Polyaniline acrylic coatings for corrosion inhibition: the role played by counter-ions

The interest in providing new corrosion protection systems for different metals and metal alloys is related to restrictions to the use of chromium (VI) compounds owing to their environmental problems. This work presents the electrochemical behavior of a polymeric blend formed by camphorsulphonate or phenylphosphonate-doped polyaniline (PANI) and poly(methyl methacrylate) used for iron corrosion protection in sulphuric acid solutions with or without chloride ions. Results indicate that these blends act by a two-step protection mechanism. First, a redox reaction between Fe and PANI takes place leading to PANI reduction and concomitant anion release. Then, iron cations form a passivating complex with the PANI doping anion (camphorsulphonate or phenylphosphonate) that simultaneously acts as a second physical barrier to avoid penetration of aggressive ions. So, considering these processes, it is possible to conclude that PANI is an anion reservoir, which can release anions in a smart way when damage is produced on the surface of the coating.