EIS Studies of Protective Polymer Coatings on Low-Carbon Steel in Sulfuric Acid

—The electrochemical properties of protective epoxy polymer coatings with different designs are studied with electrochemical impedance spectroscopy upon exposure in water and aqueous solutions of sulfuric acid. It is found that both the active resistance of the epoxy coating R ct and resistance of the transition layer R dl decrease upon long-term exposure both in water and diluted acid. There is correlation between the decrease in R ct with the depth of penetration of the acid front for multilayer coatings. For acids with moderate concentrations, the active resistance of the epoxy coating R ct also decreases in the case of increases in the time of exposure, while the resistance of the transition layer R dl goes over to a growth after the initial drop, well before the penetration of the acid to the substrate. A correlation of the resistance of the transition layer R dl with the fraction of underfilm corrosion is found for the case of corrosion in water. A model for the estimation of the fraction of underfilm corrosion by the change in the resistance of the transition layer is proposed and verified.