Dimitra Sazou

Formation of conducting polyaniline coatings on iron surfaces by electropolymerization of aniline in aqueous solutions

Abstract Electropolymerization of aniline on an iron disc electrode in aqueous solutions is studied by using various inorganic and organic acids under potentiodynamic, potentiostatic and galvanostatic conditions. Smooth, well adhering polyaniline coatings were obtained in oxalic acid solutions under potentiodynamic conditions during sequential scanning of the potential region between −0.6 and 1.5 V. Well-defined cyclic voltammograms showing the electroactivity and growth of polyaniline were observed during each cycle. The deposited coatings were characterized electrochemically, microscopically and spectroscopically. The deposition of polyaniline appears to occur over the passive metal oxide film, but electron transfer between the polymer and the metal can occur. Polyaniline coatings on iron appear to be promising for metal anodic protection in corrosive aqueous media.

Electrodeposition of ring-substituted polyanilines on Fe surfaces from aqueous oxalic acid solutions and corrosion protection of Fe

The electrochemical polymerization of several ring-substituted anilines namely of o-toluidine (o-Tol), m-toluidine (m-Tol), o-anisidine (o-Anis) and o-chloroaniline (o-ClAn) was carried out on passivated Fe surfaces. Thin polymeric films were deposited by cyclic voltammetry, potentiostatic or galvanostatic techniques on the Fe-disc electrode from aqueous oxalic acid solutions. Well-defined cyclic voltammograms showing the electroactivity and growth of polymers were observed during each potential cycle. The electrochemical response and the surface morphology of the deposited polymeric films of substituted polyanilines were characterized in comparison with the non-substituted polyaniline films on Fe. Adherent and relatively stable polymeric films were obtained for o-Tol, m-Tol and o-Anis. These films show protective properties against corrosion of Fe in sulfuric acid solutions stabilizing the Fe passive state. Their anti-corrosion behavior, although slightly poorer than that of the non-substituted polyaniline, bears strong similarities with that of the polyaniline. A less conductive polymer with a very poor stability and protective ability against the corrosion of Fe was obtained for o-ClAn.

Conducting polymers for corrosion protection: a review

Conducting polymers (CPs) such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) are used for the corrosion protection of metals and metal alloys. Several groups have reported diverse views about the corrosion protection by CPs and hence various mechanisms have been suggested to explain anticorrosion properties of CPs. These include anodic protection, controlled inhibitor release as well as barrier protection mechanisms. Different approaches have been developed for the use of CPs in protective coatings (dopants, composites, blends). A judicious choice of synthesis parameters leads to an improvement in the anticorrosion properties of the coatings prepared by CPs for metals and their alloys. This article is prepared as a review of the application of CPs for corrosion protection of metal alloys.

The dynamical behavior of the electrochemical polymerization of indole on Fe in acetonitrile–water mixtures

Abstract This study reports on the stability of polyindole (PInd) films on Fe. Current oscillations emerge during the potentiostatic polymerization of indole upon its anodic oxidation on an Fe electrode in acetonitrile containing tetrabutyl-ammonium tetrafluoro-borate (N(Bu) 4 BF 4 ). The oscillatory response of the system depends on the concentration of water, indole (Ind) and N(Bu) 4 BF 4 . For low concentrations of N(Bu) 4 BF 4 , the polymerization of Ind is possible and is enhanced in the presence of H 2 O. The addition of H 2 O leads also to an increase of the Fe dissolution. Increasing the concentration of Fe 3+ ions at the Fe|polymer interface induces a mechanical instability of the polymer film. The origin of the present dynamical behavior is discussed on the basis of a breakdown and reconstruction of the polyindole film due to the accumulation of Fe 3+ ions at the Fe|polymer interface. The kinetics of the indole polymerization coupled with the Fe electrodissolution includes the catalytic oxidation of indole by Fe 3+ ions.

Non-linear dynamics of the passivity breakdown of iron in acidic solutions

Abstract Breakdown of the iron passivity in acid solutions accompanied by current oscillations was investigated by using electrochemical techniques, which reveal the non-linear dynamical response of the system in the current–potential (I–E) and current–time (I–t) planes. Current oscillations of the Fe|electrolyte electrochemical system were studied in the (a) absence and (b) presence of chlorides. In case (a) two oscillatory regions were distinguished; one at low potentials associated with the formation–dissolution of a ferrous salt and another at higher potentials associated with the formation-breakdown of the oxide film. Chaotic oscillations appear in the former region whereas periodic oscillations of a relaxation type appear in the latter region. In case (b), complex periodic and aperiodic oscillations are induced by small amounts of chlorides due to pitting corrosion. Pitting corrosion is a multistage localized process of a great technological importance. It consists of a local breakdown of the passive oxide film and formation of individual areas of metal active dissolution. It was shown that oscillatory phenomena might be used for the characterization of pitting corrosion. Examples of the application of the non-linear dynamical response to pitting corrosion were discussed at early and late stages of the pit growth.

