The electric properties of an electrode can be evaluated by the determination of the capacitance.However, the literature reports a large panel of experimental protocols for capacitance determination, which involve either cyclic voltammetry (CV) or electrochemical impedance spectroscopy (EIS). Indeed, substantial discrepancies in the assumptions are made between both techniques as in CV, the system is usually considered as ideal (and the current-scan rate relationship is assumed to remain linear at all frequencies), whereas in EIS, the system is usually considered as non-ideal and a CPE element is introduced. In this work, the electric properties of two electrode configurations, namely an Au blocking electrode and an Al electrode with a thin oxide film were investigated using EIS and CV. The capacitive domain was determined for both systems assuming the presence of an ideal and non-ideal system. The results collected from CV revealed a limited linearity between the current and scan rate for both electrode configurations. Finally, a CPE element was introduced to determine the capacitance from CV data and compared to EIS results.
Electrochemical methods (cyclic voltammetry (CV), potential steps, and electrochemical impedance spectroscopy) were successfully combined with in situ reflectometry measurements for a detailed analysis of the passive layer evolution as a function of the electrode potential. Interestingly, both EIS and surface reflectivity allowed a film thickness in the nanometer range to be readily determined. In addition, transient analyses of the reflectivity simultaneously recorded with CVs show the formation of both FeO and FeO oxides. The image analysis showed that the steel surface reactivity is heterogeneous and presents micrometric islands coated with a thicker oxide layer than the surrounding surface. The in situ combination of these techniques thus offers a powerful analytical description of the interface on a local scale and its transient response to a perturbation.
The cathodic dissolution of aluminium in neutral medium was studied by electrochemical methods. It is shown that oxygen reduction at the surface of the metal is kinetically controlled and that the diffusion regime is never reached due to the poor conductivity of the oxide film and consequently a high potential drop across the layer. Nevertheless, at E < -1.4 V/MSE, the rate of hydroxyl production during oxygen reduction is high enough to raise the interfacial pH
Corrosion of iron exposed to H2S saturated solution at pH 4 was studied by electrochemical impedance spectroscopy, weight loss coupons and surface analysis. Hydrogen permeation was also used as indirect means of evaluating the intensity of the proton reduction reaction leading to hydrogen entry into the metal. Since corrosion in this type of test solution results in the rapid build-up of a conductive and highly porous iron sulfide scale, a specific contribution of the film has to be considered. An impedance model was thus proposed. The faradaic anodic impedance consists of a two-step reaction with charge transfer and adsorptiondesorption. An additional contribution, associated with the conductive and highly porous iron sulfide film was added in parallel. This contribution, mostly visible in the 2 low frequency domain, presents a 45° tail associated with a porous electrode behavior. This model was well adapted to describe impedance diagrams measured at various exposure times, up to 620 hours. Charge transfer resistance determined from impedance analysis allowed calculating the evolution with time of the corrosion current density. A very good correlation was found between this corrosion current density and the hydrogen permeation current density. As expected in our experimental conditions, a permeation efficiency close to 100 % is demonstrated. Corrosion rate of 490 µm/year was measured by weight-loss specimens, confirming the validity of the impedance analysis, which resulted in a calculated corrosion rate of 530 µm/year.
The increasing use of impedance for the characterization of an electrified interface is accompanied by the development of accurate models to analyze the results. In the present work, the concept of ohmic impedance is revisited using both numerical simulations and experimental results. The Havriliak-Negami equation is shown to provide a good representation of the high-frequency dispersion or complex ohmic impedance associated with the disk electrode geometry. An excellent fit to simulated complex ohmic impedance was found for both capacitive electrodes and for electrodes characterized by constant-phase-element behavior. The use of the Havriliak-Negami equation to account for the complex ohmic impedance was shown to extend the useful frequency range for regression of physical models to the impedance response for three experimental systems: a gold electrode in a 0.1 M sodium sulfate solution, an aluminum electrode in a 0.01 M sodium sulfate solution, and pure iron in a 0.5 M sulfuric acid solution.
The present study proposes an alternative eco-friendly method to prepare a thin composite coating based on graphene embedded in siloxane polymers which can be used as application for the corrosion protection of steel. The nanocomposite coatings were elaborated by a dielectric barrier discharge using a nebulized colloidal suspension of graphene nanosheets (GNs) dispersed in hexamethyldisiloxane (HMDSO) used as the precursor for the polymer matrix. After obtaining a stable colloidal solution, it was nebulized into the plasma reactor to form a plasma polymer (pp) coating from HMDSO (ppHMDSO) in which GNs were incorporated (GN@ppHMDSO) on the mild steel substrate. The chemical structure of the hybrid coatings was characterized by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry. Raman spectra of GNs and GN@ppHMDSO coatings suggest the existence of charge transfer between the GNs and the HMDSO matrix. Furthermore, scanning electron microscopy confirms the synthesis of micro/nanocomposite with a fairly homogeneous dispersion of the GNs in the polymer matrix. The corrosion resistance of the samples was evaluated by electrochemical impedance spectroscopy which showed that the hybrid coatings GN@ppHMDSO deposited by a one-step atmospheric pressure plasma process, presented excellent anticorrosion performance with 99.99% of protection efficiency.
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