Studies of impedance models and water transport behaviours of epoxy coating at hydrostatic pressure of seawater

• In simulated deep-sea environment, EIS characteristic of epoxy coating was studied.
• Water transport followed the Fickian diffusion behaviour during the initial stage.
• High pressure shortened the period of coating water uptake that reached saturation.
• High pressure of seawater only accelerated the coating physical failure.

In simulated deep-sea environment, the evolution of impedance models and water transport behaviours of epoxy coating on 907A steel surface were studied by electrochemical impedance spectroscopy (EIS). By the fitting analysis of EIS, three equivalent electrical circuits (EEC) were proposed to the epoxy coating/907A steel system at 25 atm hydrostatic pressure for different immersion stages: good barrier property of the coating (Circuit A), active corrosion period at the beginning (Circuit B), and finally emergency of diffusion impedance (Circuit C). The evolution of coating capacitance indicated that water transport followed the Fickian diffusion behaviour during the initial stage at 25 atm hydrostatic pressure and at atmospheric pressure. High pressure accelerated diffusion rates of seawater into epoxy coatings, this shortened the period of water uptake of the coating that reached saturation. The similar chemical structures of epoxy coatings at two kinds of hydrostatic pressure were found from the Fourier transform infrared spectroscopy (FT-IR) – attenuated total reflection (ATR) measurements. High pressure of seawater accelerated the coating physical failure, and chemical structures of the coating did not change during the experiment.