The improved Franck—FitzHugh model for the electrodissolution of iron in sulphuric acid solutions: linear stability and bifurcation analysis. Derivation of the kinetic equations for the forced Franck—FitzHugh model

Abstract A linear stability and bifurcation analysis is carried out on the improved mathematical model proposed by Franck and Fitzhugh for the oscillations observed during the electrodissolution of iron in sulphuric acid solutions. For the improved Franck—Fitzhugh model a logarithmic instead of a linear dependence of the Flade potential on H + concentration is considered. Also a correcting function, o, related to the transference number of the H + , is incorporated into the Nernst-Planck equation, which is applied between the bulk of the solution and the diffusion layer, to account for the effect of the charge transport by Fe 2+ and SO 2− 4 . This type of analysis together with the inherent physico-chemical constraints gives the conditions and the limits of the control parameters for stable, decaying or periodic solutions of the dynamical equations which describe the system. The transition to periodic solutions occurs through a Hopf bifurcation. The advantage provided by the improved model is that the control parameters which are involved in the bifurcation condition include the H + concentration of the solution, h 0 , the applied potential E p , and a function related to the transference number of the H + ions, o. Therefore, a direct comparison of the bifurcation results with the experimental data can be made to determine more explicitly the parameters for which the model provides not only qualitatively but also quantitatively reasonable predictions. The kinetics equations for the periodically forced improved and simplified Franck—FitzHugh model are derived.

A point defect model for the general and pitting corrosion on iron∣oxide∣electrolyte interface deduced from current oscillations

Abstract Analysis of the passive–active oscillatory region of the Fe∣0.75 M H2SO4 system, perturbed by adding small amounts of halide species, allow the distinction between pitting and general corrosion. Complex periodic and aperiodic current oscillations characterize pitting corrosion whereas monoperiodic oscillations of a relaxation type indicate general corrosion. A point defect model (PDM) is considered for the microscopic description of the growth and breakdown of the iron oxide film. The physicochemical processes leading to different types of corrosion can be clarified in terms of the PDM. Occupation of an anion vacancy by a halide ion results in the localized attack of the passive oxide and pitting corrosion. On the other hand, the formation of surface soluble iron complexes is related to the uniform dissolution of the passive oxide and general corrosion.

Distinction between general and pitting corrosion based on the nonlinear dynamical response of passive iron surfaces perturbed chemically by halides

We report on different types of current oscillations induced by fluoride and chloride species in the Fe|H2SO4 electrochemical system. On the basis of the nonlinear dynamical response of the system, certain criteria are deduced, which allow the distinction between general and pitting corrosion. We use a point defect model (PDM) for the description of the iron oxide film along with the formation of surface complexes between the oxide iron cations and halides in order to explain the difference between general and pitting corrosion.

The influence of chloride ions on the dynamic characteristics observed at the transition between corrosion and passivation states of an iron electrode in sulphuric acid solutions

Abstract The role of chloride ions on the current oscillations of relaxation type at the transition of iron between the active and passive states in sulphuric acid solutions is investigated. Pitting corrosion occurs in the presence of chloride ions. The periodic relaxation oscillations observed in the absence of chloride become aperiodic and finally disappear under stationary conditions of the Fe disc electrode in the presence of chloride. However, under rotational conditions, quasiperiodic and chaotic oscillations are observed upon gradually increasing the chloride additions. The rich dynamical response of the system Fe/2M H 2 SO 4 + x M Cl − , under various conditions of chloride concentrations, electrode rotation and potential has been characterized by using the current—time series and power spectra. Examples of quasiperiodicity and chaos are given.

Current oscillations and mass-transport control during electrodissolution of iron in phosphoric acid solutions

This work presents a characterisation of the global electrochemical and non-linear dynamical behaviour across the active-passive transition region of the Fe/H3PO4 system. The Fe/H3PO4 system is a new electrochemical oscillator displaying a rich dynamical response on varying several controllable parameters such as the phosphoric acid concentration (CH3PO4), the Fe-disc rotation speed (ω) and the applied potential (E). The current-potential (I-E) curves show two transition points. The first transition occurs from the active to the limiting current plateau. On increasing CH3PO4, the limiting current (IL) decreases. The IL is governed by full mass-transport conditions and is associated with the electropolishing process of iron in concentrated phosphoric acid solutions as well as with current oscillations. The second transition occurs from the limiting current to passivity and is accompanied by current oscillations for moderate and high CH3PO4 (4 M ≤ CH3PO4 < 14.8 M). The oscillatory region is located within a loop that is formed in the I-E curve when a forward and a subsequent backward potential scans were applied, because reactivation occurs at lesser positive potentials than the potential at which the system leaves the limiting current region. The observed current oscillations can be classified into two main classes: The complex oscillations (small amplitude, aperiodic, mixed-mode and periodic) that occur at the least positive potentials (end of the plateau) and the periodic relaxation oscillations that occur at the most positive potentials (on the site of the passive state). It is concluded that the iru-potential drop (where Ru is the solution uncompensated resistance) must be taken into account along with the electrode and surface processes for understanding the origin of the current oscillations in the Fe/H3PO4 system. In order to explain the rich dynamical response of the Fe/H3PO4 system, the effect of the electrolyte composition on the kinetic of the electrode and surface processes should be considered.