Role of doping ions in the corrosion protection of iron by polypyrrole films

Duplex polypyrrole (PPy) films have been electrochemically synthesized on iron to be used as protective coatings against corrosion. The inner film was doped with tetraoxalate anions and the outer one with dodecyl sulfate anions. The protection against corrosion of this coating was tested in sodium chloride solutions: an increase in the protection time by one order of magnitude with respect to an oxalate-doped PPy film of the same total thickness was measured. The proposed mechanism was based on a change in the permselectivity of the film from anion exchange to cation exchange, thus preventing from the ingress of corrosive chloride anions.