Delaminated areas beneath organic coating: A local electrochemical impedance approach

Local electrochemical impedance mapping was used to investigate delamination phenomena at the steel/epoxy-vinyl primer interface. The delamination occurred from an artificial defect(cutter scribing) and from ageing in a salt spray chamber. The samples were taken from the salt spray chamber after 20, 30 and 50 days of exposure. To observe delamination after ageing, the corrosion product layers were removed by a cathodic polarization at −1.5 V/SCE for 4 h. A non-aged reference sample was tested for comparison. Mapping was performed at 5 kHz. Initiation and propagation of the delamination were clearly observed. The delaminated surface areas measured by visual observations after the removal of the coating were lower than those determined by local electrochemical impedance mapping. The delamination mechanisms were discussed with reference to literature data

Comparison of local electrochemical impedance measurements derived frombi-electrode and microcapillary techniques

In the present paper, local electrochemical impedance spectrawere obtained on a 316L stainless steel from two configurations: a dual microelectrode (bi-electrode) and microcapillaries. With the bi-electrode, the local impedance measurements were made from the ratio of the applied voltage to the local current density calculated from the application of the ohm’s law. With the use of microelectrochemical cells, the specimen surface area in contact with the electrolyte is limited by the use of glass microcapillaries and the local impedance was defined fromthe ratio of the local potential to the local current restricted to the analysed surface area. Differences and similarities observed in local impedance spectra obtained with the two configurations were describe

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

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

Corrosion protection of carbon steel by an epoxy resin containing organically modified clay

This study focusses on the use of montmorillonite clay (MMT) treated with an organic compound (aminotrimethylphosphonic acid (ATMP)) and dispersed in an epoxy resin to improve corrosion protection of carbon steel. X-ray diffraction was performed to verify that the individual silicate layers were separated and dispersed in the epoxy resin. Corrosion resistance of the coated steel was evaluated by electrochemical impedance spectroscopy (EIS) and local electrochemical impedance spectroscopy (LEIS). Three systems were tested: the epoxy clear-coat, the epoxy resin containing 2 wt.% clay and the epoxy resin containing 2 wt.% clay modified byATMP (ATMP-modified clay). From conventional EIS, it was shown that the incorporation of clay or ATMP-modified clay in the epoxy matrix significantly improved the barrier properties of the coating. The corrosion resistance of the carbon steel coated by the epoxy resin containing ATMP-modified clay was higher than that obtained for the system containing non-treated clay. Local electrochemical measurements performed on scratched samples revealed the inhibitive role of ATMP at the carbon steel/coating interface

Influence of normal and radial contributions of local current density on local electrochemical impedance spectroscopy.

A new tri-electrode probe is presented and applied to local electrochemical impedance spectroscopy (LEIS) measurements. As opposed to two-probe systems, the three-probe one allows measurement not only of normal, but also of radial contributions of local current densities to the local impedance values. The results concerning the cases of the blocking electrode and the electrode with faradaic reaction are discussed from the theoretical point of view for a disk electrode. Numerical simulations and experimental results are compared for the case of the ferri/ferrocyanide electrode reaction at the Pt working electrode disk. At the centre of the disk, the impedance taking into account both normal and radial contributions was in good agreement with the local impedance measured in terms of only the normal contribution. At the periphery of the electrode, the impedance taking into account both normal and radial contributions differed significantly from the local impedance measured in terms of only the normal contribution. The radial impedance results at the periphery of the electrode are in good agreement with the usual explanation that the associated larger current density is attributed to the geometry of the electrode, which exhibits a greater accessibility at the electrode edge

Corrosion protection mechanisms of carbon steel by an epoxy resin containing indole-3 butyric acid modified clay

This work is an extension of studies into the mechanisms of corrosion protection of mild steel by an epoxy resin containing organically modified clay (Hang et al. [1]; Truc et al. [2]). In a previous study (Truc et al. [2]), it was shown that indole-3 butyric acid (IBA)-modified clay improved the corrosion performance of epoxy. In the present study, it was shown that the IBA is an anodic inhibitor and its efficiency was about 93%. Exfoliation and dispersion of the IBA-modified clay in the epoxy coating were checked by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The improvement of the corrosion performance of the epoxy coating containing IBA-modified clay by comparison with pure epoxy coating was confirmed for a low film thickness (10 μm). Local electrochemical impedance measurements performed on scratched samples revealed the inhibitive action of IBA at the carbon steel/coating interface. It was shown that the inhibitor release from the IBA-modified clay was favoured for high pH values. Thus, in neutral media, the corrosion process which induces a local increase of pH will increase the self-healing performance

Contribution to a better understanding of different behaviour patterns observed with organic coatings evaluated by electrochemical impedance spectroscopy

In the present paper, three different coatings (epoxy, alkyd, polyurethane paints) were characterized by electrochemical impedance measurements, permeability tests, free-standing film impedance and local impedance measurements. The increase in resistance with immersion time of alkyd paint was linked to the nature of the polymeric network and not to phenomena occurring at the metal/paint interface. For polyurethane paint, local impedance was not able to detect defects in the paint, which was attributed to the fact that the defects are smaller than those observed in alkyd paint and also that they are less active and homogeneously distributed through the coating. Although electrochemical impedance methods (global and local) are excellent tools to monitor the behaviour of organic coatings, these techniques alone are not sufficient to screen different paints

Tool Use by Four Species of Indo-Pacific Sea Urchins

We compared the covering behavior of four sea urchin species, Tripneustes gratilla, Pseudoboletia maculata, Toxopneustes pileolus, and Salmacis sphaeroides found in the waters of Malapascua Island, Cebu Province and Bolinao, Panagsinan Province, Philippines. Specifically, we measured the amount and type of covering material on each sea urchin, and in several cases, the recovery of debris material after stripping the animal of its cover. We found that Tripneustes gratilla and Salmacis sphaeroides have a higher affinity for plant material, especially seagrass, compared to Pseudoboletia maculata and Toxopneustes pileolus, which prefer to cover themselves with coral rubble and other calcified material. Only in Toxopneustes pileolus did we find a significant corresponding depth-dependent decrease in total cover area, confirming previous work that covering behavior serves as a protection mechanism against UV radiation. We found no dependence of particle size on either species or size of sea urchin, but we observed that larger sea urchins generally carried more and heavier debris. We observed a transport mechanism of debris onto the echinoid body surface utilizing a combination of tube feet and spines. We compare our results to previous studies, comment on the phylogeny of sea urchin covering behavior, and discuss the interpretation of this behavior as animal tool use

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm¡1) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm¡1) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field