Effect of composition on the performance of epoxy paints

Water-diffusion coefficient, water solubility and oxygen permeability measurements have been performed on free-standing membranes of epoxy-based organic coatings for different compositions. Concomitant direct (d.c.) and alternating (a.c.) current measurements were carried out on coated mild steel in order to investigate protection in neutral aerated 0.5 M NaCl. A general model has been used to analyse the a.c. data in terms of reactions occurring during the interaction of the coated metal with the aggressive environment. A correlation between transport properties of free films and the protective properties of paints has also been attempted. No relationship was found between the water permeability and the coating performance over exposure time, whereas the oxygen permeability seems to be responsible for coating effectiveness at later stages. The electrical resistivity of the coatings and its dependence on the composition and water content is related to early failure.