Organic coating capacitance measurement by EIS: ideal and actual trends

The coating capacitance is one of the most frequently studied parameters obtained by EIS measurements for characterising the protective properties of organic coatings, because by analysing this parameter it is possible to measure the water uptake phenomena (diffusion till saturation and further increase) which are very important in barrier coatings. The ideal trend described in many cases is not representative of the actual evolution measured on coatings. Often one phase is missing or a reduction of the coating capacitance is measured. In this work the ideal trend of coating capacitance is compared with some real experimental results, discussing the reasons of disagreement and the limits of the model which seem to be in many cases insufficient and too simple to explain the actual water uptake processes, in particular, when a further increase of the coating capacitance occurs after saturation. The presence of different water uptake mechanisms, the heterogeneous distribution of the water in the coating and dimensional variations make the water uptake phenomenon quite complex.

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