Water uptake in protective organic coatings and its reflection in measured coating impedance

Water uptake in commercially available epoxy coating on mild steel using impedance spectroscopy and gravimetry was studied. The water content in the coating was determined using the Brasher-Kingsbury equation and various methods of coating capacitance calculation used in the literature. The obtained results were compared with reference gravimetric data. An overestimation of values obtained from impedance data in relation to gravimetric data was observed. It was found that the calculation of the effective coating capacitance needs to use both constant phase element (CPE) parameters. Using principal component analysis (PCA) a strong correlation between them and gravimetric data was found. The use of formula of effective capacitance in the Brasher-Kingsbury equation has allowed to obtain significant compliance with gravimetric data.