* W. J. Hamer, J. H. W. de Wit, Section Corrosion Technology & Electrochemistry, Faculty of Applied Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft (The Netherlands) L. Koene, Corrosion Prevention & Antifouling, TNO Institute of Industrial Technology, P.O. Box 505, 1780 AM Den Helder (The Netherlands) Formation and electrochemical behaviour of poly(pyrrole) coatings on steel substrates W. J. Hamer*, L. Koene and J. H. W. de Wit This paper focuses on the deposition process and the semi-con- ductive properties of PPy layers on steel. The polymerisation of PPy was performed galvanostatically on steel in an aqueous solution of 0.1 M pyrrole and 0.1 M oxalic acid (H 2 C 2 O 4 ) using a current den- sity of 1 mA/cm 2 . The build-up of a ferrous oxalate (FeC 2 O 4 ) layer, which seals the seel substrate surface, is initially observed. PPy layers made by this method are always doped with oxalate anions. A mechanism is proposed in which PPy deposits at sites where fer- rous oxalate is dissolved. PPy is a semiconductor, just like oxide films formed naturally on some metals. Mott-Schottky diagrams for 9.0, 11.6 and 15.0 kHz suggests a frequency dependence of the dielectric constant of polypyrrole (e PPy ). The flatband potential E FB was about  0.39 V vs. NHE and the charge carrier density was estimated to be about 5  10 20 m 3 . Also, in earlier work by Ram- melt et al. and by Miller and Bockris (using different electrolytes and substrate materials) a frequency dependence has been found for a constant phase element that was used to describe the space charge capacitance C SC . The frequency dependence observed can partly be ascribed to surface roughness, which seems experimentally sup- ported by SEM-research. 1 Introduction For several years, the key environmental issue in corrosion research has been the search for effective corrosion protection without the use of heavy metals. Among the alternatives under investigation, intrinsically conducting polymers (ICPs) show interesting results. For corrosion protection, three ICPs are considered: polyaniline (PAni), polypyrrole (PPy) and poly- thiophene (PTh). In the research described here, PPy was used. Two different mechanisms have been discussed in literature to explain the expected corrosion protection of ICP materials, both of which result in passivation of the ferrous substrate ma- terial. In these mechanisms, passivation is reached by a high redox potential (i.e. anodic protection) or by catalytic reduc- tion of oxygen on the ICP’s surface. Both of these mechanisms have been questioned, most recently in an article by Rammelt et al. [1]. The electrochemical behaviour of ICP materials is a vital factor in understanding their corrosion protective cap- abilities. Unfortunately, the results published have neither been very consistent nor easily reproducible [1], which indi- cates the difficulties involved when synthesising and examin- ing these materials. Because ICP materials are infusible and insoluble in almost all cases, they are difficult to process re- producibly. Although literature reports on the corrosion protection of metals by semiconducting coating materials are not very fre- quent, a mechanism proposed by Jain [2] offers an interesting insight. In most cases, the semiconducting properties of ICP materials are not incorporated into the mechanisms describing the corrosion protective behaviour of ICP materials. However, Jain et al. report on semiconductor layers on aluminium sub- strates. They suggest that the metal-semiconductor contact poses a barrier to charge transport across it, with the barrier height depending on the electronic structure of the materials used. In their article, Jain et al. state that the concept of an active electronic barrier could also be applied to other semi- conductor coatings on metals. We expect the electrochemical behaviour of ICP materials to be strongly dependent on the sample preparation procedure, which is therefore described in detail. We first look into the deposition of (poly)pyrrole (PPy) layers on steel substrates as described by Su and Iroh [3]. We propose a more detailed me- chanism for this deposition process, which may be a small step towards future use of ICPs on a larger scale. The samples ob- tained are then used to examine semi-conducting properties of PPy on steel electrodes. A thorough understanding and de- scription of semiconducting behaviour in these materials might be a key to their practical use in corrosion protective applications. 2 Electrodeposition of PPy on steel 2.1 Experimental In order to make the systems examined both representative and well-defined, it was decided to use electrochemical de- position of PPy on steel substrates. Previously, we employed latex-based coatings on steel substrates [4], but the vast amount of extra parameters introduced by using a complete coating system forced us towards a less complex approach. Ultra-low carbon steel (Armco, 0.018%C steel) samples were cut from a 25 mm diameter rod. Using a lathe, the ori- ginal rolling-oxide layer was machined off prior to the sample cutting. As the samples were intended to be used more than once, a sturdy contact was needed. This was obtained by dril- ling a small pit of approx. 4 mm deep and 4 mm diameter into the back of the sample, into which a thread was cut. This hole accepts a wire that will both serve as a handle and provide a Materials and Corrosion 2004, 55, No. 9 Poly(pyrrole) coatings on steel substrates 653 F 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/maco.200303778