Corrosion Protection Mechanism of Polyaniline Blended Organic Coating on Steel

Epoxy–coal tar coatings are widely used to protect steel structures exposed to marine atmosphere due to their good barrier property. However, the presence of micropores and microcracks formed during the coating formation leads to failure of the coating due to permeation of corrosive ions. In recent years, it has been established that the coatings containing polyaniline (PANI) is able to protect pinholes and defects due to its passivating ability. Hence, a study has been made on the effect of polyaniline content (1 and 3%) in epoxy–coal tar coating on the corrosion protection of steel in 3% NaCl solution by electrochemical impedance spectroscopy (EIS) studies. Both phosphate- and chloride-doped polyanilines were prepared by a chemical oxidative polymerization method. From EIS studies, it has been found that the resistance value of the coatings containing 1 and 3% phosphate-doped polyaniline and 3% chloride-doped polyaniline pigmented coatings are even after exposure to NaCl solution, which are two orders high in comparison to that of conventional coal tar epoxy coatings. Besides, the conducting state of polyaniline has been found to be decreased after exposure to NaCl solution due to redox property of PANI. X-ray photoelectron spectroscopy studies have shown that polyaniline forms a complex layer with iron beneath the coating along with iron oxide.