Hisasi Takenouti

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.

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.

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.

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.

Electrochemical behaviour of zinc-rich epoxy paints in 3% NaCl solution

Electrochemical impedance spectroscopy (EIS) in the 100kHz–1mHz frequency range was employed as the main electrochemical technique to study the corrosion protection behaviour of a zinc-rich epoxy paint in 3% NaCl solution. The EIS results obtained at the open-circuit corrosion potential have been interpreted using a model involving the impedance of particle to particle contact to account for the increasing resistance between zinc particles with immersion period, in addition to the impedance due to the zinc surface oxide layer and the electrical resistivity of the binder. Galvanic current and dc potential measurements allowed us to conclude that the cathodic protection effect of the paint takes some time to be achieved. The loss of cathodic protection is due to a double effect: the decrease of the ZnFe area ratio due to Zn corrosion and the loss of electric contact between Zn to Zn particles. Even when the cathodic protection effect by Zn dust became weak, the substrate steel is still protected against corrosion due to the barrier nature of the ZRP film reinforced by Zn corrosion products.

Reaction Model for Iron Dissolution Studied by Electrode Impedance II . Determination of the Reaction Model

The electrode impedance determined experimentally shows a certain number of time constants as capacitive or inductive features. These impedances arise from various processes occurring at or near the electrode interface, such as charge transfer, adsorption‐desorption of reaction intermediate species, changes of the roughness factor, changes of the number of active sites or of volume concentration in the vicinity of the electrode, etc. The origins of impedances are examined on the basis of experimental results given in Part I . Then, hypotheses describing reaction rates, such as the Tafel law, reaction reversibility, and the adsorption isotherm law, are analyzed in order to translate reasonably the reaction models into mathematical expressions. With hypotheses retained, it is concluded that the experimental results should be interpreted by a model including three adsorbed reaction intermediate species. Forty possible reaction schemes, as the complete set of prospective models, were written and examined according to both steady‐state polarization curves and electrode impedances. The appearance of two current maxima implies at least two dissolution paths in the reaction models and allowed us to eliminate 10 of the 40 reaction schemes. On the other hand, the appearance of inductive impedance was found to constitute a very selective criterion. Only one model, given in Part I , was found to simulate suitably the whole set of experimental results.

Anodic behaviour of manganese in alkaline medium

Voltammetry, Electrochemical Impedance Spectroscopy (EIS), Rotating Ring-Disk Electrode techniques (RRDE), Electrochemical Quartz Crystal Microbalance (EQCM) measurements, and in-situ Raman Spectroscopy were applied to investigate the anodic behaviour of Mn in 1 M NaOH solution over a wide potential range. Prior to these experiments, for EQCM, an improved plating bath was designed for coating the thin gold electrode of the quartz sensor with Mn. The results obtained revealed clearly that various oxides, depending on the electrode potential, cover the electrode surface. The oxidation–reduction processes between these different oxides and the associated exchange of species with the solution constitute the main characteristic of this electrode. When the Mn electrode is left in 1 M NaOH solution, it becomes spontaneously passive through two consecutive steps. In-situ Raman spectroscopy indicated that the electrode surface is covered by Mn3O4, Mn2O3, and MnO2 as the potential is shifted towards more anodic values. The polarisation curves showed two anodic current peaks, in agreement with the two-step passivation process. EIS spectra exhibited the typical shape of passivation reactions with a large capacitive loop in the low frequency range. The double layer capacitance and the faradaic capacitance determined from EIS data indicate the increase in expanded surface area and bulk volume of the surface oxide with anodic potential. From RRDE measurements, the dissolution of Mn through Mn2+ and Mn3+ species were evaluated. EQCM measurements corroborated the growth of surface oxide species with the potential, and gave valuable information on the nature of the chemical species involved in the oxidation–reduction processes. A reaction mechanism of the Mn electrode in 1 M NaOH in a wide potential range is proposed.

Quantitative characterization of protective films grown on copper in the presence of different triazole derivative inhibitors

Abstract The protective films developed on copper by anodic polarization in a borate-buffered solution containing benzotriazole (BTAH), 1-hydroxybenzotriazole (BTAOH) or 3-amino 1, 2, 4-triazole (ATA) have been characterized using coulometric experiments and an electrochemical quartz crystal microbalance (EQCM). The combination of these two techniques has allowed the CuO and Cu 2 O layers and the cuprous–organic layer to be analyzed quantitatively. In the presence of BTAOH, the oxide layers were very similar to those formed in inhibitor-free solution and BTAOH appeared to be adsorbed on the oxide film. In the presence of BTAH, a thick Cu 2 O film was covered by a Cu–BTA film containing 8% Cu + ions. Cupric oxide appeared on Cu 2 O areas uncovered by Cu–BTA. In the presence of ATA, the Cu 2 O layer was very thin and the greater part of Cu + ions (75%) was involved in a thick Cu–ATA film.

Impedance of a porous electrode with an axial gradient of concentration

When the impedance is measured on a battery, an inductive impedance is often observed in a high frequency range. This inductance is frequently related to the cell geometry and electrical leads. However, certain authors claimed that this inductance is due to the concentration distribution of reacting species through the pores of battery electrodes. Their argument is based on a paper in which a fundamental error was committed. Hence, the impedance is re-calculated on the basis of the same principle. The model shows that though the diffusion process plays an outstanding role, the overall reaction rate is never completely limited by this process. The faradaic impedance due to the concentration distribution is capacitive. Therefore, the inductive impedance observed on battery systems cannot be, by any means, attributed to the concentration distribution inside the pores. Little frequency distribution is found and the impedance is close to a semi-circle. Therefore depressed impedance diagrams in porous electrodes without forced convection cannot be ascribed to either a Warburg nor a Warburg-de Levie behaviour.