Electrochemical characterisation of organic coatings for the automotive industry

Companies producing organic coatings for the automotive industry often need to test newly formulated products or materials, which may have different production parameters, to know the actual behaviour before a large scale production of the coating. Proper choice of the test for this characterization and the method of evaluation of the test results are critical. This choice is important because it is often necessary to distinguish coatings with very similar performances. Electrochemical techniques such as electrochemical impedance spectroscopy (EIS) are useful tools for this purpose. In this work, different organic coatings on mild steel, as used by the automotive industry, were studied by means of EIS measurements made in aqueous 3.5% sodium chloride solution. The coatings differ by curing parameters (change in the time or temperature of curing, kind of curing oven), and they were studied both unexposed and after weathering. The weathering was obtained (i) naturally, by 2 months of exposure in a field in Florida, and (ii) artificially, by exposure in a field in Florida, and (ii) artificial, by exposure in a UV chamber (ASTM G53). The comparison of the different materials was obtained by analysis of only two parameters: the dielectric constant of the coating ε and the normalized coating resistance ρp. Despite many difficulties (high data dispersion for ρp, small differences for ε), it was possible to obtain information useful for optimizing the curing conditions. Moreover, the characterization of the weathered samples showed very small changes in the barrier properties of the coatings, suggesting that the time of exposure (2 months) is really too short for inducing relevant weathering in materials with high performance, like coatings for the automotive industry.