Electroactive-conducting polymers for corrosion control - 4. Studies of poly(3-octyl pyrrole) and poly(3-octadecyl pyrrole) on aluminum 2024-T3 alloy

Chromate surface treatments and chromate-containing epoxy primers are often used for corrosion control of aluminum alloys. However, due to environmental concerns and adverse health effects surrounding such use of chromates, there is an intensive effort to find suitable replacements for chromate-based coatings. 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, with potentials that are positive of iron and aluminum. Thus, as with chromate, interesting and potentially beneficial interactions of ECPs with active metal alloys are anticipated. Our group has been investigating several ECPs for their corrosion-protective properties, with emphasis on solution processable polymers. In this paper, we describe results of long-term immersion and preliminary results of Prohesion(R) exposure studies on coating systems that incorporate either poly(3-octyl pyrrole) (POP) or poly(3-octadecyl pyrrole) (PODP) as a primer coating of 2-3 mum thickness, with a polyurethane topcoat of ca. 20 mum thickness, on Al 2024-T3. In control experiments, a chromated-epoxy primer of 20 mum thickness replaced the ECP coating. The immersion solution was dilute Harrison's solution, consisting of 0.35% (NH4)(2)SO4 and 0.05% NaCl. Coating performance was followed by electrochemical impedance spectroscopy (EIS), although some results using the electrochemical noise method (ENM) will also be described. At this stage of immersion, approaching 600 days for the POP samples, four of six controls (chromated-epoxy primer/polyurethane topcoat) have failed, while none of the six POP/polyurethane topcoat systems has failed. Although blisters are apparent in all samples at this stage of immersion, the size (and density) of the blisters present on the control samples (nun dimensions) is much greater than observed on the POP/polyurethane samples (a few tens of micrometers in dimension). The POP appears to have the ability to arrest blister development, and although PODP shows a similar ability, it is not as pronounced. Results of experiments on samples with only the ECP coating (no topcoat) will also be discussed, including EIS, linear polarization measurements, and pH changes occurring in the immersion solution. (C) 2001 Elsevier Science B.V. All rights reserved.