Nadine Pébère

The corrosion of pure magnesium in aerated and deaerated sodium sulphate solutions

Abstract The corrosion behaviour of pure magnesium in aerated and deaerated Na 2 SO 4 solutions (0.01 and 0.1 M) was investigated by steady-state current–voltage curves and electrochemical impedance measurements with a rotating disc electrode. It was shown that the anodic current densities were lower and the resistance values higher in deaerated media. It was demonstrated that the absence of HCO 3 − due to the absence of CO 2 , which is always present in the natural environment was responsible for the results observed. The magnesium surface was covered with a porous film (MgO, Mg(OH) 2 ) which became thicker with time. The protection afforded by the layer was low and the corrosion rate remained constant as the immersion time increased. In addition, experiments carried out at different NaHCO 3 concentrations in deaerated media revealed that the corrosion rate of magnesium was accelerated when the NaHCO 3 concentration increased due to dissolution of the film (MgO, Mg(OH) 2 ). It was concluded that O 2 did not influence magnesium corrosion.

Enhanced Graphical Representation of Electrochemical Impedance Data

Bode plots, corrected for Ohmic resistance, logarithmic plots of the imaginary component of the impedance, and effective capacitance plots are shown to be useful complements to the more traditionally used complex-plane and Bode representations for electrochemical impedance data. The graphical methods are illustrated by synthetic data and by experimental data associated with corrosion in saline environments. Bode plots are shown, in particular, to be confounded by the influence of electrolyte resistance. The plots proposed here provide useful guides to model development for both reactive and blocking systems. The logarithmic plots of the imaginary component of the impedance and effective capacitance plots are useful for all impedance data, and the correction for Ohmic resistance in Bode plots is useful when the solution resistance is not negligible.

Investigation of magnesium corrosion in aerated sodium sulfate solution by electrochemical impedance spectroscopy

Abstract The electrochemical behavior of magnesium in aerated 0.5 M Na 2 SO 4 solution was investigated by plotting steady-state current—voltage curves and by measurement of electrochemical impedance. The low value of capacity associated with the high frequency loop, and the increase in protection with immersion time provide evidence for the existence of a protective layer over the surface. The layer was thought to consist of Mg(OH) 2 and/or MgO. The protective action of this film is dependent on anodic or cathodic polarization, the latter affording better protection. The electrochemical determination of the corrosion rate was based on the Stern and Geary relationship, into which were introduced the value of the charge transfer resistance (diameter of the high frequency loop in the impedance diagram) and the anodic and cathodic Tafel coefficients. The results obtained are in agreement with those obtained by direct assay of dissolved magnesium by atomic absorption spectroscopy.

AC Impedance Spectroscopy in Characterizing Time-Dependent Corrosion of AZ91 and AM50 Magnesium Alloys Characterization with Respect to Their Microstructures

The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E corr ) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the α-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al 2 O 3 in the magnesium oxide (or hydroxide) layer a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance, This was attributed to an increase of the number of Mg 2 Si particles, accelerating the dissolution n of eutectic areas.

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.

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 Ω 1/2 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 CuCl 2 - . 0n 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

The Apparent Constant-Phase-Element Behavior of a Disk Electrode with Faradaic Reactions

Geometry-induced current and potential distributions modify the global impedance response of a disk electrode subject to faradaic reactions. The problem was treated for both linear and Tafel kinetic regimes. The apparent capacity of a disk electrode embedded in an insulating plane was shown to vary considerably with frequency. At frequencies above the characteristic frequency for the faradaic reaction, the global impedance response has a quasi-constant-phase element (CPE) character, but with a CPE coefficient alpha that is a function of both dimensionless frequency K and dimensionless current density J. For small values of J, alpha approached unity, whereas, for larger values of J, alpha reached values near 0.78. The calculated values of alpha are typical of those obtained in impedance measurements on disk electrodes. For determining the interfacial capacitance, the influence of current and potential distributions on the impedance response cannot be neglected, even if the apparent CPE exponent alpha has values close to unity. Several methods taken from the literature were tested to determine their suitability for extracting interfacial capacitance values from impedance data on disk electrodes. The best results were obtained using a formula which accounted for both ohmic and charge-transfer resistances.

An investigation of the corrosion inhibition of pure aluminum in neutral and acidic chloride solutions

Abstract This work is devoted to the corrosion inhibition of pure aluminum by 8-hydroxy-quinoline (8-HQ). In neutral solutions, steady-state current-voltage curves and electrochemical impedance measurements show that the passive alumina layer is strengthened, particularly for long immersion times. The formation of a complex aluminum chelate explains the electrochemical results. Surface analysis indicates that in the presence of 8-HQ, the oxide film is very thin; the adsorption of the organic compound prevents the adsorption of chloride ions and the destruction of the aluminum oxide layer. In acidic solutions, 8-HQ does not modify the corrosion mechanism of aluminum. This result is explained by the solubility of the aluminum chelate in acidic media. This is confirmed by XPS analysis which also reveals a significant adsorption of the organic molecule on the surface.

The Apparent Constant-Phase-Element Behavior of an Ideally Polarized Blocking Electrode

Two numerical methods were used to calculate the influence of geometry-induced current and potential distributions on the impedance response of an ideally polarized disk electrode. A coherent notation is proposed for local and global impedance which accounts for global, local, local interfacial, and both global and local ohmic impedances. The local and ohmic impedances are shown to provide insight into the frequency dispersion associated with the geometry of disk electrodes. The high-frequency global impedance response has the appearance of a constant-phase element CPE but can be considered to be only an apparent CPE because the CPE exponent is a function of frequency.

Local Electrochemical Impedance Spectroscopy Applied to the Corrosion Behavior of an AZ91 Magnesium Alloy

Local electrochemical impedance spectroscopy (LEIS) has been applied to study the corrosion behavior of an as-cast AZ91 magnesium alloy. The results are compared with those obtained from traditional electrochemical impedance spectroscopy and discussed in relation with previously published results. They confirm the specific time-dependent corrosion of the AZ91 magnesium alloy: due to the high aluminum content and to the presence of a eutectic constituent in the alloy, the corrosion begins in the grain body and then spreads to the eutectic areas. This leads to aluminum enrichment of the corrosion product layer. When an aluminum threshold concentration is reached, the properties of the oxide layer become predominantly determined by the properties of the alumina component and there is a change in the impedance diagrams for both global and local impedance spectroscopies. However, the space resolution of the local electrochemical impedance setup was not sufficient to give additional results on the influence of the alloy structure.