Ekcrrochimicrr Acra, Vol. 43, Nos 34, 309-313. pp. 1998 6 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain zyxwvuts PII: S0013-4686@7)00052-2 0013-4686/98 $19.00 + 0.00 Investigation of corrosion protection of steel by polyaniline films J. R. Santos, Jr,” L. H. C. Mattosob and A. J. Motheoa* “Institute de Quimica de SBo Carlos, Universidade de ‘Go Paula, Cx.P. 780, Sao Carlos, SP, 13560-970, Brazil bCNPDIA/EMBRAPA, Cx.P. 741, Sao Carlos, SP. 13560-970, Brazil (Received 14 October 1996; in revised form 22 January 1997) Abstract-In the present study potentiodynamic polarisation curves were obtained for carbon and stainless steel in contact with 3% sodium chloride aqueous solution saturated with air in order to evaluate the capacity of polyaniline in the emeraldine oxidation state to protect the surface against corrosion processes. A high stability of the PAni films was observed with a gain of the corrosion potential around 270 mV more positive in the substrate covered with PAni than in the case without it. Corrosion of steel could be prevented using the conducting polymers as a protective layer. 0 1997 Elsevier Science Ltd Key njor&: polyaniline, corrosion protection, steel. INTRODUCTION Metal corrosion processes can be controlled by different methods such as the modification of the corrosive medium or the metal and the use of protective agents. In this last case, they act like corrosion inhibitors and they can be a single substance or a mixture that, when present on the surface in a specific concentration, can reduce or eliminate the corrosion process. The most effective inhibitors are organic compounds having in the molecular structure double bonds (n bonds). The efficiency of an organic compound to act as a corrosion inhibitor is strongly dependent on its adsorption on the metal surface, ie its capability to remove water from the surface, which can be expressed as Org,,i + n H20ads+-+Orgads + n HlO,,i. The adsorption of these materials is influenced by the presence of functional groups such as: =NH, -N=N--, -CHO, R-OH and -R=R--, in the inhibitor molecule. It was already observed that the area [I] and the molecular weight [2] of the inhibitor play an important role on the corrosion protection. It is well known that the conducting polymers are an extremely important class of material because they present a combination of properties of classical ~~______. ___ *Author to whom correspondence should be addressed. metals, semiconductors and polymers. In particular, the polymers derived from aniline present some kind of interactions involving dopants which are respon- sible for the solubility of the polyanilines. These soluble polyanilines, besides their application in advanced technologies, can have a big impact in corrosion control by being used as inhibitors as well as for covering metals as a very thin protection layer. In the last two decades, the studies involving conducting polymers have been in different areas of application, such as electrochomic windows [3], displays [4], chemically modified electrodes [5] and, more recently, soluble polyaniline (PAni) films as corrosion inhibitors [&lo]. To be an ideal protective cover against atmospheric and/or electrochemical corrosion, a polymer must present a structure without free ions, water or oxygen. Also, the mobility of the ions must be equal to zero as well as the oxygen and water diffusion coefficients. In 1985, DeBerry [1 I] studying the electrodeposition of PAni on stainless steel observed an anodic protection that reduced significantly the corrosion rate in sulfuric acid solutions. The author [l l] also verified that before PAni electrodeposition a layer of passivated oxide was formed on the steel surface, and so he proposed that the PAni film, in contact with the oxidized surface, stabilised the oxide against the dissolution and/or reduction process. Troth-Nagels et zyxwvutsrqpon al. [12], 309