Direct measurements of ionic diffusion in protective organic coatings

The transport of species within a polymer matrix is an important feature that influences anticorrosion properties of an organic coating (i.e. paint) film. In particular, the rates at which migration of both inhibitive (e.g. phosphate) and aggressive (e.g. chloride) anions are key parameters that are difficult to measure in situ on an intact coating adherent to a substrate. Here, the diffusion rates for these key species through an epoxy coating were measured using attenuated total reflection infrared spectroscopy (ATRIR) (for phosphate) and ion selective electrodes (for chloride) and were found to be essentially identical to that for the diffusion rate of water in the same coating. This confirms that for epoxy coatings during equilibration immersed in an electrolyte solution, the ionic species diffuse into the epoxy along with the aqueous phase.