Epoxy model coatings were investigated while immersed in a 3% NaCl solution using Electrochemical Impedance Spectroscopy (EIS) during the first stage of the exposure where water uptake is the main process and during long term exposure. where the corrosion starts. The analysis of the impedance measurements showed the necessity of the use of the Constant Phase Element (cpe) in the equivalent circuit of the impedance. A short review of literature on cpe-interpretations is given. From literature it is derived that deviation from ideal dielectrical properties, that is represented by a cpe, originates from interaction (shielding) of polarisable groups.
The results of impedance measurements on the water uptake of the coatings, show that the entering water not only affects the dielectrical properties but also yields swelling of the coating polymer. The water uptake and the swelling of the coating polymer were analyxed and quantified from the results of the impedance measurements using a novel method. Impedance measurements during the long term immersion of the coating show that the start of the corrosion process under high impedant barrier type coatings can be detected by changes of the dielectrical properties of the coating, resulting in changes of the parameters of the cpe. The difference in results interpreting the impedance measurements using a capacitor of a cpe in the equivalent circuit is also discussed.