Performance and mechanism of action of self-priming organic coating on oxide covered steel surface

Abstract

Performance and mechanism of action of a self-priming organic coating formulated by incorporating a linear vinyl terpolymer with appended hydroxyl, acetyl and formal groups, a short oil alkyd resin, alkoxy amino resin cross-linkers, extenders and pigments, applied on abraded rust free and rust covered steel surfaces are discussed. During the period of test (4400 h, exposed in 3.5% sodium chloride) it was observed that the coating maintained polarization resistance in the range of giga ohms. Under all conditions of tests, it was noted that the coating applied on steel surface covered with rust performed superior than applied on abraded rust free surface. The studies reveal that phosphoric acid added in coating during its preparation reacts with polyvinyl alcohol to form ester. The ester is diffused to rust present on steel substrate during exposure of coated surface in aqueous environment and transforms originally present mixed phases of rust into a stable phase of maghemite. This transformed layer of rust adheres strongly with O–P–O bond available in binder of coating and forms a passive film resulting in its improved performance.