Electrohydrodynamic Impedance Research Articles

Characterization of Biofilms Formed on Gold in Natural Seawater by Oxygen Diffusion Analysis

Studies using a gelatin layer showed the feasibility of using combined steady-state and electrohydrodynamic (EHD) impedance techniques to determine the diffusion layer thickness (δf), diffusion coefficient of oxygen (Df), and porosity (Ε) of the biofilm in describing diffusion processes from the bulk of the solution to the surface of a gold rotating disk electrode (RDE). Biofilm modeling as a porous layer seemed appropriate as indicated by EHD diagrams and steady-state measurements. The diffusion rate through the coating layer was equal to Df/δf and was obtained from variation of the limiting current vs disk rotation speed. By analyzing the transfer function between the current response and the sinusoidal modulation of the rotation speed, the diffusion time (δ2f/Df) also could be obtained. With these techniques, δf and Df could be determined. This procedure was tested on a gelatin layer modeling the physical behavior of a biofilm. Good agreement was shown between the model and experimental data. This method then was used to characterize a biofilm developed in natural seawater as a function of immersion time.

Anodic dissolution of nickel in concentrated sulfuric acidic solutions

The anodic dissolution of nickel in concentrated sulfuric acidic solutions is characterized by two diffusion plateaux related to the active and transpassive regions. For the same speed of rotation of a rotating disc electrode, the two plateaux currents are almost identical. However, the less anodic one gives rise to a rough electrode surface while a polished surface is observed at the more anodic one. For nickel as-received, mass transport influences the current over the whole potential range. After heat treatment, only the current for the two plateaux were mass transport controlled. Electrochemical impedance diagrams show that the lowest frequency capacitive loop is influenced by mass transport. Electrohydrodynamic (EHD) impedance diagrams show two different regions. In the low frequency range, the results follow the theoretical curve corresponding to a uniformly accessible electrode with a very high Schmidt number around 107. At high frequency, the EHD impedance may correspond to an interface covered by a gel layer formed from the products of the anodic dissolution.

EHD and steady state measurements for characterization of transport properties in a gel layer

Mass-transfer to a rotating disk electrode has been studied in the case of an interface covered by porous inert layers of thickness δ f , through which species are transported by molecular diffusion with a diffusivity D f . In this study, the porous layer consists of a gel deposited on a gold electrode and the species used as tracers are Fe(CN) 6 3− and oxygen. The method is based on the analysis of tracer reduction current under total mass transport control (diffusion plateau of the species). The tracer concentration gradient is distributed between the porous layer where mass transport is only determined by molecular diffusion and the electrolyte solution where mass transport is governed by convective diffuson. Two techniques are used here namely electrohydrodynamical impedance (EHD) and a steady flow technique. From the proposed physical model, the diffusion time ( δ f 2 D f ) is obtained by regression of the impedance data while the steady state current data yields the diffusion rate ( D f 2 δ f ) through the porous layer. By combining the EHD and steady state data, a separate determination of δ f and D f , is available and allows the calculation of the porosity of the film. A comparison between the two tracers on the same layer has been made.

Anodic dissolution of metals under mass transport control

This paper is focussed on anodic dissolution processes controlled by mass transport. After the recall of the results obtained for a classic redox system, four examples are discussed : 1. (i) iron dissolution in sulphuric acid for which a liquid layer of high visosity is formed at the interface; 2. (ii) copper dissolution in hydrochloric acid and 3. (iii) in phosphate acid, where in both solutions a solid salt layer is existing at the electrode surface; and 4. (iv) nickel dissolution in sulphate acid media, where a very high Schmidt number is involved in the mass transport. On the basis of the analysis of steady-state results and electrohydrodynamic impedance, it is evidenced that each system needs a particular description.

