THE DETERMINATION OF COATING PERFORMANCE WITH IMPEDANCE MEASUREMENTS .2. WATER-UPTAKE OF COATINGS

Corrosion prevention by organic coatings is not only obtained by resistance inhibition. The transport of water and corrosive species is also important. A low permeability of water gives no guarantee for an optimal anti-corrosion performance, as this is likely to cause blistering when osmotic pressures occur due to surface contamination etc. A certain permeability is necessary to prevent blistering. Electrochemical Impedance Spectroscopy (EIS) can provide useful information in this field. This article is part two in a series. The water uptake of coatings is discussed and it is shown that the water taken up in the coating affects the coating polymer and results in swelling, which is reflected by a change of the dielectrical properties. This phenomenon shows the necessity of using a novel method of calculating the water uptake from the results of impedance measurements. For the study of the water uptake of the epoxy coatings, the constant phase element (CPE) was found to be a valuable component in the equivalent circuit used for EIS. In part I [VAN WESTING et al., Corros. Sci. 34, 1511 (1993).], the values of the CPE parameters Y0 and n were related to the properties of the network of the coating polymer, here Y0 and n are related to the amount of water taken up by the coating and the interaction (e.g. swelling) of the coating polymer.