J.C. Galván

The charge transfer reaction in Nyquist diagrams of painted steel

Abstract When applying the AC impedance technique to metal-paint systems, it is of interest to identify the corrosion reaction of the metallic substrate in the arcs appearing on the complex plane of the Nyquist diagrams. This article shows how difficult it sometimes is to obtain direct information on this reaction on the basis of the diagrams obtained. The use of peelable films has been demonstrated to be very useful for this research. With these films it is possible to separate the effect of the metallic substrate from that of the film in the impedance diagrams, first measuring with the film adhered to the metallic surface and then with the same film detached from the substrate.

An interpretation of electrical impedance diagrams for painted galvanized steel

Abstract The physical significance of various arcs which appear on impedance diagrams for painted galvanized steel exposed to an aggressive medium is discussed. These diagrams undergo various changes during exposure, i.e. when the paint coating is still intact the response is almost purely capacitive, but as the coating degrades the impedance value decreases and the plots exhibit two arcs. Measurements with detached films confirm the relationship between the single arc appearing in the impedance diagrams during early stages of paint degradation and the intrinsic paint properties. However, it is more difficult to find an interpretation for the two-arc diagrams. The hypothesis that the low-frequency arc arises from effects associated with the double-layer capacitance and charge-transfer control at the pore base leads to unacceptable conclusions. Various arguments seem to support the view that the low-frequency arc is determined by a finite diffusion impedance.

Ionic conductivity in layer silicates controlled by intercalation of macrocyclic and polymeric oxyethylene compounds

Abstract This contribution concerns intercalation materials based on the formation of intracrystalline polymer—salt complexes obtained by insertion of poly(ethylene oxide) (PEO) and crown-ether compounds in a layer silicate (montmorillonite), containing Na + exchangeable cations in their interlayer space. Polyoxyethylene compounds (crown ethers and PEO) are able to associate interlayer cations modifying dramatically the ionic conductivity of the natural silicate. The new organo–inorganic materials exhibit a two-dimensional structure able to induce an anisotropic character in their electrical properties.

A new silver-ion selective sensor based on a polythiacrown-ether entrapped by sol–gel

Abstract We report a new electrode based on a selective complexing agent for Ag + ions, the 6-oxa-3,9-dithiabicicle[9.3.1]pentadeca-1(15),11,13-triene (MAO), incorporate into an organopolysiloxane matrix prepared via sol–gel from a mixture of γ-methacryloxypropyltrimetoxysilane (MAPTMS) and tetramethoxysilane (TMOS). The modified electrode is prepared by coating the surface of graphite electrodes with a thin film of the nanocomposite material. SEM and EDX were used to determine the continuous coverage and composition of the films constituting the active phase of the electrodes. The application of electrochemical impedance spectroscopy (EIS) and potentiometric techniques, allows establishing the selective response of the electrode towards the Ag + ions. The selectivity over other cations as Cd 2+ , Pb 2+ , NH 4 + , Na + , etc. is explained by the molecular recognition character of the electrode introduced by the presence of the MAO ionophere into the xerogel film. These new hybrid organic–inorganic systems constitute an alternative to classical PVC-matrices for preparation ion-selective electrodes showing also additional advantages such as higher reproducibility, lifetime, detection limits and adhesion with the electrode.

Synthesis of pillared clays assisted by microwaves

Abstract Microwave (MW) irradiation was used to synthesize aluminum-pillared clays (MW-Al-PILCs) by replacing the conventional calcination of Al-PILCs precursors (pre-Al-PILCs). X-ray diffraction (XRD) patterns and nitrogen adsorption/desorption isotherms showed that the MW-irradiated pre-Al-PILCs gave a series of MW-Al-PILC materials exhibiting some characteristics comparable to Al-PILCs obtained by conventional heating. However, differential thermal analysis (DTA), thermogravimetric analysis (TGA), impedance spectroscopy, 27 Al MAS NMR, and intercalative adsorption of methylene blue (MB), indicated that the MW-Al-PILC samples were intermediate between Al-pre-PILCs and conventional Al-PILCs. The 27 Al MAS NMR spectra showed that in MW-Al-PILCs obtained after 18 min of MW irradiation, the Al IV /Al VI ratio was 1/9, which is comparable to that in Al-pre-PILCs (8% Al IV ; 92% Al VI ) and significantly different from the typical values of conventional Al-PILCs (23% Al IV ; 77% Al VI ). It is assumed that the MW-induced calcination gives PILCs containing a fraction of residual, hydroxylated Al-polycations. The good thermal stability of the MW-Al-PILCs supports their possible use as acid catalysts.

Corrosion of Rusted Steel in Aqueous Solutions of Tannic Acid

Abstract The use of tannic acid to inhibit metal corrosion has been a controversial issue, particularly in relation to its application to rusted steel prior to painting. In this work, the protectiv...

Reactive nanocomposites based on pillared clays

This work concerns the preparation of novel nanocomposite materials deriving from pillared clays, which have been functionalised by treatment with either vanadium pentoxide or p-toluenesulfonic acid, with the aim to obtain porous materials with oxidant or strong acid character, respectively. The synthesis involves the use of montmorillonites and saponites, as starting 2:1 charged phyllosilicates to give the alumina pillared clays employed as host matrices for reagent inclusion. Characterisation of the resulting materials was provided by chemical and thermal analyses, XRD, SEM-EDX, FTIR and XPS spectroscopies, specific surface area and porosity measurements, and electrochemical impedance technique. The reactivities of the nanocomposite materials have been tested with positive results in the oxidation of alcohols to carbonyl compounds, in particular in the oxidation of benzyl alcohol to benzaldehyde, and also in the syntheses of methyl tert-butyl ether (MTBE) and bisphenol A. Good correlation between the proton conductivity obtained from electrochemical impedance and the acid catalytic activity of nanocomposites containing p-toluenesulfonic acid has been found.

Reproducibility of electrical impedance data for a metal/paint system

The corrosion protection characteristics of a paint/metal combination have been investigated via measurement of the electrical impedance, and specifically via an interpretation of the electrical parameters comprising the systems equivalent circuit. Unfortunately, the reproducibility of the ionic resistance of the coating and the charge transfer resistance of the metal was affected by the uncertain character of macroscopic failures. In contrast, good reproducibility was obtained for the coating capacitance values which depend almost exclusively upon changes occurring at the microscopic level.

Intercalation mechanism of nitrogenated bases into V2O5 xerogel

It has been shown by i.r. and XPS techniques that protonation of nitrogenated bases (ammonia, pyridine and pyrazine) is the major process that occurs on adsorption of these compounds in the intracrystalline environment of V2O5· 1.5 H2O xerogel. At room temperature the intercalation process takes place mainly by a proton-transfer reaction involving protons inherent to the solid and interlayer water molecules. Thermal treatment of the intercalated compounds gives rise to coordination of guest species to vanadium ions included in the host lattice, and VV/–VIV transformations are found to different extents depending on the nature of the intercalated molecules. The protons of the solid and the interlayer water appear to be the origin of the protonic conductivity on the V2O5· 1.5 H2O xerogel, as shown by the impedance spectra. The V5+/V4+ couples formed in the intercalation process produce the electronic conductivity detected when the bases are intercalated in the layered solid.

Effect of treatment with tannic, gallic and phosphoric acids on the electrochemical behaviour of rusted steel

Abstract The present paper studies the effect of different treatments with tannic, gallic and phosphoric acids, which are applied on a rusted steel surface for stabilizing the rust layer before a paint coating is applied. Impedance, polarization resistance and chemical analysis data suggest that none of the applied treatments effectively block the activity in the rusted steel surface.