2016 Volume 56 Issue 11 Pages 2029-2036
The degradation of epoxy coating for ballast tanks was examined by electrochemical impedance spectroscopy (EIS). EIS spectra were measured in LiCl solutions using approximately 200-µm-thick epoxy-coated steel in the temperature range 30–70°C. To clarify the effect of water activity (aw), three different LiCl concentrations were used: 0.01 M (aw = 1), 5 M (aw = 0.7), and 10 M (aw = 0.3). The EIS spectra in 0.01 M LiCl solution revealed capacitive behavior over the whole 1 kHz–1 mHz frequency range at 30°C, whereas distinct resistive behavior arose in the low-frequency region above 40°C. Increasing temperature and water activity slightly increased the epoxy film capacitance (Cf) and drastically decreased the film resistance (Rf). The long-term monitoring of the coating degradation by EIS was also performed under thermal cycling in the 0.01 M LiCl solution, from which it was found that Rf decreased and Cf increased gradually with increasing cycle number. Observation by scanning electron microscopy after 250 thermal cycles revealed that several defects (voids and cracks) that could act as water absorption sites formed inside the coating and at the coating/steel interface. The epoxy coating was removed from the steel substrate and a trace amount of Fe3O4 was confirmed to have formed over the whole surface of the steel substrate. EIS is extremely useful for monitoring the degradation of thick polymer coatings under thermal cycling.