Influence of tungstate ion dopants in corrosion protection behavior of polyaniline coating on mild steel M. Sabouri, T. Shahrabi * and M. G. Hosseini Electropolymerization of polyaniline (PANI) and polyaniline- tungstate (PANIW) coatings on mild steel were successfully per- formed using cyclic voltammetry technique. Processes were carried out in aqueous electrolyte solutions of 0.3 M oxalic acid þ 0.1 M aniline and 0.3 M oxalic acid þ 0.1 M aniline þ 0.001 M sodium tungstate dehydrate. Corrosion protection of PANI and PANIW coatings was evaluated with the help of open circuit potential (E ocp ) monitoring and electrochemical impedance spectroscopy (EIS) methods. All the results reveal the influence of additional doping agent (i.e., tungstate) in corrosion protection behavior of PANI coating. 1 Introduction Conductive polymers are being extensively researched for their application in a lot of new technologies. Among the well-known conducting polymers, polyaniline has drawn particular interest of many investigations owing to its electronic application areas [1–6]. One of the applications is corrosion protection [7–16]. For example, Thompson et al. [8] proposed mechanisms for corrosion protection of mild steel by both doped and de-doped forms of PANI in brine solutions and dilute hydrochloric acid. Wessling [9] reported that proper coating of metals with PANI from dispersion (Versicon TM ) leads to a significant shift of corrosion potential to more noble values. Lu et al. [10] proposed a mechanism for corrosion protection of steel using PANI coatings where the passivation of steel surface occurred with the formation of Fe 2 O 3 and Fe 3 O 4 layers. The formation of specific oxide layers occurred when the polymer is formed on the surface during galvanostatic synthesis. Troch-Nagels et al. [11] showed any protective behavior by PANI coatings on steel substrate. Calle and MacDowell [12] observed the galvanic mechanism of PANI/tetracyanoethylene (TCE) and PANI/p-toluenesulfonic (PTSA) coatings on carbon steel. In conclusion, it seems that the mechanism of protection by conducting polymers may depend on many parameters, such as the composition of electropolymerization solutions, synthesis method, type of corrosive solution, and synthesis condition. This study is a continuation of our previous investigations to find the rules of passivator ions on corrosion protection performance of the conducting polymers that parts of it have been published elsewhere [13–15]. PANI and PANIW coatings were electropolymerized on mild steel electrodes in aqueous oxalic acid solution by employing the cyclic voltammetry technique. In the case of polymer-tungstate coating, sodium tungstate was added in electropolymeriza- tion solution and at the end of the electropolymerization process, smooth and adherent coatings were obtained. Open circuit potential monitoring as well as electrochemical impedance spectroscopy technique was employed to evaluate influence of tungstate ions as additional dopants in the corrosion protection behavior of prepared coatings. 2 Experimental methods 2.1 Electrodes and chemicals Commercial mild steel sample with its elemental composition is given in Table 1. In our electrochemical studies, mild steel specimens embedded in resin with an exposed surface area of 0.25 cm 2 were used as the working electrode. Before each experiment, working electrodes were carefully polished with sequence emery papers of various grades (400–1200 grit) and finally rinsed with distilled water, and then electropolished at 4.0 V in a solution containing 50 g/l NaOH at 50 8C which was followed by rinsing them in distilled water and activated by immersion in 0.1 N HCl for 3 s. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a platinum sheet with a large surface area. A Luggin capillary was used to assure the correct measurement of the working electrode potential. The same cell configuration was used in all experiments (electropolymerization and corrosion). In this work, all potentials are reported versus that of the SCE. All purchased chemicals were of analytical reagent grade (Merck) and used without further purification and double-distilled water. 2.2 Electropolymerization A Potentiostat/Galvanostat (PAR EG&G Model 273A) was used. PANI coatings were electropolymerized in a 814 DOI: 10.1002/maco.200804170 Materials and Corrosion 2008, 59, No. 10  T. Shahrabi, M. Sabouri Corrosion & Protection Research Laboratory, Department of Materials Science and Engineering, Tarbiat Modares University (TMU), P.O. Box 14115-143, Tehran (Iran) E-mail: tshahrabi34@modares.ac.ir M. G. Hosseini Electrochemistry Research Laboratory, Department of Physical Chemistry, Chemistry Faculty, Tabriz University, Tabriz (Iran) www.wiley-vch.de/home/wuk ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim