M. Keddam

Reaction Model for Iron Dissolution Studied by Electrode Impedance I . Experimental Results and Reaction Model

Steady‐state polarization curves and electrode impedances were measured during the dissolution of iron in solution acidified by the addition of . These experiments were performed within very widepH (0–5), current density (up to 0.1 A cm−2), and frequency (10−3–105 Hz) ranges. Three time constants, in addition to the high‐frequency capacitive loop attributed to the double layer capacity and the charge transfer resistance, were observed before the onset of the passivation process. The experimental results were quantitatively interpreted by computer simulation on the basis of a reaction model including three dissolution paths. At low current densities, the dissolution path, which can be related to the consecutive mechanism, controls the overall rate. At higher current densities, a self‐catalytic path, implying a ferrous intermediate, determines the overall current. Another self‐catalytic path, with monovalent iron, plays an important role in the electrode impedance and the prepassivation process although its contribution to the current is not prevalent at any pH.

Use of impedance measurements for the determination of the instant rate of metal corrosion

The various methods of evaluating corrosion rate and charge transfer resistance have been critically reviewed on the basis of the recent developments on the measurement and interpretation of faradaic impedances. It is demonstrated that the entity which is most accurately correlated with corrosion rate is the transfer resistance, the limit of the faradaic impedance at infinite frequency. In the case of iron, with and without inhibitor (propargylic alcohol), it has been ascertained, under various experimental conditions, that the measurement of this resistance constitutes nowadays the best electrochemical test for corrosion, and allows thea priori calculation of corrosion rate.

Noise Resistance Applied to Corrosion Measurements I. Theoretical Analysis

The measurement of current and voltage fluctuations on the same electrochemical cell allows the evaluation of a quantity R{sub n} known as noise resistance, which has been proposed as an indication of the corrosion resistance of the material under study. The theoretical basis for the relationship between R{sub n} and the electrode impedance Z is developed, taking into account the various measurement schemes currently in use. Parameters such as cell geometry, solution resistance, and electrodes with different kinetics are considered. It is shown that, in general, the modulus of the electrode impedance can be derived by measuring the power spectral densities (PSD) of the voltage and current noises. The circumstances in which R{sub n} is equal to the polarization resistance of the electrode are also discussed.

Electrochemical behaviour of steel rebars in concrete: influence of environmental factors and cement chemistry

Four series of reinforced concrete specimens have been studied over 3 years exposure in a 100% relative humidity atmosphere. Addition of CaCl2, NaNO2, and a mixture of CaCl2 and NaNO2 changed the cement chemistry with respect to an ordinary portland cement series of samples used as reference series. The study, based on low-scan rate cyclic voltammetry and electrochemical impedance spectroscopy, confirms previous results obtained in alkaline medium, i.e. the redox activity in the rebar’s oxides layer greatly influences the electrochemical behaviour of rebars in the passivity potential domain. Different redox processes also influence the active and cathodic protection domains that make corrosion rate estimations very difficult. The possibilities of estimating corrosion rate are discussed in terms of cement chemistry and corrosion potential of the system.

Use of EIS, ring-disk electrode, EQCM and Raman spectroscopy to study the film of oxides formed on iron in 1 M NaOH

Abstract Different electrochemical techniques (electrochemical impedance spectroscopy (EIS), ring-disk electrode, electrochemical quartz crystal microbalance (EQCM), and in situ Raman spectroscopy) have been employed to study the behaviour of the passive film formed on iron in alkaline medium simulating pore solution in fresh concrete. The study, based on low scan rate cyclic voltammetry performed over the entire electrolyte stability domain, allows for establishing the influence of the redox activity developing in the oxides layer on the electrochemical behaviour of the system and, thus, to get valuable information on the applicability of classical electrochemical techniques employed to assess corrosion of steel in concrete. The passive film is based on a magnetite-type structure which, in partially reversible processes, can be oxidised and reduced depending on the electrode potential. Those redox processes mask the corrosion process itself.

Magnetic water treatment for scale prevention

A home-made magnetic device was built with permanent magnets for treating scaling waters. Its efficiency was evaluated by measuring the remaining ionic calcium at the output of the device by means of an ion selective electrode. The scaling power of the treated water was estimated through an electrochemical scaling test. Chroamperometric curves and chronoelectrogravimetric curves were plotted to obtain the scaling time and the nucleation time of the scale deposition. The variation of the efficiency of the magnetic treatment was studied when the length of treatment, the flow velocity of the scaling water in the device, the material of the pipe where the scaling water flowed were changed. An empirical relationship, which gives the value of the efficiency in function of the length of treatment and the flow velocity, was proposed. Possible mechanisms of action of the magnetic treatment were discussed.

