Conducting Polymers and Corrosion: Part 2 — Polyaniline on Aluminum Alloys

Abstract

The electrochemical behavior of conducting polyaniline coatings on various aluminum alloys subjected to immersion in dilute Harrison solution (0.35% ammonium sulfate [(NH4)2SO4], 0.05% sodium chloride [NaCl]) was studied. Electrochemical impedance spectroscopy revealed that the charge-transfer resistance (Rct) of polyaniline-coated alloys increased as a function of immersion time. Polyaniline-coated platinum did not exhibit a significant increase in impedance under similar conditions, indicating that an active metal in contact with the polyaniline is required for the observed increase in Rct. A similar pattern of increasing Rct was observed for Alodine† (Product A)-treated Al 7075T-6 (UNS A97075) alloys. Mean current and mean potential values obtained from electrochemical noise measurements also suggest a substantial electrochemical interaction between the polyaniline and the aluminum alloy during the early stages of immersion. Polarization experiments and open-circuit potential measurements revealed an ennobling of aluminum alloys to higher potential in the presence of polyaniline coatings. The corrosion protection afforded by a polyaniline/epoxy two-coat system on Al 2024T-3 (UNS A92024) alloy also was evaluated using impedance spectroscopy and compared with that for a single coat of epoxy on untreated and Product A-treated Al 2024T-3 alloy. The Product A treatment and the polyaniline coating were found to increase the lifetime of the epoxy topcoat, although these two-coating systems exhibited rather different variations in low-frequency impedance with immersion time. A mechanism consistent with these observations was suggested.