G. Reinhard

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.

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.

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.

Application of corrosion inhibitors in water-borne coatings

Abstract Modification of an acrylate-based model dispersion by different additives which should act as corrosion inhibitors during the flash-off time of the dispersion film on a mild steel surface has been investigated. It is shown that mercapto compounds with a second functional group are capable of interaction with both the substrate surface and the functional groups of the polyacrylate film. Such additives bring about sufficient corrosion inhibition without adversely affecting other properties of the acrylate dispersion. Thus coatings with pronounced barrier properties on steel sheets are built up. The corrosion protection properties of the additives in the liquid dispersion as well as in the coatings formed on steel sheets have been characterized by electrochemical impedance spectroscopy.

Impedance analysis of conversion layers on iron

The electrochemical behaviour of iron coated with a porous conversion layer has been studied in neutral aqueous solutions by means of Electrochemical Impedance Spectroscopy (EIS). The different structures of reactively formed layers of inorganic insoluble salts (ie phosphates) and of organic complex compounds (ie with phosphonates) can clearly be distinguished with this method. Using nonlinear least squares fitting all values of the components in the equivalent circuit model can be evaluated. In order to determine successively the components it is useful to combine the least squares fitting with a sequence of deconvolutions.

Surface characterization of iron and steel prior to coating

Abstract This report presents the latest knowledge on surface properties by which the mechanism and intensity of the adhesion of organic coatings can be influenced on iron materials. By this, it is to be made clear above all that these surface properties depend on a large number of single factors. Since, however, their complex combined action has not been quite clear, it is not yet possible to reconcile the partly different conceptions of the most favourable condition of steel surfaces for a subsequent coating [1 -14]. On the basis of experimental results there are given reasons for the conclusion that greater attention than has been the case so far should be paid to the hydroxyl group-containing primary oxides being formed spontaneously on steel surfaces following the mechanical or chemical pretreatment during contact with air, when coating materials and techniques are selected. If such a continuous, homogeneous primary oxide layer exists, its amphoteric surface hydroxyl groups can be utilized for the chemical adhesion of suitable organic film-formers. On steels whose surface is heterogeneous due to the island-shaped distribution of primary oxides, non-metallic deposits and pollutants, homogenization can be achieved by producing a conversion layer. Usually, phosphate and oxide layers have amphoteric surface hydroxyl groups, too, whose isoelectric point (IEPS) varies with the composition, so that it is possible to select thermodynamically favourable conditions for the chemical adhesion of organic coatings.