Polyaniline Inhibition of Corrosion-Driven Organic Coating Cathodic Delamination on Iron

Abstract

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 (catbodic 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 (φpa) on delamination rate and the potential of the intact (undelaminated) coated surface (Eintact). 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 20°C, high relative humidity (93%), and φpa, = 0.3, the iron oxide layer grows at a constant rate of 5.6×10-3 nm s-, Eintact increases monotonically from 0.2 to 0.56 V vs. SHE, and coating delamination rates decrease by co. 95% as φpa 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 O2 reduction is suppressed by the ennoblement of substrate potentials and the OH- product of O2 reduction is absorbed through PAni-pTS mediated pH buffering. © 2005 The Electrochemical Society. All rights reserved.