Structural defects and electrochemical reactivity of β-Ni(OH)2

Abstract Electrochemical reactivities and structural properties of several nickel hydroxide powders were analysed by X-ray diffraction, Raman spectroscopy and extended X-ray absorption fine structure (EXAFS). It is shown that the electrochemical efficiency of β-Ni(OH) 2 is associated with the amount of proton vacancies included in the crystal lattice. The number of those proton vacancies increases when the crystallite size decreases or when the ratio of co-precipitated cobalt increases. Proton vacancies shift the oxidation potential of β-Ni(OH) 2 towards less anodic values and, therefore, improve the chargeability and the electrochemical efficiency of nickel hydroxide. It is shown that both Raman spectroscopy and X-ray diffraction techniques can be used to predict effectively the electrochemical efficiency of β-Ni(OH) 2 hydroxide. EXAFS results indicate also that the oxidation level of nickel atoms inside the hydroxide is not modified by the existence of proton vacancies. It means probably that to maintain the electroneutrality in the whole crystal induces others singularities. Finally, the influence of co-precipitated additives such as cadmium and cobalt on the rate of defects has been investigated.

Calibration of the Electrochemical Quartz Crystal Microbalance

The quartz crystal microbalance seems to be a very usefui tool in electrochemical studies, but, up to now, no attempts have been made to calibrate the microbalance under electrochemical conditions. The aim of this paper is to determine not only the average sensitivity for different active areas but also the differential sensitivity as a function of the radial position of a localized change of mass. In this way, for homogeneous mass perturbation, it is possible to calculate the change of mass (Am) from the frequency shift (AJ) for different active areas by taking into account the average sensitivity value. In the case of localized Am, the differential sensitivity value allows one to calculate hm from hfif the position of the event is known. The calibration procedure was made for an AT-6 MHz quartz crystal. Quartz crystals are important sensing devices in liquid phase. They can be used to monitor changes in electrode mass (1-3) or changes in the liquid properties (4, 5). In an electrochemical process, a shift of the value of the quartz crystal resonance frequency (fo) can be attributed to a change in mass (Am) of the electrode, provided that the true relationship between hf and Am is known. The problem of the sensitivity of the quartz crystal microbalance (QCM) has been already dealt with in the air (6-8). The differential sensitivity of the QCM has been calculated from the local damage produced in a thin polymer film by an ion beam sputtering. In the liquid phase the distribution of the vibration amplitude of the quartz crystal has been investigated by putting a tungsten wire probe in contact with the quartz crystal to provoke a change in frequency (9, 10). The results obtained in both liquid and gas phases are rather similar, in spite of the properties of liquid phase which extend the vibration of the quartz crystal even beyond the active region defined by the metal deposit. These works show the complexity of the system and that the boundary conditions used for the derivation of Sauerbreys equation (11, 12) are not always fulfilled. The problem of the QCM calibration has been extensively studied in air. on the contrary, despite the wide use of the QCM in electrochemistry, the calibration procedure of this technique in electrochemical conditions is not depicted in the literature. The aim of this work is to calibrate the electrochemical quartz crystal microbalance (EQCM), not only in the case where hm is uniformly spread on the active electrode surface, but also for localized mass changes. The last case is very relevant to all electrochemical phenomena which produce a change of mass on a small area, e.g., in localized corrosion or gaseous bubble evolution. The general case where the mass change is continuously but nonuniformly distributed across the surface is not addressed in the paper.

The characterization of porous electrodes by impedance measurements

Abstract Impedances of a gold-powder and a Raney-gold electrode were measured over a wide frequency range, and were found to be very similar to those of cylindrical pore of finite depth. The simulation calculation for sphere-packed electrode impedance was performed. It was observed that even though each sphere-layer shows the impedance related to an occluded pore-shape, the overall impedance is similar to that of a cylindrical pore electrode. When the penetration depth approaches the pore depth, the shape of the pore wall has little influence on the impedance. Thus, impedance measurement techniques can be applied to porous electrodes of more intricate pore-texture, and evaluate the radius, depth and pore number of its equivalent cylindrical pore electrode. These values determined for a Raney-gold electrode are in very good agreement with those determined by other methods. The case of a more realistic fuel cell electrode such as Raney-nickel with a metal-electrolyte-gas system, was also tentatively examined.

Protective effect of electropolymerized 3-amino 1,2,4-triazole towards corrosion of copper in 0.5 M NaCl

Abstract The electro-oxidation of 3-amino 1,2,4-triazole on a copper substrate in alkaline methanol solution produces a homogeneous and adherent polymer film. The formation kinetics of the film was investigated using cyclic voltammetry, chronoamperommetry and quartz crystal microbalance. The protection of poly-amino 1,2,4-triazole (pATA) films obtained by different electropolymerization conditions was determined by electrochemical impedance spectroscopy in 0.5 M NaCl solutions. These results were compared with those obtained with bare copper electrodes immersed in this corrosive medium. The protection of ATA reaches 99% after one month immersion test.