layer interface. The p-type character of the film is consistent with the diagnostic criteria obtained from the Point Defect Model for a passive film, in which the majority defect in the NiO barrier layer is the metal vacancy. The transpassive state is postulated to comprise a thick, porous oxide film on the surface, with the current probably being due to the oxidative ejection of Ni³⁺ species from the barrier layer and oxygen evolution within the pores, or both.     

Electrochemical impedance spectroscopic characterization of titanium during alkali treatment and apatite growth in simulated body fluid

Alkali treatment of titanium with subsequent heat treatment has been adapted as an important pre-treatment procedure for hydroxyapatite formation in orthopaedic applications. The electrochemical study during the alkali treatment process has not been explored yet. In the present work, electrochemical impedance spectroscopic (EIS) studies have been employed to analyse the electrochemical behaviour of titanium during the alkali treatment. The open circuit potential and potentiodynamic polarisation measurements were carried out in simulated body fluid (SBF) solution. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the surface morphology and to correlate the results obtained from the electrochemical studies. An optimum growth of the passive film was found to occur at the end of 17th hour of treatment by alkali treatment. The alkali treated titanium immersed in SBF solution for various durations exhibited the formation of a duplex layer structure due to an inner barrier layer and an outer gel layer during the initial periods of immersion. However, with increase in immersion time to 10 days, a stable apatite layer was formed over the barrier layer and this was confirmed from the equivalent circuit fitted for the impedance data.     

Participation of the dissolved O2 in the passive layer formation on Zn surface in neutral media

The composition of the passive layers formed on Zn electrode in naturally aerated and de-aerated 0.1 M KClO₄ solution were studied using X-ray photoelectron spectroscopic measurements (XPS). A correlation between the presence of dissolved O₂ and the formation of an interior passive layer was carried out. Librated Cl⁻ from the perchlorate reduction reaction was detected in its solutions during Zn electrode polarization. The librated Cl⁻ concentration reached its maximum value at −1.4 V. Moreover, in the studied potential range the perchlorate reduction rate increases in absence of dissolved oxygen. Chrono-amperometry and electrochemical impedance spectroscopy (EIS) were performed for the stationary and the rotating disc Zn electrodes in naturally aerated and de-aerated 0.1 M KClO₄ solution. EIS technique showed a change in the electrode impedance with the experimental conditions as a result of changing the reactions occurring in the electrode vicinity. The obtained data were fitted to three different equivalent circuits depending on the electrode potential. The protective nature of the passive layers formed in different experimental conditions was found to decrease with rotating the electrode and de-aerating the solution.     

Effect of Zn and Pb as alloying elements on the electrochemical behavior of brass in NaCl solutions

The electrochemical behavior of brasses with various Zn content (5.5–38 mass%) and brass (Cu–38Zn) with different Pb contents (1–3.4 mass%) in 0.6 M NaCl was investigated. The effects of temperature, immersion time, and concentration of chloride ions on the behavior of the different alloys were studied. The pitting corrosion behavior of Cu–Zn alloys and leaded–brass alloys in 0.6 M NaCl solution was also investigated. Open-circuit potential measurements (OCP), polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results show that the increase in the Zn content increases the corrosion rate of the brass alloys in chloride solutions, while the increase of Pb content in Cu–38Zn–Pb decreases the corrosion rate of the alloy. Long immersion time of the alloys in the aqueous electrolyte improves their stability due to the formation of passive film on the alloy surface. The breakdown potential is shifted to more negative direction with increasing the Zn content, whereas it shifts towards positive values with increasing Pb content. Equivalent circuit model for the electrode/electrolyte interface under different conditions was proposed to illustrate the electrochemical processes taking place at the interface. The electrochemical behavior of the different alloys was discussed in view of the fitting results.     

Effects of solution temperature on the kinetic nature of passive film on Ni

Effects of solution temperature on the kinetic nature of passive film on Ni were investigated using polarization test, Mott–Schottky analysis and electrochemical impedance spectroscopy to reveal why the corrosion resistance of Ni is degraded with an increase in solution temperature. The increase in the corrosion rate of Ni with solution temperature was confirmed by the increase in the passive current density and also in the steady-state current density. Mott–Schottky analysis revealed that the passive film formed on Ni exhibits a p-type semiconducting characteristics irrespective of the solution temperature, and the concentration of cation vacancy in the passive film increases with temperature. By optimizing the reduced PDM (point defect model) on the experimental impedance data, base rate constants and transfer coefficients for the charge transfer reactions occurring at the metal/film and film/solution interfaces were extracted, and the Warburg coefficient for the cation vacancy transport was also determined. According to the calculated kinetic parameters (rate constants for the interfacial reactions, diffusivity of cation vacancy, etc.), the mechanism for the degradation of corrosion resistance of Ni with solution temperature was explained.     

Electrochemical properties of metallurgical-grade silicon in hydrochloric acid

The electrochemical properties of metallurgical-grade silicon (MGS) and its native oxide (passive layer) were investigated using standard potentiostatic, potentiodynamic and impedance methods. The corrosion rate was found to reach up to 37 μm year⁻¹ in undiluted hydrochloric acid. The passive oxide layer was found to thicken with time as well as with potential. Increasing the potential from −0.5 to 1.0 V vs. SHE increases the thickness of the passive layer from approximately 2 to 6 nm. The interfacial impedance data for MGS in contact with the electrolyte was modeled. The model takes into account the capacitive behaviour of the space charge region and passive film. Due to the non-ideal behaviour of the passive film, a constant phase element (CPE) was incorporated in the model. Extraction of Mott–Schottky plot data from the EIS tests showed that the passive layer on the surface of the MGS is a p-type semiconductor. Scanning electron microscopy was used to determine that the impurities in the silicon (mostly iron, aluminum and calcium) were located almost exclusively at grain boundaries.     

Micro-electrochemical characterization and Mott–Schottky analysis of corrosion of welded X70 pipeline steel in carbonate/bicarbonate solution

The electrochemical corrosion behavior of welded X70 pipeline steel in a bicarbonate/carbonate solution was studied by scanning vibrating electrode technique and localized electrochemical impedance spectroscopy as well as Mott–Schottky analysis. The effect of applied stress on corrosion of various zones in welded steel specimen was investigated. Results demonstrated that passivity can be established on various zones, i.e., weld metal, heat-affected zone (HAZ) and base steel, of welded X70 steel. There is a higher passive current density for the passivated HAZ than that for weld metal and base steel. Applied stress enhances the anodic dissolution of steel, as indicated by the decreasing charge-transfer resistance and increasing dissolution current density of steel with the increase of stress. A maximum current density is observed in HAZ at all stressing levels. With the positive shift of anodic potential, the stability of passive film is enhanced, and the dissolution current density decreases. When the applied potential approaches the water oxidation potential, the current density increased significantly again due to the oxidation of water. The passive film formed is an n-type semiconductor. There are higher donor density and thinner space-charge layer in passive film formed at HAZ than those formed on weld metal and base steel. Moreover, the donor densities in all zones increased significantly with the applied stresses. The metallurgical transformations and formation of low temperature hardening phase transformation products during welding result in the enhanced electrochemical activity in HAZ.