Inhibition of steel corrosion by polyaniline coatings

Polyaniline (PANI) coatings were electrosynthesised on steel samples (13% and 4.44% Cr) using sulphuric and phosphoric acids as supporting electrolytes. Protective properties of PANI coatings in the supporting electrolytes were investigated by monitoring the open-circuit potential vs. time, and by applying electrochemical impedance spectroscopy. PANI layers have been found to provide corrosion protection. Thicker PANI layers at 530 mV vs. Ag/AgCl (3 mol dm(-3) KCl) exhibit pure capacitive behaviour at low frequencies, and in addition a small resistance at high frequencies. Thinner layers at 530 mV exhibit a much higher resistance attributed to a higher degree of PANI-free electrode surface and/or to a significant amount of PANI transformed from emeraldine to leucoemeraldine form. The layer deposited in a phosphate solution appears to have better protective properties than the layer deposited in a sulphate solution. Therefore, PANI from phosphate solution was tested also in 0.1 mol dm(-3) HCl. However, in the chloride-containing solution, the time of protection was significantly shorter. (C) 2002 Elsevier Science Ltd. All rights reserved.