The influence of error structure on interpretation of impedance spectra

The influence of error structure on interpretation of impedance measurements is demonstrated for electro-hydrodynamic impedance spectra. The stochastic component of the error structure was determined through use of measurement models, and a three-parameter model for the error structure was identified. The measurement model was also used to identify portions of the impedance spectra that were not corrupted by bias errors. Regression employing weighting by the stochastic component of the error structure yielded unambiguous values for physical properties such as the Schmidt number, whereas significant ambiguity was observed using modulus or proportional weighting.

High‐Rate Copper Dissolution in Hydrochloric Acid Solution

The dissolution of copper in 1 M chloride solution was studied by steady‐state (current‐potential curves for various disk rotation rates) and transient measurements [frequency analysis of the electrochemical and electrohydrodynamical (EHD) impedance]. The anodic polarization curves present four regions according to the potential: one region of mixed kinetics followed by another of mass‐transport control characterized by one plateau, followed by a second mixed kinetics region and a second plateau. The results obtained for the first domain of mixed kinetics and for the first plateau were previously published. In this work, the second mixed kinetics region is considered. From the analysis of the ac and EHD impedance measurements, it was shown that a transition between the presence and the absence of a CuCl salt layer occurs at a critical potential. The value of this critical potential depends on the rotation rate; the higher the rotation rate, the higher the potential value. A model of kinetic dissolution for this transition is presented, and a coupling between the currents corresponding to the two reactions is demonstrated.

Application of Measurement Models to Electrohydrodynamic Impedance Spectroscopy

The measurement model concept was used to assess both the stochastic and bias contributions to the error structure of electrohydrodynamic (EHD) impedance spectra. The stochastic component of the error structure was determined, and a model for the error structure was identified. Regression employing weighting by the stochastic component of the error structure yielded unambiguous values for physical properties such as the Schmidt number, whereas significant ambiguity was observed using other weighting strategies. The influence of surface phenomena on the impedance response could, through use of the measurement model for assessing consistency with the Kramers‐Kronig relations, be distinguished from nonstationary effects. The measurement model provided a quantitative means for assessing the utility of input signal filtering. The noise level in the measured impedance was found to be unaffected by use of input filters and, surprisingly, the time required to collect the data was increased slightly by using the filters. The estimated parameter values and their standard deviations were comparable whether or not filtering was used. The use of the measurement model to assess stochastic and bias components of the error structure extends the useful frequency range for EHD measurements and should enhance the application of EHD for assessing surface phenomena.

Silver reduction in a thiosulphate + nitrate solution: study of a CE reaction scheme by means of polarization curves and ac impedance

Silver electrodeposition from an electrolyte consisting of 0.1 M NaNo 3 + 0.025 M Na 2S 2O 3 + 1 × 10 −3 M AgNO 3 (pH 12) was investigated by means of polarization curves and ac impedance. In order to work under quasi-stationary conditions, measurements were performed on a rotating disk electrode, the reactive surface of which was covered with a fine silver pre-plating. It is shown that for cathodic overvoltages higher than 30 mV the reduction mechanism followed a CE reaction scheme. The rate constants of both the preceding homogeneous decomplexation reaction of Ag(S 2O 3) 3− 2 to Ag(S 2O 3) − and the heterogeneous electron transfer reaction were determined. The low-frequency limit of electrohydrodynamic impedance measurements was used to check the calculated chemical reaction parameters. In earlier publications in which higher nitrate/thiosulphate ratios were used, the influence of this decomplexation reaction could not be detected. Also, it was found that the lower NO − 3 S 2 O 2− 3 ratio used here caused a substantial difference from the current-overpotential equation at cathodic overvoltages lower than 30 mV. In this potential region, the low-frequency limit of the amplitude of the ac impedance increased with the overvoltage.

