Corrosion protection of mild steel by coatings containing polyaniline

The anti-corrosion performance of polyaniline coated mild steel samples exposed to artificial brine and dilute hydrochloric acid environments was evaluated. Samples of mild steel (UNS G10100) coated with polyaniline deposited from solution, and overcoated with an epoxy barrier paint, when scratched to expose precise areas of bare metal, exhibited corrosion rates in aqueous 3.5% NaCl solutions 2 times less, and in 0.1 N HCl solutions, 100 times less than observed on identical samples coated with epoxy paint alone. Mechanistic information, and quantitative corrosion rates were obtained by Tafel Extrapolation, Potentiodynamic Polarization, Galvanic Coupling and Electrochemical Impedance Spectroscopic techniques. These studies, in conjunction with surface analysis by ESCA and Auger techniques, indicate that the corrosion protection, even for exposed bare steel areas, occurs by the formation of passivating iron oxide (γ - Fe2O3 and Fe3O4) surface layers. The formation of these specific oxide layers occurs when the polyaniline is galvanically coupled to the steel. This is evident by the fact that the dimensions of the exposed bare steel area that can be protected in a scratch can be large, but are limited in each corrosion environment.