Galvanic coupling between copper and aluminium in a thin-layer cell

The Al/Cu coupling was investigated in a thin-layer cell formed by a large Cu electrode and an Al microelectrode embedded in an insulator placed above the Cu electrode. By using a scanning electrochemical microscope (SECM) the thickness of the thin layer was perfectly controlledwith a precision in the micrometer range. A copper deposit on an electrochemical quartz crystal microbalance (EQCM) was also used as SECM substrate to quantify the copper dissolution rate. It was shown that such an experimental set-up allows to mimic the galvanic corrosion of intermetallic particles embedded in the aluminium matrix of the 2XXX series aluminium alloys. The combination of the SECM and the EQCM permitted the evaluation of the corrosion rate of copper at the corrosion potential of the 2024 Al alloy, whereas cyclic voltammetry performed on the SECM tip indicated the enrichment in Cu2+ ions in the thin electrolyte layer

Influence of flow on the corrosion inhibition of carbon steel by fatty amines in association with phosphonocarboxylic acid salts

This work was carried out to study the inhibition mechanism of a carbon steel in a 200 mg l−1 NaCl solution by a non-toxic multi-component inhibitor used for water treatment in cooling circuits. The inhibitive formulation was composed of 50 mg l−1 fatty amines associated with 200 mg l−1 phosphonocarboxylic acid salts. Steady-state current–voltage curves, obtained with a rotating disc electrode, revealed that the properties of the protective layer were dependent on the electrode rotation rate and on the immersion time. The cathodic process of oxygen reduction was not modified in the presence of the inhibitive mixture. As expected, the current densities increased when the rotation rate was increased. In the anodic range, original behaviour was observed: the current densities decreased when the electrode rotation rate increased. The morphology and the chemical composition of the inhibitive layers allowed the electrochemical results to be explained. Two distinct surface areas were visualised on the metal surface and the ratio between the two zones was dependent on the flow conditions. This behaviour was attributed to a mechanical effect linked to centrifugal force. XPS analysis revealed that the formation of a chelate between the phosphonocarboxylic acid salt molecules and the iron oxide/hydroxide was enhanced by the increase of the electrode rotation rate

Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy

The electrochemical behaviour of an AZ91 magnesium alloy was investigated in a low aggressive medium by means of local and global electrochemical impedance measurements. The results were compared to those obtained on a pure magnesium sample. It was possible to show the individual contribution of each phase constituting the alloy, and it was observed that the corrosion mechanism of the two phases was the same, controlled by the Mg dissolution. However, local impedance diagrams clearly indicated that the kinetics dissolution of the b-phase was slower than that of the a-phase, which was in good agreement with SEM observations

CPE analysis by local electrochemical impedance spectroscopy

Constant-phase elements (CPE) are used extensively in equivalent electrical circuits for fitting of experimental impedance data. The CPE behavior is generally attributed to distributed surface reactivity, surface inhomogeneity, roughness or fractal geometry, electrode porosity, and to current and potential distributions associated with electrode geometry. In this work, different electrochemical systems showing the CPE dependence in the high-frequency range for the overall impedance were considered. Local electrochemical impedance spectroscopy was found to provide a good means for assessing the influence of local variations on the CPE behavior seen in global impedance measurements. A separation between 2D and 3D distributions could be easily observed. In the case of a 2D distribution (AZ91 Mg alloy), the origin of the CPE behavior was the distribution of high-frequency resistance associated with the geometry of the disk electrode; whereas, the capacitance was independent of position. In the case of the aluminium electrode, the CPE behavior could be attributed to a combination of 3D and 2D distributions. Geometric distributions can play a significant role in the impedance response of electrochemical systems, and these distributions can lead to CPE behavior

Localized approach to galvanic coupling in an aluminum–magnesium system

The corrosion behavior of a pure aluminum/pure magnesium couple in a weakly conductive sodium sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method algorithm. Magnesium acted as the anode of the system while oxygen and water were reduced on aluminum. Calculations predicted a large current peak at the Al/Mg interface related to a local increase in both Mg dissolution and oxygen and water reduction on aluminum, leading to a local pH increase. Optical and scanning electron microscope observations confirmed the strong dissolution of magnesium concomitantly with depassivation of aluminum at the Al/Mg interface. Local electrochemical impedance spectroscopy showed the detrimental effects of the galvanic coupling both on aluminum and magnesium

Constant-Phase-Element Behavior Caused by Coupled Resistivity and Permittivity Distributions in Films

A recent proposed model showed that the impedance of a film with a uniform permittivity and a resistivity that varies along its thickness according to a power-law is in the form of a constant phase element (CPE). This model is further considered in order to assess the effect of non-uniform permittivity profiles. It is shown that a power-law permittivity profile is also compatible with a CPE behavior when resistivity and permittivity vary in opposite ways along the film thickness. This work shows that, for important classes of materials which show CPE behavior, relaxation of the assumption of a uniform permittivity does not alter the conclusions developed in the earlier work, and the formula relating film properties to CPE parameters is shown to apply

Galvanic Coupling Between Pure Copper and Pure Aluminum Experimental Approach and Mathematical Model

The corrosion behavior of a pure aluminum/pure copper couple in a weakly conductive sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. The potential distribution predicted by the calculations was checked using a Ag/AgCl microreference electrode. A good agreement was found between experimental and theoretical results. It was shown that the reaction occurring at the copper electrode was oxygen reduction, while aluminum remote from the Al/Cu interface remained in the passive state. Moreover, calculations predicted a large cathodic current, related to an increase in oxygen reduction, restricted to copper at the Al/Cu interface. This led to a local pH increase reaching values higher than 9, allowing the dissolution of aluminum to occur close to the interface. Combining these data with optical and scanning electron microscope observations after 24 h of immersion in the sodium sulfate solution allowed a three-step mechanism to be proposed to explain the corrosion damage, and particularly the presence of a copper deposit on the aluminum surface, some distance from the Al/Cu interface, a phenomenon currently observed in commercial copper-rich aluminum alloys

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

Simulating the galvanic coupling between S-Al2CuMg phase particles and the matrix of 2024 aerospace aluminium alloy

Study of the corrosion behaviour of a magnetron sputtered Al–Cu/Al–Cu–Mg model alloy couple in sulphate solutions has been undertaken to gain insight into the galvanic coupling between the matrix and SAl2CuMg particles in the 2024 aluminium alloy (AA2024). Polarisation curves and local electrochemical impedance spectroscopy measurements (LEIS) were performed on the individual alloys and on the model alloy couple. SEM enabled correlation of electrochemical phenomena to the observed damage. The corrosion behaviour of the sputtered alloys was shown to be representative of the AA2024, with the Al–Cu–Mg alloy part undergoing localised corrosion and the Al–Cu alloy part remaining passive

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