Conducting Polymers and Corrosion III. A Scanning Vibrating Electrode Study of Poly(3-octyl pyrrole) on Steel and Aluminum

Electroactive conducting polymers (ECPs) continue to be of considerable interest as components of corrosion-resistant coating systems. ECPs, in addition to being conductive, are redox active materials, typically with potentials that are positive of iron and aluminum. Thus, as with chromate, interesting and potentially beneficial interactions of ECPs with active metal alloys such as steel and aluminum are anticipated. In this work, the scanning vibrating electrode technique (SVET), also known as the current density probe, was used to probe such interactions between a poly(3-octyl pyrrole) coating (POP) and cold-rolled steel and aluminum (Al 2024-T3) substrates. The POP coatings were scribed to simulate a defect through the coating to the metal substrate surface. The SVET was used to map the current flowing in and around the defect while the sample was immersed in either 3% NaCl (steel) or in dilute Harrison solution (aluminum), an aqueous solution consisting of 0.35% (NH4)2SO4, 0.05% NaCl. Although there were significant differences in the behavior of the POP-coated steel and POP-coated aluminum substrates, both exhibited a significant delay before the onset of any observable current compared to uncoated or epoxy-coated samples. Current density maps for the steel clearly indicate that the reduction reaction occurred on the conducting polymer surface, with oxidation confined to the defect. Current density maps for the aluminum alloy never displayed significant oxidation at the defect. Rather, reduction (after a significant delay) occurred at the defect as well as across the polymer surface, with concomitant localized undercoating oxidation of the aluminum substrate.