C. Deslouis

Electrochemical behaviour of copper in neutral aerated chloride solution. I. Steady-state investigation

The electrochemical behaviour of a Cu rotating disc electrode in neutral aerated NaCl solution was investigated in the cathodic and anodic ranges and at the corrosion potential. In the cathodic range, where the reduction of oxygen takes place, reduction peaks allow the identification and quantitative evaluation of insoluble corrosion products (CuCl and Cu2O). In the anodic range Cu is dissolved, most likely as CuCl2−. A new mechanism for the anodic dissolution is proposed after comparing our data with previously published mechanisms. Corrosion currents were found to decrease with time and to be a function of the rotation rate of the electrode. Both the mixed kinetics of the anodic partial reaction and diffusion through a porous layer seem to be relevant in controllinglcorr.

Corrosion Study of a Carbon Steel in Neutral Chloride Solutions by Impedance Techniques

The electrochemical behavior of a carbon steel in 3% solution has been investigated using a rotating disk electrode. Both steady‐state (diffusional current vs. the disk angular velocity plots) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass transport phenomena, have been carried out. It is shown that oxygen transport takes place not only in the liquid phase but also through a porous layer of corrosion products. From electrochemical impedance measurements, it was found that at the corrosion potential the oxygen reduction reaction is under either diffusional or mixed (activation + diffusion) control depending on both the electrode rotation speed and on the hold time at the free corrosion potential. In addition, it was shown that the oxygen consumption occurs not only by electrochemical reduction but also by chemical oxidation of ferrous to ferric ions. Finally, because of the possible occurrence of mixed corrosion control, it is emphasized that the use of the polarization resistance in order to evaluate corrosion rates is not always valid.

An Impedance Investigation of the Mechanism of Pure Magnesium Corrosion in Sodium Sulfate Solutions

The corrosion behavior of pure magnesium in sodium sulfate solutions was investigated using voltammetry and electrochemical impedance spectroscopy with a rotating disk electrode. The analysis of impedance data obtained at the corrosion potential was consistent with the hypothesis that Mg corrosion is controlled by the presence of a very thin oxide film, probably MgO, and that the dissolution occurs at film-free spots only. This hypothesis was substantiated both by the superposition of the EIS diagrams, obtained for different immersion times and for two Na2SO4 concentrations once normalized, and by use of scanning electrochemical microscopy in the ac mode to sense the local conductivity of the material. On the basis of the electrochemical results, a model was proposed to describe magnesium corrosion at the open-circuit potential. Simulation of the impedance diagrams was in good agreement with the experimental results.

Comparison of the AC Impedance of Conducting Polymer Films Studied as Electrode‐Supported and Freestanding Membranes

The ac impedance response of a symmetrically bathed membrane consisting of a conducting polymer is modeled. The model is an extension of a recent one proposed for a conducting polymer film contacting an electronic and an ionic conductor on its two faces (modified electrode geometry) and takes into account both diffusion-migration of ionic and electronic charge carriers inside the film and charge-transfer across metal/polymer and polymer/electrolyte interfaces. The membrane impedance is calculated for various combinations of parameters in order to stress the effect of diffusion coefficients, charge-transfer resistance, and double layer capacitance at the interfaces. The impedance responses in the modified electrode and membrane geometries are compared. It is thus shown how some ambiguities in the attribution of certain features of the impedance of modified electrodes to either the polymer/electrode or polymer/electrolyte interfaces may be eliminated by this comparison

Frequency response of electrochemical sensors to hydrodynamic fluctuations

The response of mass transfer to a small mass sink to hydrodynamic fluctuations in the concentration boundary layer has been calculated as a function of frequency. The dimensionless local flux was expressed as a series expansion of the dimensionless local diffusion layer thickness η and the dimensionless local characteristic frequency ξ in the low frequency range, and as the asymptotic power law ξ -3/2 , in the high frequency range. The two solutions overlap fairly well for 6≤ξ≤13

Interfacial pH measurement during the reduction of dissolved oxygen in a submerged impinging jet cell

A pH sensor-grid assembly is used in a submerged impinging jet cell to measure the interfacial pH during the reduction of dissolved oxygen in well controlled flow conditions. Experiments were performed in a 0.5m K2SO4 solution with and without carbonate ions. In each case a good agreement between experiment and theory is found. In particular it was confirmed that, in the absence of chemical reaction and for a totally mass transport controlled oxygen reduction reaction, stirring has no influence on the interfacial pH.

Electrochemical behaviour of copper in neutral aerated chloride solution. II. Impedance investigation

A.c. and EHD impedance measurements were performed on a Cu rotating disc electrode immersed in neutral aerated NaCl and the results were compared with simulated curves according to the kinetic model developed in Part I. The agreement is good over the whole anodic range and at the corrosion potential. A slowing down of the mass transport rate is explained by diffusion through a surface layer (Cu2O) at the corrosion potential and for the close anodic range. It is confirmed that, on the cathodic plateau, the interface may be considered as uniformly accessible for the cathodic partial reaction of oxygen reduction. However, at less cathodic potential a surface layer effect due to CuCl must also be taken into account. Diffusion coefficients for both O2 and CuCl2− were determined by EHD impedance.

Mixed ionic-electronic conduction of a conducting polymer film. Ac impedance study of polypyrrole

Polypyrrole (PPy) films were studied by ac impedance spectroscopy both in a free-standing membrane and in a modified electrode arrangement. The experimental results were analyzed by comparison with previously derived kinetic models, allowing to obtain, through a non-linear least-squares fitting procedure, both transport (diffusion coefficients and concentration of ion- and electron-exchanging sites) and interfacial parameters (charge transfer resistances and double layer capacitances). PPy film thickness, nature and concentration of the electrolytes and duration of the immersion were varied in a systematic way. The transport properties were found to depend on both anions and cations contained in the electrolyte and to be independent of polymer film thickness.

The cathodic mass transport process during zinc corrosion in neutral aerated sodium sulphate solutions

Abstract The cathodic behaviour of zinc in 0.5 M Na 2 So 4 aerated solution has been investigated using a rotating disc electrode. Since the steady-state current-voltage curves exhibit two well-separated diffusion plateaux, both steady-state (diffusion current vs. the disc angular velocity plts) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass-transport phenomena, have been carried out. It is shown that, in the vicinity of the corrosion potential (first plateau region) the dissolved oxygen reduction occures through a layer of dissolution products whereas the electrode surface appears quasi-uniformly active at the second plateau. Electrochemical impedance measurements corroborate the existence of the layer and allow us to justify the shape of the entire current-voltage curve. Finally, additional experiments have been carried out in order to gain a better understanding of the nature and the properties of the layer.

The kinetics of zinc dissolution in aerated sodium sulphate solutions. A measurement of the corrosion rate by impedance techniques

Abstract The kinetics of zinc corrosion in aerated Na 2 SO 4 solutions has been studied by an a.c. impedance technique. A dissolution model involving the relaxation of adsorbed species has been put forward and a satisfactory agreement was obtained from a comparison between the experimental and simulated I–V curves and a.c. impedance diagrams. This model allowed the deduction of the charge transfer resistance from the impedance data and led to a fast determination technique of the instantaneous corrosion rate. This procedure was validated by a direct measurement of the corrosion rate using the atomic absorption method.