Zinc-rich powder coatings characterisation in artificial sea water: EIS analysis of the galvanic action

Two zinc-rich powder coatings are applied onto phosphatised steel substrate and are studied using electrochemical impedance spectroscopy (EIS). Measurements are performed before (dry experiment) and during immersion in artificial sea water. Before immersion, EIS spectra show that percolation threshold is not attained for coating A, while coating B presents an effective percolation. In this latter case, and in order to account for the distribution of the zinc particles within the coating described as isolated, semi-isolated and percolant grains, a distributed transmission line model is proposed. When panels are immersed, the model is modified due to the electrolyte penetration through the coating, in particular with the introduction of an ionic resistance. However, it is shown that EIS spectra are not significantly modified when this ionic resistance is changed.