The Sacrificial Protection of Steel by Zinc-Containing Sol-Gel Coatings

Abstract

The scanning vibrating electrode technique (SVET), electrochemical impedance spectroscopy, and salt spray testing are used to investigate the ability of Zn rich sol-gel coatings to provide sacrificial protection to carbon steel. Three types of coatings (containing either Zn powder, a colored pigment, or both) are applied to steel. Intact coatings are shown to act as barrier layers through which electrolyte ingresses over time. Under conditions where the substrate is exposed by an artificial coating defect, SVET is used to investigate the extent to which different coatings offer sacrificial protection when the defect size is systematically changed. The total anodic current, as derived using SVET, doubles when the defect covers 25% of the total area compared to when 12% of the area is exposed. This finding is consistent with efficient sacrificial protection of the steel by the zinc based coating. This sacrificial protection is observed for up to 24 hours for cases where the defect constitutes up to 52% of total area. The protection offered in the presence of a colored pigment is delayed and it is proposed that the pigment restricts the ability of Zn to couple with the underlying steel.