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 ( ϕ 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 20°C, high relative humidity (93%), and ϕ pa = 0.3 , the iron oxide layer grows at a constant rate of 5.6 × 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 ϕ 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.