Electrohydrodynamic impedance investigation of silver electrode position in a thiosulphate ligand system

Mass transfer-limited silver plating from a thiosulphate solution has been studied by means of the electrohydrodynamic (EHD) impedance technique. The aim of the investigation was to find out how the surface topography determines the measurements and through this information possibly detect the decomplexation reaction preceding the charge transfer. To that end, the rapid reduction reaction of ferricyanide to ferrocyanide in 1 M KCl was used to compare its EHD frequency response on a smooth platinum electrode and on rough Pt-modified silver electrodes, prepared by depositing a thin Pt film on silver platings obtained after different electrolysis times. Through this method, the predominating influence of the surface roughness was elucidated, even of crystallites with a characteristic diameter of 1–3 μm. In the latter case, the EHD behaviour was similar to that of a flat, partially blocked electrode, and allowed an in situ evaluation of the average active site dimension. Also, the diffusion coefficient of Ag(S 2O 3) 3− 2 has been determined by both a dc method and from EHD data in the low frequency range. As to ac impedance measurements of the electrocrystallization process, the EHD measurements presented can support the analysis of the former in the corresponding frequency domain.

Iron dissolution under mass transport control: the effect of viscosity on the current oscillation

The current oscillation normally seen at the beginning of the plateau of polarization curves was found to be influenced strongly by the solution viscosity. The role of viscosity for iron dissolution in sulphate solution was studied by polarization curves, ac and electrohydrodynamical impedances. It was found that the charge and mass transport processes are not changed by the viscosity. However, when the glycerol was added to electrolyte the viscosity of the solution changes. So, the hydrodynamic conditions at the electrode surface also change and consequently the oscillation processes can be eliminated.

Partially blocked surface studied by the electrohydrodynamic impedance

The mass transport to a partially blocked surface on a rotating disc electrode when its angular velocity is submitted to small sinusoidal oscillations around a mean value has been analysed. From the quantitative treatment of the response of a single active site embedded in an otherwise conducting surface, it was inferred that the high frequency response of a partially blocked surface with active sites of the same size can be estimated. The response contains two time constants and its frequency analysis can provide the average size site and the active fraction of the interface. These predictions were compared with an experimental investigation using a photolithographic model of a partially blocked electrode. Good agreement between theory and experiment was obtained when the high frequency response was corrected for kinetic effects due to the finite rate of the chosen redox reaction and double-layer charging.

Corrosion inhibition of pure iron in neutral solutions by electrochemical techniques

Steady-state current-voltage curves for various disk rotation rates were combined with transient measurements (frequency analysis of the electrochemical and electrohydrodynamical impedance) in order to investigate the corrosion inhibition by an organic surfactant of pure iron in an aerated 0.5 M NaCl solution. In the cathodic range, physical adsorption of the compound with the interfacial potential was observed leading to the thickening or strengthening of the film. Thus, for short times of polarization, due to the adsorption relaxation, it was necessary to compensate the ohmic drop in the circuit to obtain the electrohydrodynamical (EHD) curves. The inhibitor acts by forming a porous layer covering the surface. From the analysis of the EHD diagrams the thickness and the porosity of the film were estimated. Nevertheless, a non-classical behaviour in the HF range of the EHD diagrams was observed and interpreted by a mechanical action of the fluctuating flow on the film. At the corrosion potential, the electrochemical impedance diagrams were essentially representative of the charge transfer process. Diffusion across the inhibitor film is very slow and requires extremely low modulation frequencies (below 1 mHz).

Mass‐Transport Study for the Electrodissolution of Copper in 1M Hydrochloric Acid Solution by Impedance

The dissolution of copper in 1M chloride solution was studied by steady‐state (current‐voltage curves for various disk rotation rates) and transient [frequency analysis of the electrochemical and electrohydrodynamical (EHD) impedance] measurements. The anodic polarization curves have presented one region of mixed kinetic and another of mass‐transport control characterized by a current plateau. The limiting current is a function throughout the whole range of rotation rates. From the EHD impedance diagrams observed below the anodic plateau, it was shown that the limitation by mass transport is due to . On the current plateau, the presence of two time constants on the EHD impedance diagram and the reductibility of the curves have been attributed to the existence of a salt layer covering the surface, the rate of formation of this layer being identical to the rate of dissolution. A layer model has been developed.

