Influence of ageing factors on the corrosion behaviour of polyester coated systems—A EIS study

The main objective of this work is to study the influence of ageing factors – UV, temperature, pH, humidity and SO2 – on the behaviour of polyester coated systems by electrochemical impedance spectroscopy (EIS).The behaviour of the systems after exposure to the ageing agents was evaluated as function of substrate (HDG and Galfan), chromium presence in the pre-treatments and polyester primers (with and without chromate) and topcoat (different gloss levels).EIS results were analysed based on estimated parameters such as the coating resistance and the low frequency resistance (Rfilm and Rf−>0, respectively), which were estimated using the value of the impedance modulus at the minimum phase angle in the frequency range in which each of these parameters manifests.The results show that the combination of ageing factors is not necessary cumulative. They could interfere, leading to synergic effects originating higher degradation levels or unexpected protective behaviour. Moreover, extrapolation of the effects resulting from isolated or combined ageing factors in one system to another one could give erroneous results, even if they are apparently similar.