Electrochemical impedance spectroscopy investigation of chlorinated rubber-based coatings containing polyaniline as anticorrosion agent A. F. Baldissera, D. B. Freitas and C. A. Ferreira * Corrosion protection of mild steel by a newly developed chlorinated rubber (CR)- based coating system containing the inherently conductive polymer polyaniline (PAni) as an anticorrosion agent was studied. The synthesis of PAni and preparation of CR-based paint containing this polymer are described herein. The corrosion behavior of mild steel samples coated with a CR resin, CR/PAni-EB (emeraldine base), CR/PAni-ES (emeraldine salt), and CR/DBSA-doped PAni were investigated in 3.5% NaCl solution. For this purpose, electrochemical impedance spectroscopy and corrosion potential versus time measurements were utilized. It was found that the addition of the two forms of PAni, doped and undoped, to the CR resin increased its corrosion protection efficiency. 1 Introduction Conducting polymers have been extensively studied in recent years due to a great variety of possible applications in several fields, such as energy storage systems [1–3], electrocatalysis [4–6], electrodialysis membranes [7–10], sensors [11–13], and anticorrosive coatings [14–27]. They exhibit different oxidation states and behave as elec- tronic or mixed conductors [28]. A polymer coating is expected to act as a surface modifier which can increase the adhesion of paints to a metal surface [29] and reduce the corrosion rate of the protection system. In some cases, the redox behavior of the coating can provide anodic protection to the substrate [30]. The degree of corrosion protection afforded by a conducting polymer coating depends on both its structural and electronic properties [16, 30]. Among the well-known conducting polymers PAni and poly- pyrrole drawn particular interest from many researchers owing to their electronic applications, and also because they have been used as anticorrosive coatings for almost two decades [16, 31, 32]. PAni is one of most readily prepared conducting polymers [33], and has controllable electrical conductivity, excellent environ- mental stability, and easy processability [34, 35]. An organic coating protects a metal substrate from corrosion primarily via two mechanisms: by forming a barrier against reactants (water, oxygen, ions, etc.) and by acting as a reservoir for corrosion inhibitors. The barrier properties of the coating can be improved by the presence of a pigment (chromate, lead oxides, etc.) [36]. Rohwerder et al. [22–24] discussed that continuous coatings of conducting redox polymer will fail to provide corrosion protection in the presence of larger defects, which they cannot passivate, and will show a fast break-down of the whole coating by fast reduction. This phenomenon is caused by high cation mobility in the reduced polymer, which is due to the gradual transformation of the polymer into an ‘‘autobahn’’ for fast cation transport with increasing progress of the reduction front. It is proposed that this is true for all kinds of redox polymer, regardless the kind of dopant, polymerization conditions, etc. It is assumed that this fast cation transport will also occur in composite coatings containing pigments or filaments of conducting polymer in a non-conductive matrix polymer where high conductivity is reached by extended percolation networks of the conducting polymer. PAni-containing paints offer high corrosion resistant coat- ings for steel surfaces [37–40]. Wessling [41] established a relationship between the corrosion protection offered by PAni along with an increase in the corrosion potential and the redox catalytic activity of the conducting polymer, which was attributed to the formation of a passive layer of metal oxide. Riaz et al. [26] discussed that the presence of the conducting polymeric nanoparticles (PAni and (poly(1-naphthylamine) - PNA) dis- persed in alkyd coatings) neither seems to alter the strength of the passive oxide film nor the polymer undergoes deprotonation, which results in the simultaneous release of the doping anion. The nanoparticles seem to act as ‘‘effective binders’’ and enhance the crosslinking of the alkyd matrix with the mild steel. The uniform dispersion of conducting polymer cements the pores in the alkyd matrix and helps in the formation of a well adherent, dense and continuous network-like structure which impedes the penetration of the corrosive ions until the metal substrate and protects the mild steel from the attack of the corrosive species. Sabouri et al. [25] have described that using PAni and PAni-W coatings, the dominant protection mechanism will be galvanic as well as barrier mechanisms. Also, it was demonstrated that the protection mechanism of the prepared PAni layer will depend on the nature of the species present into the electropolymerization 790 DOI: 10.1002/maco.200905254 Materials and Corrosion 2010, 61 No. 9 C. A. Ferreira, A. F. Baldissera, D. B. Freitas LAPOL/PPGEM, Universidade Federal do Rio Grande do Sul, P.B. 15010, CEP 91501-970, Porto Alegre (Brazil) E-mail: ferreira.carlos@ufrgs.br ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com