Omar Aaboubi

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.

Magnetic Field Effects on Nickel Electrodeposition

The effects of a superimposed magnetic field B on the structure of nickel electrodeposits prepared from a quiescent Watts solution were studied by scanning electron microscope and transmission electron microscopy investigations. It was observed that B can induce a change of the surface morphology and of the preferential growth direction of the nickel grains. In the absence of organic inhibitor, it is shown that these effects result from an inhibition of nickel electrocrystallization in relation to the mass-transport enhancement of H + ions promoted by the magnetic field. The phenomenon is much more important in the presence of a strong inhibitor such as 2-butyne-1,4-diol, the activity of which is also under mass-transport limitation. The presence of the magnetically induced convective flow is directly illustrated by the perturbations of the relief of the deposit in the vicinity of attached bubbles. These results demonstrate that most structural modifications of nickel electrodeposits observed in the presence of a magnetic field are consequences of convection phenomena induced by a magnetohydrodynamic effect.

Magnetic Field Effects on Nickel Electrodeposition II. A Steady‐State and Dynamic Electrochemical Study

A very recent paper, referred to below as Paper I, had been devoted to the description of magnetic field effects on the structural features of nickel electrodeposits grown from a Watts solution, either pure or in the presence of small amounts of a primary brightener and leveling agent such as 2-butyne-1,4-diol (BD). The conclusions of Paper I stressed the major effect of the magnetic field which was supposed to come down to a magnetohydrodynamic forced convection close to the cathodic interface. This convection enhances those among the cathodic reactions which are likely to be mass-transport controlled, which is the case for the hydrogen electrode reaction as well as for the catalytic hydrogenation undergone by BD on the cathodic surface. The aim of this paper is to describe the results of electrochemical studies which have been carried out with the same system so as to check the validity of our assumption.

Thermoelectrochemical impedances—I. A new experimental device to measure thermoelectrical transfer functions

Abstract The temperature is an important parameter in electrochemical processes and thus dynamical investigations are of great significance. Frequency response analysis to a thermal perturbation must be obtained without any artefact in experimental results. We describe an experimental device allowing to control the electrode temperature with high accuracy and to obtain the thermoelectrical transfer functions ∂V/∂T in a galvanostatic regime and ∂I/∂T in a potentiostatic one. The electrochemical impedance, the electrode processes and characteristic parameters can be inferred from these new impedances.

Thermoelectrochemical transfer function under thermal laminar free convection at a vertical electrode

An analysis is presented for the frequency response of the limiting diffusion current to a thermal perturbation under conditions of simultaneous heat and mass transfer at a heated vertical electrode. At the limiting diffusion current plateau, this response is the sum of three terms: the first one corresponds to the diffusion coefficient and is real, the second one is purely imaginary, and the third one corresponds to the flux of electroactive species at the electrode surface. The dimensionless local flux due to the thermal modulation of the electrode temperature is expressed as a series expansion of the dimensionless local characteristic frequency ξ in the low-frequency range, and is proportional to ξ -3/2 in the high-frequency range. With 80 terms in the series, the two solutions are shown to overlap fairly well for 1.6 ≤ ξ ≤ 11 The overall transfer function is then obtained for two electrode geometries, rectangular and circular. For a circular electrode, the experimental results obtained for different steady temperature differences, T e - T 0 , between the electrode and the solution and different bulk concentrations, c 0 , are successfully tested with these theoretical predictions. The characteristic parameters are determined with a fitting procedure.

Impedance of Laminar Free Convection and Thermal Convection at a Vertical Electrode

Simultaneous heat and mass transfer under thermal laminar free convection to a vertical electrode was investigated. On the basis of Marchiano and Arvias theory developed in steady state, the ac diffusion impedance for a circular or a rectangular vertical electrode was calculated as a function of frequency. For this calculation, the dimensionless local flux was expressed as a series expansion of the dimensionless local characteristic frequency ξ in the low-frequency range and proportional to ξ +1/2 in the high-frequency range. With 80 terms in the series, the two solutions were shown to overlap fairly well for 1.6 ≤ ξ ≤ 10. The overall impedance was then obtained for the different electrode geometries. For a circular electrode these theoretical predictions were successfully tested with different steady temperature differences, ΔT, between the electrode and the solution (0° ≤ ΔT ≤ 2.65°).

Inhibitive Effect of Molybdate Ions on the Electrochemical Behavior of Steel Rebar in Simulated Concrete Pore Solution

Several compounds tested as corrosion inhibitors have proven to possess good effectiveness in protection of steel rebar in concrete. However, most of them are considered as pollutant compounds, which limits their use. The aim of this work is to investigate the inhibitive effect of sodium molybdate, which is considered as a nonpollutant compound, against pitting corrosion of steel rebar in simulated concrete pore solution. Corrosion behaviors of steel in different solutions were studied by means of corrosion potential, potentiodynamic polarization, and electrochemical impedance spectroscopy. The results indicate that the addition of sodium molybdate to the chlorinated solution decreases significantly the corrosion rate of steel. Due to its passivating character, the sodium molybdate inhibitor promotes the formation of a stable passive layer on the surface of steel, acting as a physical barrier against chloride ions, on one hand, and consolidating the passivation mechanism of steel, on the other. The optimal inhibition rate is given by the concentration of molybdate ions, corresponding to a [MoO42−]/[Cl−] that is equal to 0.5.

Influence of zinc (II) ion concentration on Ni–Zn–P coatings deposited onto aluminum and their corrosion behavior

Abstract Various Ni–Zn–P alloy coatings were deposited onto aluminum substrates using electroless plating from a sulphate bath containing varying amounts of zinc (II) ions. Different techniques, such as scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, were employed to characterize the morphology, composition and structural properties of the resulting films. The effect of varying Zn content on the corrosion properties of the coatings was tested in 3 wt.% NaCl media, by means of current–potential curves, potential transients and electrochemical impedance spectroscopy measurements. It was found that increasing ZnSO4 concentration from 5 to 25 g l−1 in the plating bath, increases the Zn content in the film from 9.90 wt.% to 12.7 wt.%, reduces the film thickness, modifies the surface morphology and significantly reduces the corrosion rate.

Thermoelectrochemical impedance (TEC) —II. Validation for an electrochemical mass transport-controlled system by stationary and dynamic investigations

Thermal gradient effects on mass transport-controlled system have been investigated. Results have been analyzed under accurate experimental conditions to determine the thermal gradient. They show that this gradient leads to a limiting diffusion current dependence as it was written by Marchiano and Arvia. Electrochim. Acta 13, 1657 (1968), 14, 741 (1968). Il = 0.77 nFD23 C∗α034C∗34Sc14 ± β034 ΔT34Pr1413g4 v2314d74 . This equation is proved true thanks to classical impedance and thermoelectrochemical transfer functions. By this way, those functions are shown to be efficient.