Lateral and interfacial diffusion in coating systems

摘要

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 bottleneck for corrosion advance and 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. The diffusion rate is also impacted by adhesion between the substrate and primer, as well as the adhesion between the primer and the topcoat. 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 of a diffusion front experimentally using electrochemical impedance spectroscopy (EIS) will allow the interrogation of the adhesion of the coating to the substrate and adhesion between the primer and topcoat. Delamination of the topcoat from the primer or loss of adhesion allow fast transport of water and corrosion catalyzing ions laterally and could be identified electrochemical impedance spectroscopy by judicious experimental design and simulation directed interpretation. 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 both the transient diffusion and solution of Maxwell's equations to simulate the response of electrochemical impedance spectroscopy.