Cation diffusion at the polymer coating/metal interface

Abstract

Theoretical and experimental studies were carried out on the transport of cations in the channel between a polymer coating and a metal substrate from a defect in the absence of an applied electrical potential. The model consists of two stages: an initial period during which ions diffuse in the coating/metal interfacial 'channel' and adsorb on the coating surface, and a propagation period during which ions also diffuse into the coating. The mathematical models were solved to predict the cation concentration and flux under the coating and the relative rate of diffusion between the initial and propagation periods. Model parameter values were derived from the results of an experiment conducted in a specially designed diffusion cell. The experiment measured the depletion of Na+ ions in a cylindrical, central reservoir, which was placed within the perimeter of a defect through the coating of an epoxy-coated steel panel. Model predictions of concentration versus time agreed well with the experimental results, which showed that most of Na+ ions were removed by lateral diffusion from the reservoir during the initial period. Further, the transport during the initial period was much faster than that during the propagation period. The results also indicated that during the propagation period, the rate-limiting step was the lateral diffusion along the coating/metal interface rather than diffusion through the coating.