Cathodic Delamination and Anticorrosion Performance of an Epoxy Coating Containing Nano/Micro-Sized ZnO Particles on Cr(III)-Co(II)/Cr(III)-Ni(II) Posttreated Steel Samples

Abstract

In this study, a coating system including conversion coating and organic paint was used on the steel samples. The steel samples were treated by Cr(III), Cr(VI), Cr(III)-Co(II), and Cr(III)-Ni(II) conversion coatings. Epoxy coatings were reinforced with nano-zinc oxide (ZnO, 3.5 wt%) and micro-ZnO (critical pigment volume concentration [CPVC]/pigment volume concentration [PVC] = 0.2) particles. The epoxy coatings were applied on the steel samples treated by conversion coatings. The surface properties of the samples were studied by a contact angle measuring device. The values of dry and wet pull-off strengths of the epoxy coatings were measured using a pull-off adhesion tester. The cathodic delamination and anticorrosion performance of the epoxy coatings were investigated on the pretreated steel substrates by the cathodic disbonding test and electrochemical impedance spectroscopy (EIS), respectively. Results showed that all conversion coatings increased dry and wet pull-off strengths. The increase in pull-off strength was more pronounced on the samples treated by Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings. It was shown that conversion coatings can reduce the cathodic delamination rate of the epoxy coatings. The decrease in the rate of cathodic delamination was more pronounced on the substrates treated by Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings. Results showed that nano-ZnO particles could improve epoxy coating resistance against corrosion and cathodic delamination higher than micro-ZnO particles.