Monitoring the Early Stage of Degradation of Epoxy-Coated Steel for Ballast Tank by Electrochemical Impedance Spectroscopy

Abstract

The early stages of the degradation of thick epoxy-coated steels used for water ballast tanks were monitored by electrochemical impedance spectroscopy (EIS). 200-μm-thick epoxy-coated steels were exposed to accelerated corrosion tests containing humid air at 70℃ for 246 days before immersion in a NaCl solution at 70℃ for 79 days. They were removed from the corrosion test chamber during the test so that EIS measurements could be performed in order to determine the coating resistance (R_f) and the coating capacitance (C_f). In addition, the water volume fraction (X_V) and the diffusion coefficient (D_H2O) in the coating were determined from the C_f vs. t^1/2 curves during the water desorption process. These parameters were compared with the degradation morphology of the coated steel after a pull-off test. Only “physical aging” of coating was observed for first 80 days. After 190^th day, R_f decreasing, C_f and X_V increasing started. Underlying corrosion also initiated between 80^th and 190^th day. On the other hand D_H2O did not indicate the clear change associated with the degradation. The changes in R_f, C_f, and X_V were closely correlated with the progress of the degradation of the coated steels.