An investigation of the corrosion of pure iron by electrochemical techniques and in situ observations

The electrochemical behaviour of pure iron in an aerated 0.5 M NaCl solution was studied by steady-state determination of current-voltage curves for various disc rotation rates and transient measurements of frequency analysis of the electrochemical and electrohydrodynamical impedance. During the experiments, in situ photographs were taken of the electrode surface. Whatever the rotation rate, iron corrosion is initiated by pits and spreads out in the form of corrosion product layers, one of which is poorly adherent. In the cathodic range and for short immersion times, the electrode surface appears to be almost uniformly active from the viewpoint of mass transport whereas for long immersion times oxygen reduction occurs through the porous layers of the corrosion product. Concerning these immersion times, it was shown that the role of the occluded pits is negligible by comparison with the porous film effect. It is emphasized that at the corrosion potential the electrochemical impedance diagrams show two diffusion processes: the very low frequency loop corresponds to the diffusion across the layer and the medium frequency loop is essentially representative of the mass transport in the liquid phase. The degree of coupling between the two processes is dependent on the immersion time and the rotation rate.

Oxygen as a tracer for measurements of steady and turbulent flows

The reduction of dissolved oxygen has been studied over a wide conductivity range for use in steady or nonsteady hydrodynamic measurements. Mass transfer fluctuations can be analysed statistically to obtain the power spectra of hydrodynamic fluctuations. This requires a consideration of the proton reduction, which prevents diffusion from limiting the current. The transfer function for deducing the hydrodynamic spectra from mass transfer spectra includes not only transport effects but also kinetic effects which account for the finite rate of the reaction. The experimental study was performed using electrohydrodynamic (EHD) impedance.

Electrohydrodynamic impedance study of anodically formed salt films on iron in chloride solutions

The effect of applied potential and disk rotation speed on the electrohydrodynamic (EHD) impedance of mass-transfer-limited iron dissolution into a 4.0 M FeCl 2 electrolyte is presented. The results are interpreted with a model of a duplex film whose growth rate is limited by the flux of chloride ions at the solution—film interface. A combination of EHD impedance and a.c. impedance measurements allows for the determination of both the porosity and thickness of the outer, porous layer.

Anodic Dissolution of Iron in Acid Sulfate Under Mass Transport Control

The dissolution of iron in 1 and 1.8M of sulfate at was studied by steady‐state and electrohydrodynamical impedance. The experimental results were simulated by integration of the diffusion equation with the migration term together with an empirical viscosity profile from the electrode surface to the bulk solution. This model could be explained either by the presence of a porous and thin film or by a colloidal dispersion of .

Application of the Electrohydrodynamical Impedance Method to the Study of Corrosion Inhibitors

Application of the Electrohydrodynamical Impedance Method to the Study of Corrosion Inhibitors

Magnetic Field Effects on Mass Transport

It has been shown that the stationary limiting diffusion current on a steady electrode is proportional to where is the electroactive species concentration and 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 . 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.

Kinetical and morphological effects during CdTe and Te electrocrystallization on electrohydrodynamical impedance

E.H.D. impedance technique was used to investigate two typical electrocrystallization cases where the current is, at least partially, limited by mass transport. In the case of Te electrodeposition, it is shown that the E.H.D. responses of microdendritic layers are similar to that of a flat partially blocked surface, and allow an in situ evaluation of a characteristic size d of the dendritic structure. The transition between smooth and dendritic layers as well as the variations of d vs various parameters were investigated in order to illustrate the possibilities of the method. In the case of CdTe electrodeposition, E.H.D. impedances indicate that a slow surface process is involved in the definite compound formation. The surface concentration of cations differing from zero, the deposits are smooth and compact.