Corrosion protection of low carbon steel with polyaniline: passivation or inhibition?

The mechanism by which polyaniline-containing coating formulations afford corrosion protection towards carbon steel has been investigated using potentiodynamic polarization techniques. Polyaniline-containing coatings were found to reduce corrosion rates in 0.1 mol dm−3 NaCl by approximately a factor of 2. More importantly, however, the data reveals that anodic protection of carbon steel in such media cannot explain the effect, as no evidence of an active–passive transition is observed upon anodic polarization; at sufficiently high potential the steel simply reaches a state of rapid diffusion-limited dissolution. Overall, the results of our kinetic study suggest protection stems from an inhibitory rather than a passivating effect. A mechanism is proposed which assigns the effect to inhibition by dopant ions released upon reduction of the polyaniline emeraldine salt to its non-conducting leuco counterpart.