Anodic growth of passive layers on steel rebars in an alkaline medium simulating the concrete pores

Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques have been used to study passive layers anodically grown on steel rebars in an aqueous alkaline solution simulating the electrolyte of the concrete pores. Nyquist diagrams recorded by EIS at the different stabilization potentials show a diffusional tail at low frequencies. The analysis of the impedance measurements has been made by means of an equivalent circuit with a Warburg component and within the framework of the point defect model (PDM) theory. It is observed that the calculated concentration of vacancies is a function of the potential in accordance with the theoretical prediction of the PDM.     

Electrochemistry and surface chemistry of stainless steels in alkaline media simulating concrete pore solutions

Despite the increased use of stainless steel for concrete reinforcement in harsh chloride environments, comparatively little is known about the surface chemistry of these materials in alkaline media simulating concrete pore solutions. This work is concerned with a combined electrochemical and XPS surface analytical investigation on austenitic, ferritic and duplex stainless steels in simple NaOH and more complex alkaline concrete pore solutions. The results show that the passive films on these materials change with immersion time, the ferritic and duplex stainless steels becoming enriched in chromium oxy-hydroxide, the austenitic steel strongly enriched in nickel hydroxide. The composition of the metal layer beneath the surface film is strongly enriched in nickel and depleted in chromium and iron. The results are discussed with respect to the relation between the Fe(II) content in the films and the open circuit potential (OCP) during exposure, the film growth mechanism and localized corrosion resistance.     

Impedance spectroscopic determination of effect of temperature on the transport resistances of an electro-electrodialysis cell used for concentration of hydriodic acid

Effect of temperature on different transport resistances of an electro-electrodialysis (EED) cell used for concentration of hydriodic acid (HI) was found by equivalent circuit modeling of the measured impedance response of the cell. The EED cell consisted of two compartments separated by Nafion 117 membrane and each compartment had a platinum electrode. Both the compartments were filled with aqueous solution of 55?wt% HI containing 0.5?M iodine. Impedance measurements were carried out at four different temperatures in the range of 308?C353?K. Equivalent circuit for the cell consisted of a resistor for ohmic resistance of the cell, a Warburg element for the resistance due to diffusion boundary layer and a constant phase element for the resistance to transport of ions due to non-electro neutral heterogeneous transport layer at the membrane. Effect of temperature on impedance due to heterogeneous transport was lower than the Warburg impedance and the solution and membrane resistance. Effective capacitance of the heterogeneous transport layer was found to reduce with temperature. The dynamics of the heterogeneous transport layer along with the diffusional boundary layer were found to reduce with increase in the cell operating temperature.     

Nickel-free manganese bearing stainless steel in alkaline media—Electrochemistry and surface chemistry

The use of austenitic nickel-containing stainless steels as concrete reinforcement offers excellent corrosion protection for concrete structures in harsh chloride bearing environments but is often limited due to the very high costs of these materials. Manganese bearing nickel-free stainless steels can be a cost-effective alternative for corrosion resistant reinforcements. Little, however, is known about the electrochemistry and even less on surface chemistry of these materials in alkaline media simulating concrete pore solutions. In this work a combined electrochemical (ocp = open circuit potential) and XPS (X-ray photoelectron spectroscopy) surface analytical investigation on the austenitic manganese bearing DIN 1.4456 (X8CrMnMoN18-18-2) stainless steel immersed into 0.1 M NaOH and more complex alkaline concrete pore solutions was performed. The results show that the passive film composition changes with immersion time, being progressively enriched in chromium oxy-hydroxide becoming similar to the conventional nickel-containing stainless steels. The composition of the metal interface beneath the passive film is strongly depleted in manganese and enriched in iron; chromium has nearly the nominal composition. The results are discussed regarding the film growth mechanism (ageing) of the new nickel-free stainless steel in alkaline solutions compared to traditional austenitic steels. Combining the results from pitting potential measurements with the composition of the passive film and the underlying metal interface, it can be concluded that the resistance against localized corrosion of the new nickel-free stainless steel relies on the strong chromium(III) and molybdenum (VI) oxy-hydroxide enrichment in the passive film.     

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.     

