Conducting polymers have been used in a variaty of applications. Due to the redox property, conducting polymers have also been investigated for anticorrosion coatings on metals. Here the conducting polymers are expected to provide advantages such as anodic protection and self-repairing ability. Most promising conducting polymers for use in anticorrosion coatings are polyaniline and polypyrrole, and nowadays they are blended with conventional polymer coatings or used as additives to conventional organic paint. One crucial requirement is that the conducting polymer must posses oxidizing power that enables anodic ennoblement or induce passivity of the metal.
In this study, a conducting polymer based on polyaniline (Pani) was prepared in the presence of methane sulfonic acid (MeSA) as dopant by chemical oxidative polymerization, using ammonium peroxodisulfate. Then the conducting polymer (MeSA-Pani) was dispersed in weight percentage of 1.25%, 2.5% and 5%, respectively, in polyvinyl acetate (PVAc) and coated on carbon steel samples by a dipping method. The anticorrosion properties of these coatings on carbon steel in 3% NaCl solution were investigated by electrochemical methods including open-circuit potential (OCP) vs. time of exposure and electrochemical impedance spectroscopy. The results show a large anodic ennoblement and also indicate interaction between the conducting polymer and the metal (Fe) surface. The coating containing 2.5 wt.% MeSA showed the best anticorrosion performance, a stable high OCP value during the 60 days exposure.
To elucidate the corrosion protection mechanism and explain the reason for best performance of the coating containing 2.5 % MeSA, conductivity mapping of the coatings was performed by using an atomic force microscope with conductive probe. The AFM conductivity mapping revealed that the conducting polymer particles are electrically interconnected throughout the coating and are aseembled in different ways in the coatings. Moreover, the coating containg 2.5 % MeSA exhibited the highest local conductivity among the three coatings. The results indicate that the distribution of conducting polymer particles has a strong influence on the anticorrosion performance of the coatings containing the conducting polymer.
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