Characterization of water uptake by organic coatings used for the corrosion protection of steel as determined from capacitance measurements

Abstract

Water absorption by a coating on a metal is a major factor affecting its corrosion protection efficiency. It is usually determined from the increase of coating capacity with time, which is taken as a measure of the water volume ratio in the system. In this communication, the early stages of water uptake by three paint systems applied on galvanized steel have been investigated. The models proposed by Brasher and Kingsbury (BK), and the discrete (DM) and the continuous (CM) models proposed by Bellucci and Nicodemo, were employed to evaluate water absorption. It has been found that the amount of water adsorbed in the coating depended on both the nature and the thickness of the film, though different results were delivered by these methods, with convergence occurring only in the saturation stage. Next, water diffusion coefficients were determined, and they were observed to vary with the thickness of the polymer film regardless their actual composition. The time evolution of the diffusion coefficients could be adequately described using the DM model.