Effect of bare metal surface on the dissolution in aqueous citrate solutions of magnetite films on carbon steel

The potential–time dependence of magnetite under varying conditions of pH and Fe2 concentration and of magnetite layers on carbon steel with initially exposed small bare metal areas have been studied. The experiments simulate the case of magnetite scales partially removed from surfaces in the course of chemical cleaning when coupling conditions occur with area ratios variable with time. The interpretation is based on an equivalent electric circuit composed of internal current generators of faradaic origin and capacitors simulating the electric double layer at the metal/solution and oxide/solution interfaces.     

Modelling of the steel–concrete interface to obtain information on reinforcement bar corrosion

The selection of an equivalent circuit that faithfully models the steel–concrete interface response to the application of electrical signals is a fundamental aspect of the electrochemical determination of reinforcement corrosion rate. Experimental evidence is provided in favour of using a modified Randles circuit in which the corroding interface is characterised by a parallel combination of a constant phase element and a charge transfer resistance in series with a Warburg element. An advantage of having an appropriate model of the steel–concrete system is the possibility of carrying out studies of quality of the information that is extracted from the experimental data. A sensitivity-algorithm is applied with the object of identification of the conditions in which the systems response is dominated by certain parameters or combinations of them.     

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.     

铝合金在碱性和中性溶液中电化学行为的研究

通过对铝阳极进行交流阻抗、塔菲尔曲线、析氢速率、开路电压测试,研究了多种铝合金在碱性和中性溶液中的电化学行为。发现Al-Ga-Sn-Mg和Al-Ga-Sn-Zn具有较好的电化学性能。与纯铝相比,在碱性电解液中,Al-Ga-Sn-Mg的开路电位负移70mV,抑制析氢速率为37.5%;在中性电解液中,Al-Ga-Sn-Mg合金的开路电位负移447mV。铝阳极在碱性和中性溶液中的反应机理不同。     

丙三醇对钒液流电池电解液影响的交流阻抗研究

采用交流阻抗法研究了添加剂丙三醇对全钒氧化还原液流电池阳极电解液电极反应影响的内部作用机理。通过RS(Cd(RpW))形式的等效电路对其阻抗谱进行了较好的模拟解析，研究结果发现，随着丙三醇含量的依次增加，溶液电阻、双电层电容和极化电阻均出现最小值，浓差极化电阻出现最大值，此时添加量均为1%，说明含有一定量供电子基团结构的丙三醇能够有效结合钒离子，均匀分散于溶液中，使得溶液电阻降低，当钒离子结合丙三醇后体积增大，双电层电容距离增加，从而双电层电容降低，当钒离子结合丙三醇后更利于向电极表面吸附，提高阳极电解液的催化效率，加大了电极表面反应物浓度与本体溶液中的差值，使得浓差极化电阻增强，电极反应效率提高。     

Thickness and structure of passive films on iron in acidic and basic solution

This paper describes the effect of solution pH on the passive film on iron which was investigated by ellipsometry and by cathodic reduction combined with chemical analysis. Anodic two-step passivation was employed to exclude anodic deposition of ferrous ion, resulting from anodic oxidation of iron. The film in acid solution is almost anhydrous oxide, whereas the film in neutral and alkaline solution has an outer hydrated oxide layer, whose thickness decreases with decreasing pH. The hydrated outer layer depends also on the anion present in solution. The transition layer model of the film is proposed to explain the results.     

Electrochemical behaviour of cadmium in NaOH solution

The electrochemical behaviour of cadmium in 1 M NaOH solution was studied. Different electrochemical methods such as potentiodynamic measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. During the course of polarization, several characteristic features were observed, including prepassivation and a relatively wide passive range extending over 1.4 V. Equivalent circuit models for the electrode–electrolyte interface under different applied potentials were proposed. The impedance data measured at steady state were fitted to calculated data according to proposed equivalent circuits. The relevance of the proposed equivalent circuits to the different phenomena occurring at the electrode–solution interface was discussed. The passive film formed on Cd is composed of two layers: a barrier layer in contact with metal and a deposit on the barrier layer. The barrier layer appears to form directly from the metal through a solid state reaction, rather than by a dissolution–precipitation mechanism. For an applied potential >+0.7 V the relative passive film thickness and resistance decrease to lower values indicating transpassive dissolution at this potential.     

