For the development of a method to evaluate the residual lifetime of a heavy-duty coating on the surfaces of the internal bottom plates of oil storage tanks， the impedance characteristics of the coating were investigated by the laboratory immersion test of the coated steel panel in a 3 wt. % NaCl solution for 85 days and the field study of an actual oil storage tank that had stored oil over 266 months. As a result， the impedance spectra of the coating could be analyzed with the electrical equivalent circuit of one CPE (Constant-phase element). It was found that the CPE behavior in the impedance spectra is caused by the electrolyte solution permeation in the coating. In addition， it was suggested that the degradation with age of the coating can be evaluated quantitatively by using the fitted parameter values of the CPE.
