Degradation of heavy-duty coating applied to inner bottom plates of oil storage tanks and its evaluation method based on electrical impedance analysis

摘要

Inner bottom plates of crude oil storage tanks are generally exposed to mixtures of corrosive drain water and sludge.In order to isolate the steel plates from aggressive environments,high-performance anticorrosion coatings such as a vinyl ester resin organic coating containing glass flakes have been recently utilized.However,the barrier property gradually deteriorates in a long in-service period.In this study,the degradation processes were investigated using acceleration test which imposed the temperature gradient along the thickness direction on coating samples.The glass flake exhibited a good blistering resistive performance and a thick coating system of about 1 mm thickness did not show any sign of blistering in our experimental condition.However,black rust was detected at the interface between coating and steel substrate even in the case of thick coating system.For tracing degradation process in more detail,electrical impedance measurement was adopted and the field data were acquired from actual oil storage tanks.It was found that the impedance behaviors of degraded coating was well explained by using electric equivalent model including CPEs(Constant Phase Elements].The data of extracted CPEs were then analyzed by using extreme value statistics as a tool for diagnosis of heavy-duty coatings.