A Novel Application of EIS for Quantitative Coating Quality Assessment During Neutral Salt Spray Testing of High-Durability Coatings.

The aim of the study was to exemplify the benefits of using electrochemical impedance spectroscopy (EIS) for quantification of coating quality during accelerated corrosion testing before the appearance of visual changes on the surface of the coating. The study included an innovative application of the commercial flexible gel electrodes in a two-electrode setup for following the impedance of the two high-durability offshore coating systems exposed to 1440 h of neutral salt spray (NSS) according to EN ISO 9227. The results were compared to those of the EIS measurements during 8000 h long exposure to a liquid electrolyte in a press-on cell according to the established EN ISO 16773 method. Complementary methods of differential scanning calorimetry, Fourier transform infrared spectroscopy and thermogravimetric analysis have been used for surface, interface and depth profiling of coating characteristics that were commented with respect to the EIS results. The technique was applied on high-durability coatings Zn (R) + EP + EP or PUR intended for the protection in the coastal and offshore areas corresponding to C5- (H) and CX corrosivity categories in accordance with EN ISO 12944. EIS has been found to give sound quantitative results for coating impedance and a good estimation of long term coating behaviour within the first 100 h of accelerated exposure. It can be considered a promising tool for the early detection of coating damage during weathering tests.