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Deposition of Polypyrrole on Porous Aluminum Oxide

Aluminium (Al) alloys, such as Al 2024-T3, are extensively used in the aircraft industry because of their light weight and excellent mechanical properties. Chemically pure Al is naturally protected from corrosion by an oxide layer. In the case of alloys this layer contains intermetallic impurities capable of causing galvanic corrosion. Corrosion of Al alloys is a serious industrial problem that typically is remedied by using hexavalent chromium (Cr 6+ ) coatings. Cr 6+ coatings are a hazard for human health and the environment and due to new OSHA requirements these coatings have to be replaced. A possible alternative to Cr 6+ coatings is conjugated polymer (CP) coatings that have shown to provide a very good corrosion protection on Al. Our research group has been exploring various conjugated polymers (e.g., polyaniline, polypyrrole and polythiophene) for use as corrosion control coatings, particularly for aluminum alloys [1-6]. Depositing a CP on Al alloy is a challenge because of a simultaneous growth of oxide layer. We have used an approach based on electron transfer mediators (ETMs) [1, 2] to deposit CPs on a surface of Al. In a recent publication we reported the influence of ETMs on the deposition of the CP polypyrrole (PPy) on Al alloy surface and have shown that depending on the type of substitution on benzene ring (carboxyl or sulfonate) and its position, mediating properties of the compound dramatically vary [7]. In this paper we report a successful approach for depositing PPy onto artificially grown Al oxide in the presence of electron transfer mediators, such as Tiron. The rationale for depositing PPy on the oxide layer is to combine the barrier properties of Al oxide with corrosion inhibiting properties of PPy. The Al oxide is a dielectric and in order to deposit CP onto oxide of a few microns thickness, it has to be porous. In this paper Al oxide was obtained electrochemically by a method that is usually used to obtain anodic porous alumina (APA), such as described in [8]. However, due to variations in the composition of the alloy 1 , in our experiment the oxide was not comprised of regular distributed pores like in APA, but contained small pores of different size randomly distributed over the entire surface of the sample. Porous