The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of PMo12O403- (PMo12) and HPO42- anions for stabilization of the passive oxide film at the metal–polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo12. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy–PMo12/PPy–DHNDS < PPy–PMo12/PPy–NDS < PPy–PMo12/AnqDS < PPy–PMo12/PPy–DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.