Abstract: The effect of free volume and water affinity of coating polymers on the corrosion protection of epoxy resin coatings was evaluated using a series of epoxy resin coatings obtained by curing o-cresol novolac epoxy resin with mixed hardener comprising novolac resin (NOV) and novolac acetate resin (NOVA) in different molar ratios. It was found that, as the molar content of NOVA increased, the room- temperature density of the cured resins decreased with increase in free volume and decrease in water affinity. The results of the water sorption of the resins suggested that water affinity is more important than free volume in determining water sorption of the coatings. While the water sorption of the coating decreased linearly with the decrease of water affinity of the coating, the corrosion protection of the coatings increased much more significantly. The great improvement in corrosion protection of the coating with less water affinity can be mainly attributed to the less amount of water that reaches the coating/metal interface as demonstrated by electrochemical impedance spectroscopic measurement. However, owing to the larger diameter of the water passages in the NOVA cured epoxy resin matrix, diffusion of hydrated ion to the polymer/coating interface is easier which resulted in an increase of the concentration of corrosive ions in the interfacial region, and therefore, the corrosion tendency of the metal substrate at the wetted places under the coating is larger than that of general coating.

Key words: Epoxy resin, Free volume, Water affinity, Water sorption, Corrosion protection