Electrochemical study for the characterisation of wet silicon oxide surfaces

Because wet ultra-thin silicon oxides are extensively used in the microelectronic industry, we have investigated the growth of these oxides in various aqueous solutions using three main electrochemical techniques: (i) open circuit potential variation with time; (ii) linear voltammetry in a narrow range of potential; and (iii) electrochemical impedance spectroscopy under various polarisation potentials, to collect quantitative data regarding the growth kinetics of silicon oxide passivating layer, mainly at room temperature (r.t.). In oxidising alkaline solutions, the surface silicon oxide layer reached a limiting thickness value with time, related to oxidation/dissolution stationary behaviour. This observation was confirmed using ellipsometry. It was possible to reach with electrochemical techniques and ellipsometry the etching rate of the silicon substrate under the oxide layer in alkaline solution. Another interesting observation in this study was that the oxide layer showed a pronounced permeability to ions and oxidising agents in alkaline media, while this phenomenon vanished in acidic solutions.     

Chloride induced reinforcement corrosion: Rate limiting step of early pitting corrosion

Transition from passive state to stable localised corrosion of reinforcement steel in concrete owing to chloride ingress takes place over a period of time rather than being a one-step-occurrence. The depassivation process was characterised by frequent measurements of corrosion potential, polarisation resistance, and macro-cell currents when short-circuiting the working electrode with additional cathode surface. In addition, the concrete resistivity was continuously monitored and cathodic and anodic polarisation curves were measured. The results are consistent and imply that the localised corrosion process is in the initial phase of pit growth under mixed anodic/ohmic control. With time, the anodic reaction kinetics become more limited and the corrosion rate gets almost entirely determined by anodic control. The observed relationship between achieved maximum corrosion current and concrete resistivity indicates that the extent to which the anodic reaction kinetics are restricted is determined by the concrete microstructure and its ability to retain ionic movement.     

Electrochemical and ellipsometric study of the oxide films formed on copper in borax solution: Part I: Effect of oxygen

The formation and reduction of passive layers on copper in weakly alkaline solutions saturated with N2 and O2 were studied. Voltammetric and ellipsometric techniques were employed to examine the structural characteristics of the layers formed in the –0.32 to 0.75 V vs RHE potential region. Optical measurements at open circuit potentials (Eoc) were also made to simulate operational conditions. The passive layer consists of a duplex structure: an outer hydrated copper oxide film and an inner dehydrated film. This inner layer is composed of Cu2O with a surface excess of Cu(ii) ions. The growth rate of the oxide layers at controlled potentials is higher in O2 saturated solution. The corrosion resistance of copper depends on the presence of O2 in the electrolyte, on the stirring rate and on the Eoc value.     

Electrochemical behaviour of high strength steel wires in the presence of chlorides

The present work summarises results of the electrochemical behaviour of high strength steel wires in a high alkaline medium simulating the solution present in concrete. The aim is to expose the possible factors affecting the initiation period of stress corrosion cracking, the most frequent form of failure of pre-stressed concrete structures.Cold drawn steel wires were subjected to constant stress at 70% of their ultimate tensile strength and exposed to corrosive environments in order to study the stability of the passivating layer. The effects of chloride concentration, temperature, presence of oxygen, and cavitation induced by ultrasonic waves were studied. Two different techniques (cyclic voltammetry and impedance spectroscopy) were employed to study the effects of those parameters on the formation/breakdown of the passive layer. The results show that variations in the studied parameters can provoke breakdown of the passivating layer even at very low chloride concentrations. Chlorides, oxygen availability, temperature changes, and vibrations have been found to be important factors in the corrosion initiation of pre-/post-tensioned tendons.     

