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 Na 2 SO 4 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.

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The electrochemical behavior of a carbon steel in 3% NaC1 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.In a previous study (1), steady-state electrochemical techniques (voltage-current curves and polarization resistance measurements) were used to study the corrosion of carbon steel in a stirred and aerated chloride solution (3% NaCI). It was concluded that the corrosion rate is controlled by the reduction of dissolved oxygen, and that the corrosion rate deduced from electrochemical techniques is in fair agreement with that obtained from an absolute measurement such as the determination of the amount of iron species in solution.In the literature, many stud/es have been devoted to corrosion (and inhibition) in acidic media in order to measure the corrosion rate or to identify the elementary processes. However, relatively few studies have been carried out in neutral media (2), probably because of the formation of insoluble corrosion products which adhere to the metal surface (3a, b).In this work, we aimed firstly to be more precise about the quantitative influence of mass transport in the corrosion process. This was achieved by the use of specific analytical methods, such as plotting of the diffusional component vs. the angular velocity Of a disk electrode, and by the frequency analysis of the so-called EHD impedance {4).Secondly, in order to separate the elementary anodic and .cathodic contributions near to the corrosion po-Key words: interface, disk electrode, porous films, activationdiffusion.tential, and, therefore, to get a better insight of the mechanism, we carried out electrochemical impedance measurements for various polarization conditions.

The interface [BuMeIm][Tf2N]/electrode, where [BuMeIm][Tf2N] stands for the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was characterized by electrochemical impedance spectroscopy at different temperatures and for different electrode materials: platinum (Pt, metallic), glassy carbon (GC, high conductivity), carbon nitride (a-CNx, mean conductivity), and boron-doped diamond (BDD, semiconducting with a quasimetallic character). For Pt, GC, and a-CNx, the behavior of the interface could be described by the same equivalent electrical circuit. In the case of BDD, a parallel combination of Rsc and Csc was introduced into the circuit to take into account the potential drop due to the development of a space charge region within the material. The Mott-Schottky plots have confirmed the polycrystalline semiconductor character of the BDD material, and the boron concentration estimated is fully consistent with the B amount introduced for the synthesis. The variations of the double-layer capacitance as a function of potential were found to be camel shaped for all electrode materials at the highest studied temperature. This is consistent with the prediction of Kornyshev's theory as low values of the packing parameter γ were estimated by simulation (lower than 0.33). An increase of the double-layer capacitance is found with the temperature similarly to most of the results obtained for molten salts.

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