Inhibition of corrosion-driven organic coating delamination from a zinc surface using polyaniline pigments

Abstract

In-coating dispersions of the para-toluene sulfonic acid emeraldine salt (ES) of polyaniline (PAni-pTS) are shown to be highly effective inhibitors of corrosiondriven organic coating delamination on zinc surfaces. However, this inhibition is achieved without the characteristic ennoblement of substrate potentials typically observed for ferrous metals. At high humidity, galvanic interaction of oxidizing PAni-pTS pigments with the underlying zinc causes the growth of an interfacial oxide film. The period of oxide growth coincides with the duration over which the Zn surface is temporarily ennobled by the PAni-pTS. The results of experiments designed to elucidate the inhibition mechanism are presented, where substrate potentials, along with delamination and oxide growth kinetics are compared with equivalent in-coating compositions of ES and emeraldine base (EB). It is demonstrated that the basic nature of the interfacial oxide layer causes PAni-pTS re-oxidation to the non-conducting EB form, accounting for the loss of ennoblement. Efficient inhibition is achieved by a combination of a PAni-pTSinduced oxide layer and a zinc para-toluene sulfonate phase, acting as a reservoir of underfilm Zn2+ ions, producing a suppression of cathodic oxygen reduction. Availability of Zn 2+ ions at the organic coating.metal interface both reinforces and prevents the dissolution of the amphoteric zinc oxide surface layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.