Corrosion inhibition of polyaniline and poly(o-methoxyaniline) on stainless steels

Polyaniline films were grown by electrochemical deposition on 316 and 304 stainless steels and their corrosion performance monitored by following the open circuit potentials in acidic solutions. Poly(o-methoxyaniline) was successfully polymerised on stainless steel electrodes, as shown by cyclic voltammetry and impedance spectroscopy, and provided corrosion inhibition in a similar manner to polyaniline. In 0.5 M H2SO4 the potential climbed to over 0.4 V (SHE), to values typical of the partially oxidised form of the polymer and of the metal substrate in a passive state with low rates of corrosion. In 0.5 M HCl, the steels were maintained in a passive state for some hours to days (lasting longer with a thicker polymer film), prior to a drop in potential to −0.15 V after the onset of pitting corrosion. The fluctuations of potential seen in 0.5 M HCl are explained by regions of the oxidised polyaniline, produced by dissolved O2, reaching the metal and causing an increase in the potential, and by pits formed at higher potentials rapidly reducing an already oxidised film leading to a drop in the potential.