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

摘要

The early stages of the degradation of thick epoxy-coated steels used for water ballast tanks were monitored by electrochemical impedance spectroscopy (EIS). 200-mu m-thick epoxy-coated steels were exposed to accelerated corrosion tests containing humid air at 70 degrees C for 246 days before immersion in a NaCl solution at 70 degrees C 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 190th day, Rf decreasing, C-f and X-V increasing started. Underlying corrosion also initiated between 80th and 190th 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.