Effect of AC interference on failure mechanism of zinc-rich epoxy coatings in alkaline environment

Abstract

In this paper, the failure mechanism of Zn-rich epoxy coating under the AC interference in alkaline environment was revealed based on EIS tests. Using X80 steel as matrix, the coated X80 sample was formed by hand-brushing coating with the thickness of 25 ± 5 μm determined by five-point test. The EIS curves of the coated X80 sample in 3 wt% NaOH solution with immersion time was studied under (no) AC interference, and the failure evolution mechanism of Zn-rich epoxy coating was established through the fitting parameters. The results showed that during the curing process of the Zn-rich epoxy coating, Zn reacted with O2 in the air to generate ZnO, which coated the surface of Zn particle to form the ZnO-Zn structure. In alkaline environment, ZnO dissolved to form Zn(OH)2, namely the activation process of Zn particles, which was inhibited by AC interference. Furthermore, the activated Zn particles reacted to form Zn(OH)2 in alkaline environment, that is, the electrochemical reaction process of activated Zn particles, which was significantly promoted by AC interference. In conclusion, the non-conductive Zn(OH)2 generated in alkaline environment under AC interference, together with un-activated ZnO, can not only isolate the electrical connection between activated Zn particles, but also cut off the cathodic protection between Zn and Fe. Meanwhile, the matrix Fe was passivated in alkaline environment, which weakened the wet binding force between Fe and epoxy coating, resulting in coating stripping.