Electrical equivalent circuit of an ion-exchange membrane system

Usually, the current flowing through an electrochemical cell is divided into the faradaic current going to an electrochemical interface reaction, and the current charging electric double layer (EDL). This division leads to the Randles–Ershler equivalent circuit with an EDL capacitance in one branch, and the faradaic impedance in the other, specific for each particular system. However, the physics of the separation of the impedance into faradaic and capacitive components for different electrochemical systems is not sufficiently clear. The most of derivations resulting in the formal construction of the Randles–Ershler or similar equivalent circuits are based on the a priori separation of the electroneutral and the double-layer regions.In this paper, we derive an equation for the impedance of a three-layer system consisted of an ion-exchange membrane and two adjoining diffusion boundary layers (DBL) starting from the Poisson equation. The system is polarized by a constant electric current over which a small sinusoidal signal is applied. The equation shows that the impedance of the considered system can be formally interpreted via an equivalent circuit with a frequency dependent capacitance in one branch and a finite-length Warburg-type impedance in the other. To take into account this dependence, the impedance of the system may be presented as a series connection of five circuits. Three of them are consisted of a geometric capacitance connected in parallel with an ohmic resistance, respectively, for both diffusion layers and for the membrane bulk; the two others being the double-layer capacitance in parallel with the finite-length Warburg impedance for the left and the right interfaces, respectively. The comparison of the impedance spectra calculated within our analytical approach with those obtained by the full numerical solution of the Nernst–Planck–Poisson (NPP) equations shows a good agreement. Different possible situations, which might arise in real systems (different stationary current densities, different thicknesses of the left-hand and right-hand DBLs), are analysed when applying the approximate solution.     

非贵金属催化碱性硫离子-空气燃料电池

以碱性硫离子电解液作为阳极燃料构建了硫离子-氧气燃料电池体系,采用粉末活性炭材料制备了涂膏电极,将碱性硫化钠溶液作为阳极燃料,通过建立电化学三电极模型对电极在碱性硫离子溶液中的放电性能进行研究,主要考察了硫离子浓度、体系温度对开路电位以及放电平台的影响。电极在碱性硫离子溶液中具有较负的开路电位和稳定的放电平台;通过单体电池测试在0.24V电压下获得11mW/cm2的最大功率密度,此时电池的电流密度为46mA/cm2,证明碱性硫离子燃料电池在阳极不使用贵金属催化剂的情况下表现出良好的放电性能,是一种具有潜在研究价值和广泛应用前景的电化学体系。     

Understanding aluminum behaviour in aqueous alkaline solution using coupled techniques: Part II: Acoustic emission study

This work focuses on the behaviour of pure aluminum in alkaline media, by coupling both acoustic emission (AE) and direct hydrogen voltammetry to electrochemistry. We notably monitored, recorded and analyzed the acoustic emission activity generated by the aluminum electrode as a function of its polarization during a linear sweep voltammetry (from anodic to cathodic potentials) on pure aluminum in 4 M aqueous potassium hydroxide solution. Such in situ coupling of electrochemistry and acoustic emission shows a perfect correlation between the two signals. After careful analysis of the AE signal using a statistical treatment, and based on five relevant AE parameters (rise time, duration, amplitude, absolute energy, maximum frequency), we could separate various groups of AE signals occurring at the aluminum electrode. We further linked them to the different (and possibly concomitant) electrochemical phenomena, which are taking place upon polarization of the aluminum electrode in strong alkaline medium.First, we confirmed that hydrogen evolution initiates for potentials positive to aluminum open circuit potential in 4 M potassium hydroxide solution; such small but non-negligible hydrogen production occurs in parallel to aluminum oxidation. Second, aluminum oxides are present only around the open circuit potential; whereas they are eroded for high aluminum oxidation potentials, they are flaked off at high hydrogen evolution potentials. Such latter process is probably accelerated by the hydrogen evolution-induced alkalization of the electrolyte. Third, two modes of hydrogen evolution are recorded: one on the oxide, the other one on bare aluminum, the latter being the most efficient. This strong hydrogen evolution at very low electrode potential probably assists the removal of the brittle residual oxide/passive film present on aluminum (which we denote as hydrogen-assisted aluminum exfoliation corrosion), therefore causing the rapid erosion of the aluminum electrode. As a result, aluminum is never in immunity conditions in strong alkaline medium.     

Effect of an organic additive on the impedance of cadmium in alkaline solution

The a.c. impedance of a planar cadmium electrode in KOH solution was measured over the active and passive region using a potentiostatic technique. Different concentrations of polyvinyl alcohol (PVA) were subsequently introduced into the alkaline solution and the impedance remeasured. It was found that the addition of PVA markedly increased the double layer capacity of the electrode and decreased electrolyte resistance,R _sol. The impedance spectra in the presence of PVA were analysed in terms of an equivalent circuit involving the series combination of two double layer capacitances and a resistance proportional to the conductivity of the electrolyte.