In Japan, according to the national policy, oil is stockpiled to keep the life of the people around 180 days. Stockpiling method are ground base tank, marine base tank, and underground bedrock base tank. Especially, the inspection of the ground tank is carried out every 8 years by Fire Defense Law. A high performance organic coating is used as the corrosion protection for inner bottom plate of the tank. Regarding the degradation of the coating, blisters and scratches are mainly found by visual inspection. In addition, as a non-destructive inspection of the degradation level of the coating, there is an evaluation using the tanδ1–2) value by the electrochemical impedance method. However, the tanδ value is the result of only one frequency region, and complicated degradation phenomenon cannot be explained. Therefore, in this study, two Constant Phase Element (CPE)3)-5) parameters were applied to the equivalent circuit analysis. CPE is a distributed constant element and may replace complicated degradation phenomena. We designed the elements separately for overall slow degradation phenomena (soundness) and specific degradation phenomena (damage). As a result of analysis based on the analysis parameter (T, p) using the data of the inspection of actual tanks, the characteristics of each tank could be evaluated with new indices.

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