Ursula Rammelt

On the applicability of a constant phase element (CPE) to the estimation of roughness of solid metal electrodes

The dispersive behaviour observed at rough electrodes can be described by a constant phase element (CPE) of the form Z(ω) = K(jω)−α, where α has a value between 0·5 (porous electrodes) and unity (ideally flat electrodes). It is shown experimentally, that α unequivocally reflects the roughness of solid electrodes and is independent of the electrode material. Using the concept of fractal geometry the parameter α, often denoted only as a fit parameter, is characterized as a purely geometric size. If the rough surface has self-similar scaling characteristics, the CPE is directly related to the so-called fractal dimension D and therefore α can be understood as a measure of microscopic irregularities.

The influence of surface roughness on the impedance data for iron electrodes in acid solutions

It is verified by impedance measurements on iron electrodes pretreated in various manner that the frequency dispersion occurring during the initial phase of immersion in acid solutions, represented by the depression angle Φ, is a quantity which depends upon the roughness of the electrode surface only. It is shown that Φ is more appropriate for assessing the initial state of a metal for the purpose of subsequent corrosion studies than the polarization resistance Rp or the apparent double layer capacity C′dl or that corrected for its dispersion elements Cdl.

Corrosion protection by ultrathin films of conducting polymers

A new experimental model based on the electrochemical impedance spectroscopy for investigating the reactions taking place in a film of intrinsically conducting polymers formed on iron or steel substrate is developed, in which the reaction at the injured surface area of the coated iron and reaction of polymer could be studied separately. It is evident that a combination of adhesion promoting self assembling monolayer and a film of conducting polymer gives the protection against the delamination, but the catalytic action of the conducting polymer for the oxygen reduction and for the metal passivation is not found.

Physical and electrochemical characterization of nanocomposites formed from polythiophene and titaniumdioxide

Abstract A simple method for covering titanium dioxide particles with a polythiophene film by chemical preparation was developed. The resulting nanocomposites consisted of a titanium dioxide core with a grain size of 25–250 nm and a polythiophene shell between 1 and 2 nm thickness. The composites were characterized by scanning electron microscopy, thermogravimetry, X-ray photoelectron spectroscopy, cyclovoltammetry, impedance spectroscopy and photocurrent spectroscopy. The content of polythiophene in the composite (determined by thermogravimetry), was between 2% and 5%. Disk-like electrodes were prepared by pressing and then characterized by various electrochemical methods. A reversible redox potential of the polythiophene of +1.0 V (NHE) was determined by cyclic voltammetry. The reduced form of polythiophene behaved as a p-type semiconductor so that the composite with n-type TiO 2 contained the properties of a p/n-junction. In the photocurrent spectra (depending on the applied potential), the characteristic anodic peaks of the TiO 2 at λ =320 nm and cathodic peaks of the polythiophene around λ =500 nm were found. A new cathodic peak observed at 370 nm was explained as a new feature of the pn interface.

Protection of mild steel by modification with thin films of polymethylthiophene

The electrodeposition of polymethylthiophene films on mild steel from organic solution is reported. It is shown that homogeneous and very adherent polymethylthiophene films are formed after a special pretreatment with 2(3-thienyl)ethylphosphono acid. Polymethylthiophene films reduce the corrosion rate of mild steel but they cannot passivate the metal surface. A model is proposed which can explain the excellent adhesion properties of polymethylthiophene films, necessary for corrosion protection of mild steel.

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Abstract Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz ⩽ f ⩽ 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.

Investigation of polybithiophene/n-TiO2 bilayers by electrochemical impedance spectroscopy and photoelectrochemistry

Extremely adhesive polybithiophene (PBT) films were grown on TiO2 in a new, two-step procedure using a silyl substituted thiophene derivative as a surface coupling agent and a subsequent chemical polymerization process. Electrochemical impedance and photocurrent measurements were carried out in order to achieve information on the semiconducting properties of both materials. When a PBT film deposited onto TiO2 is reduced it can be described as a p-type semiconductor with a flatband potential of about 0.65 V (vs. SCE), whereas the n-type semiconductor TiO2 gives a flatband potential of about −0.4 V (vs. SCE). The photocurrent measurements show an anodic photocurrent at wavelength of between 300 and 400 nm and a cathodic photocurrent of between 400 and 600–700 nm, confirming the impedance measurements. The results of the impedance and photocurrent measurements are summarized in a band structure model which explains the behaviour of the semiconductor electrode Ti/n-TiO2/p-PBT.

Impedance dispersion on coated metal electrodes

Abstract Using the example of the steel/Zn/polyacrylate layer coil-coating system, it is demonstrated that, subsequent to chemical intensive load impedance, dispersion may occur on electrodes consisting of a metal substrate susceptible to corrosion and a defective polymer layer. For the resulting states of such electrodes, a complex equivalent circuit is developed and verified by experiments. The mathematical statements derived on this basis permit individual impedance elements and parameters to be calculated. They explain the cause of the impedance dispersion qualitatively as a relaxation time distribution of the entire electrode process.

Analysis of impedance spectra on corroding metals

Abstract The dispersion occurring during impedance measurements on corroding metal surfaces can be mathematically represented by the introduction of a frequency-dependent impedance Z 2 ( ω ). In order to determine this impedance physically, an analysis of the term Z 2 ( ω ) = K 2 −1 ( jω ) − α 2 is carried out. Supplementary to the conventional transmission line model for solid electrodes, a new model is presented. The occurrence of frequency-dependent elements is explained here as a superposition of many partial reactions in the form of R p — C p elements in series circuit. The frequency of the individual relaxation times is determined by means of the empirical distribution expression by Cole and Cole. It is shown that the results of impedance measurements on zinc-plated electrodes with a defective polymer layer after an intensive chemical loading can be represented by this model. The causes of dispersion are discussed in a general way.

On the role of the salts of weak acids in the chemical passivation of iron and steel in aqueous solutions

Abstract It is shown that the passivating action of dissolved oxygen in aqueous media for iron and mild steel depends on the presence of the salt of a weak acid above a critical minimum concentration, there have been proved to be valid, as a generalization, that for the critical pH value, pH c , should be guaranteed: pH c ≥ (pKa + 1). Exceptions result in the case of salts of acids which in the pH range 5 ≤ pH s ≤ 8 of interest for corrosion protection still have a buffering effect or form very soluble complexes with iron ions, so that no oxidic passive layer can develop.