Polyaniline inhibition of corrosion-driven organic coating cathodic delamination on iron

The polyaniline emeraldine salt of paratoluenesulfonic acid (PAni-pTS) is dispersed in polyvinylbutyral coatings adherent to an iron surface. Such dispersions are shown to effectively inhibit corrosion-driven coating delamination (cathodic disbondment) when 5% w/v (0.86 M) aqueous chloride electrolyte contacts a penetrative coating defect. A scanning Kelvin probe is used to quantify the influence of PAni-pTS volume fraction (phi(pa)) on delamination rate and the potential of the intact (undelaminated) coated surface (E-intact). Secondary ion mass spectroscopy and atomic force microscopy are used to determine the time-dependent thickness of an oxide layer developing at the coating-iron interface. At 20degreesC, high relative humidity (93%), and phi(pa) = 0.3, the iron oxide layer grows at a constant rate of 5.6 x 10(-3) nm s(-1). E-intact increases monotonically from 0.2 to 0.56 V vs. SHE, and coating delamination rates decrease by ca. 95% as phi(pa) is increased from 0 to 0.25. Addition of 0.01 M Na-pTS to the experimental electrolyte has no effect. An inhibition mechanism is proposed in which through-coating cathodic O-2 reduction is suppressed by the ennoblement of substrate potentials and the OH- product of O-2 reduction is absorbed through PAni-pTS mediated pH buffering. (C) 2005 The Electrochemical Society. All rights reserved.