Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.