EIS Studies On Gegradation Of Epoxy Heavy-Duty Corrosion Protection Coating Systems

Electrochemical impedance spectroscopy (EIS) has been widely utilized in the last few years as a tool to investigate the corrosion protection of organic coaing systems, due to its ability to provide the behavior of the coatings and the faradaic reactions at the metal/coating interface. In this dissertation, the failure process of zinc-rich epoxy/epoxy micaceous iron oxide/chlorinated rubber coatings on Q235 steels was investigated by EIS. The main contents are listed as follows:The influence of the zinc-rich epoxy primer thickness on the protection effect of coatings was studied by EIS. In the initial period of exposure, EIS spectra showed that the protection effect of the coatings mainly came from the chlorinated rubber topcoat. But after longer time of exposure, with increasing the zinc-rich epoxy primer thickness properly, the porosity of the coatings, the diffusion rate of ions and the absorbance of water in the coatings decreased remarkably. The best protection, as evinced with EIS, was obtained for the coating systems with the thickness ratio of primer/topcoat about 2:1. For the systems of single primer or single topcoat, the protection effect of the coatings decreased quickly.The failure process of zinc-rich epoxy/epoxy micaceous iron oxide/chlorinated rubber coatings on Q235 steels with three different surface pretreatments was investigated by EIS. By simultaneous interpretation of the Bode plots, the evolution of the coating resistance, breakpoint frequency and its water uptake calculated from the imaginary part of impedance at a fixed high frequency, it can be concluded that coatings on manual polished steel provided superior protection as compared to others due to wider saturation range, longer protective life and greater adhesion, and coatings on rusted steel ranked secondly, and coatings on steel without pretreatments ranked thirdly. When the breakpoint frequency increased to 1400Hz approximately, coating resistance decreased sharply, and water uptake increased quickly, which caused steel corroded aggravatingly.The influence of different types of artificial defects on the corrosion resistance of coatings was studied by EIS. EIS results showed that, artificial scratches hardly influenced the corrosion resistance of coatings during initial stage of immersion. Once the coatings were saturated with water, |Z|0.01Hz values decreased rapidly and coating capacitance increased slowly, which showed the corrosion resistance of coatings decreased evidently. But as immersion time further increased, the corrosion resistance of coatings decreased slowly due to the protection of the zinc power. As for the coatings with pores, its |Z|0.01Hz values decreased rapidly and coating capacitance increased quickly at initial immersion time, which showed its corrosion resistance decreased sharply.By simultaneous interpretation of the Nyquist plots and the phase angle trends at middle and high frequencies, the corrosion resistance of different coating systems was studied by EIS. The results showed that phase angle could serve as a good criterion of coating resistance in many cases. When the phase angle at 10 Hz decreased sharply, the corrosion resistance of the coatings decreased evidently, and when it was below 40°, the corrosion happened. Moreover, when the phase angle at 15 kHz decreased to 70°, the coating systems had already failed.