Evaluating Coating Degradation of Ballast Tank By Electrochemical Impedance Spectroscopy

Abstract

Water ballast tank (WBT) of commercial cargo ships is exposed to severe corrosive environment. In WBT, high significant coating degradation was observed at the part exposed to high temperature [1]. Corrosion of WBT steel causes the deterioration of structure’s strength. Therefore it is important to evaluate the protective performance of WBT coating. The inspection is usually carried out by visual observation, but more powerful toll is necessary to detect the coating degradation at earlier stage. Electrochemical impedance spectroscopy (EIS) is one of useful method to evaluate coating degradation. In the application, coating resistance was usually used as an index of the coating degradation. On the other hand, thick heavy-duty coatings like WBT coating have too high coating resistance to measure in applicable frequency range. Some studies proposed the evaluation methods by coating capacitance or constant phase element (CPE) [2,3]. In the present study, blast-processed steel panels applied with commercial epoxy paint of approximately 200 micro-meter thickness were used as specimen. The specimens were kept in the humid air (RH = 100%) at 343 K for 246 days and subsequent immersion in 0.05M NaCl solution at 343K for 79 days (total 325 days). Before the test and at 80th, 190th, 246th, 325th days after the test, EIS was measured in the frequency of 1 k to 1 mHz in 0.01M LiCl (water activity, aw = 1 ). After that, the solution was changed to 10M LiCl solution (aw = 0.3) and the coating capacitance was monitored by continuous measurements of impedance at 1 kHz to evaluate the change of water uptake in the coating due to the degradation. The EIS measurements were carried out at room temperature. The coating resistance decreased and the coating capacitance increased in the test. It was also confirmed absorbed water calculated by Brasher and Kingsbury’s equation [4] increased. Pull off test and SEM observation of the coating cross section after the corrosion test indicated the coating degradation and a trace amount of corrosion of the underlying steel. EIS will be useful for evaluating coating degradation of thick-coated steel at early stage. Acknowledgement This work was supported by JSPS KAKENHI Grant Number 25820427.