Corrosion Protection Mechanism of Polyaniline Blended Organic Coating on Steel

Abstract

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 ͑PANI͒ 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 ͑EIS͒ 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 ϳ10 9 ⍀ cm 2 even after 90 days 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. Epoxy-coal tar coatings are widely used for corrosion protection of steel structures exposed to marine atmosphere due to their good barrier property. However, the lifetime of these coatings is decreased due to the formation of micropores and microcracks when epoxy resin and coal tar are cross-linked with hardeners. In recent years, it has been established that the presence of polyaniline ͑PANI͒ in organic coating on steel is able to passivate the pinholes and coating defects by passivation of iron exposed.1,2 Besides, several studies have shown that PANI-containing coatings are able to protect steel more effectively due to its passivation ability. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Recently, it is shown that the addition of a small amount of titanium powder in an epoxy-coal tar coating system is able to improve the corrosionresistance property of the coating. 23 Furthermore, it has been reported that the dopants in PANI play an important role in corrosion protection due to the formation of an iron-dopant secondary layer. 8,10 Hence, a study has been made on the effect of the addition of polyaniline with phosphate and chloride dopants in the epoxycoal tar system on the corrosion-protection performance of steel in sodium chloride solution by the electrochemical impedance spectroscopy ͑EIS͒ method in order to elucidate the role of inhibitive phosphate and corrosive chloride dopants. ExperimentalPreparation of polyaniline-phosphate (PANI-PO 4 ) and polyaniline-chloride (PANI-Cl) pigments.-One molar of distilled aniline was dissolved in 500 mL of 1 M solution of phosphoric acid or 1 M solution of hydrochloric acid. ...

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