Water Diffusion to and Along the Substrate Interface in Coating Systems

The transport rate of water and ions in the plane of a coating impacts the corrosion protection ability of the coating. The ease of diffusion along the substrate defines the quality of a coating in delaying the connectivity between anodic and cathodic regions in metallic substrates thus prevent electrochemical corrosion initiation and progression. Additionally, the lateral transport of water and ions is often the cause of adhesion loss. The lateral diffusion rate through a primer impacts both how quickly and how much of an active corrosion inhibitor can reach the site of corrosion initiation, and also how quickly that inhibitor is lost to ambient water, thus defines the protective lifetime. Therefore the quantification of water advance in the lateral direction is of significant value in predicting the initial quality and predicting useful lifetime of a protective coating system.

Being able to quantify the lateral advance rate of a diffusion front experimentally using electrochemical impedance spectroscopy (EIS) will allow the comparison of surface treatments, surface preparation standards including acceptable contamination levels, and coating adhesion modifiers.  Lateral diffusion also gives a metric for the initial quality of the adhesion region and any hysteresis for subsequent wet/dry cycles will define protective lifetime. A laboratory EIS setup has been developed to investigate lateral diffusion through several primer and topcoated systems. The interpretation of the electrochemical impedance spectroscopy results are being directed by a parallel simulation effort. The multiphysics simulation includes the transient diffusion and electromagnetic wave propagation to simulate electrochemical impedance spectroscopy.