Revisiting Corrosion Protection Mechanisms of a Steel Surface by Damaged Zinc-Rich Paints

Abstract

The evolution of corrosion products that develop within the scratched region of a 96 wt% zinc-rich paint exposed to continuous salt spray for 1,000 h has been investigated by both electrochemical and materials characterization techniques. Electrochemical impedance spectroscopy suggested the corrosion resistance of the scratch region increases with the length of exposure to the salt spray, with Raman spectroscopy indicating that this was due to the formation of zinc corrosion products over the exposed steel substrate. The experimental results revealed that the center of the scratch is under sacrificial protection, but the oxygen reduction reaction is inhibited on the steel surface at the scratch corners. Nevertheless, the steel at the corners of the scratch is under barrier protection for up to 168 h, due to the formation of a thin, but dense, layer of zinc corrosion products, as evidenced by the lack of iron corrosion products up to this time. Overall, the results suggest that mechanism is not simply a pure sacrificial protection followed by a pure barrier protection, but rather both mechanisms could occur at the same time, where sacrificial protection dominates in the scratch center and barrier protection dominates at the scratch corners until iron corrosion products appear, which is the point that the paint is unable to protect the scratch region fully.