Conducting polymer films in 1985 deberry observed

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Conducting Polymer Films In 1985, DeBerry observed that like certain oxi- dizing inorganic compounds, conducting organic compounds could also anodically inhibit steel under certain conditions. 154 Since that time, a great deal of effort has been spent applying conducting organic polymers as corrosion protective films, culminating in a number of recent patents. 155-159 Some work has been directed specifically at corrosion protection of aluminum and aluminum alloys. 160-163 A very recent patent claims the formation of a composite metal oxide-conducting polymer anodic film. 163 Reports have claimed that electrochemically deposited polyanaline (PANI) improves the corrosion resistance of Al 2024-T3 by 3 orders of magnitude. 164 Parallels that exist between conducting polymer films and chromate may reveal the corrosion inhibi- tion mechanism of such films. The doped and oxi- dized form of conducting polymers, such as PANI, conduct electrons. Upon reduction, these films become nonconducting. 165 Hence, they maintain oxides in the passive condition while suppressing cathodic activity. 166 Scanning vibrating electrode measurements show that exposed metal in a large defect in a conducting polymer film do not corrode significantly during exposure to sulfate-chloride solutions. 167 Recent work has demonstrated another impor- tant property of doped and oxidized forms of con- ducting polymers. Materials doped with an inhibiting anion release the anion upon reduction. Both the re- lease of an anionic pitting inhibitor and the decrease in conductivity of the cathode contribute to the inhi- bition. 157,168 Others suggest that the negative poten- tial of Al dominates the mixed PANI/Al potentials and that PANI coupled to Al 2024-T3 reduces to form a poor cathode. 166 This latter work also notes that CSA- doped PANI appears to inhibit pitting of a manufac- tured defect and that the inhibition process does not result from anodic stabilization. This supports the hypothesis that inhibiting anions play a role in pitting inhibition. A hypothesis for the role of conducting polymer films emerges as the following: —conducting polymer films maintain passivity (anodically protects as per DeBerry 154 and Wessling 157 ) —conducting polymer films short circuit the ORR and become a nonconducting “poor” cathode 166 —conducting polymer films release an inhibiting anion 168-169 This proposed model for corrosion inhibition by conducting polymers suggests investigating easily- processed materials doped in the oxidized state by a
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING C ORROSION—Vol. 59, No. 5 397 suitable corrosion (pitting) inhibiting anion. The choices for environmentally benign anions could in- clude those oxoanions of molybdenum or tungsten, 124 permanganate, 127 phosphate, borate, and a number of heteropolyanions comprised of the oxo- com- pounds of Mo, W, Si, Al, Zr, V, and P, and combina- tions thereof. 122 The possible combinations of inorganic anionic inhibitors as anion dopants for conducting polymer cations represent a large class of compounds.
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