C. Gabrielli

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.

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.

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.

Impedance analysis of electrodes modified with a reversible redox polymer film

The study of the kinetics of an electrode coated with a redox polymer film has been carried out using an impedance technique. A theoretical model is proposed which takes into account the diffusion of the active centres. Experimental results are presented for a glassy carbon electrode coated with a [Ru(bpy)2 poly(4′-vinylpyridine)Cl]Cl film. There is good agreement between these results and the model.

Magnetic Field Effects on Mass Transport

It has been shown that the stationary limiting diffusion current on a steady electrode is proportional to {ital B}{sup 1/3}{ital C}{sup 4/3} where {ital C} is the electroactive species concentration and {ital B} the magnetic field intensity. A new impedance technique is developed which consists of the frequency response analysis of the limiting diffusion current to a sinusoidal magnetic field perturbation. In the low frequency range, all the impedance diagrams can be reduced, in Bode coordinates, by {omega}{ital B}{sup {minus}2/3}{ital C}{sup {minus}2/3}. This response is due to convective mass transport and is similar to the electrohydrodynamical impedance obtained through the modulation of the rotation speed of a rotating disk electrode.

Investigation of Hydrogen Adsorption and Absorption in Palladium Thin Films II. Cyclic Voltammetry

Hydrogen absorption into palladium thin films (10 monolayers and 100 μm) was studied using cyclic voltammetry. The behavior of thin layers prepared by electrodeposition is different from that of the bulk Pd. On very thin films hydrogen adsorption, absorption, and evolution processes are separated. Adsorption kinetics depends strongly on applied potential and layer thickness. A new method is proposed for the determination of the quantity of adsorbed hydrogen in the presence of the absorption process.

A model of the anodic behaviour of iron in sulphuric acid medium

Our previous work showed that regulation by potentiostat is not adequate for the thorough control of the transition between active and passive states of iron in acid medium and that only a regulation device having a negative output impedance permits a continuous and reversible transition[l]. The experimental results are compared in this short communication with those of simulation calculation based on the reaction model proposed. The polarization curves obtained with different rotation speeds of a disc electrode showed that the diffusion plateau precedes the region of polarization curve corresponding to the passivation. One notes that in this potential zone, there are three steady current values, at a given potential, since the polarization curve is Z-shaped. These results relevant to the steady state behaviour of iron in acid medium are completed by the study of its

Impedance investigation of the charge transport in film-modified electrodes

Abstract Two kinetic models which can explain the coupling between electrochemical charge transfer and the charge transport in polymer films of film-modified electrodes are analyzed. The first one is based on a diffusion-like process due to the mutual electron exchange between adjacent redox centers. The second one assumes a finite conductivity of the film. The electrochemical impedances characteristic of the models have been derived and also simulated numerically. Some experimental impedance measurements which were obtained from a glassy-carbon electrode modified with a redox-polymer film are reported and discussed in terms of these two models. The influence of film inhomogeneities on the impedance characteristics is also discussed.

Nucleation and growth of calcium carbonate by an electrochemical scaling process

Calcium carbonate scale was electrochemically deposited from carbonically pure hard waters on gold electrodes of an electrochemical quartz crystal microbalance. Various data concerning the rate of the scaling process were deduced from the chronoelectrogravimetric responses. The nucleation rate of CaCO3 crystals and the surface coverage were evaluated from SEM image analysis. The effects of a surface pretreatment, the oxygen concentration and the degree of hardness of the water were investigated. Transmission electron microscopy was used to identify the crystal structure of small nuclei. Calcite, aragonite or vaterite crystal forms were obtained depending on experimental conditions. It was shown that at room temperature calcite was predominantly formed for conditions which were favorable to a fast nucleation rate whereas the vaterite form is systematically obtained for experimental conditions leading to a slow nucleation rate. The temperature increase favored the formation of either aragonite needles or vaterite crystallites with a different morphology.

Noise Resistance Applied to Corrosion Measurements II. Experimental Tests

Electrochemical noise (EN) measurements have been carried out on several corroding systems, utilizing the technique of simultaneous recording of current and voltage fluctuations, and calculating the frequency dependent spectral noise impedance R{sub sn}(f) as the square root of the ratio of the power spectral densities (PSD) of the voltage to current noise. The results are compared with the theoretical predictions stemming from an analysis of the technique, which is presented in the preceding paper. It is shown that, over a quite large range of conditions, R{sub sn}, coincides with the impedance modulus of the electrodes under study. From the relationship between R{sub sn} and the noise resistance R{sub n}, which is usually obtained calculating the ratio of the standard deviations of the voltage and current fluctuations, it is shown that the latter quantity can be equal to the zero frequency limit of the impedance only if certain conditions are satisfied.