  Fickian diffusion is often used to model moisture transport through barrier
coatings, where the goal is to protect an underlying substrate from the onset
of corrosion caused by buildup of water or other aggressive species. Such
coatings are often exposed to cyclic moisture, either in laboratory testing or
in service due to natural environmental fluctuations. In this paper, we use
lattice Boltzmann numerical techniques to investigate the effects of reservoir
cycling on moisture propagation and concentration at the substrate where
corrosion onset occurs. We examine both the simple case of constant
diffusivity, representing idealized Fickian diffusion, and diffusivity that
depends on concentration via either a step or linear function, representing
polymer network swelling. The use of a coating subject to swelling is shown to
lead to highly variable equilibrium behavior. We show that the nature of the
functional diffusivity has large effects on water concentration at the
substrate, and has implications for material design and analysis to avoid
corrosion.
