Comparative EIS study of different Zn-based intermediate metallic layers in coil-coated steels

The effect the nature of the intermediate zinc-containing layer in coil-coated steels (CCS) has on their anticorrosive performance was examined by electrochemical impedance spectroscopy (EIS). The zinc-based metallic intermediate coatings considered in this work were namely galvanised (Zn), galfan (Zn–5Al) and aluzinc (Zn–55Al). Corrosion tests were performed by exposing the coated panels to a 3% NaCl aqueous solution in a horizontal flat-cell. EIS data were analysed considering the values of the distinct elements in equivalent electrical circuit models, which allowed the long term evaluation of the coating condition for the different CCS. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all three cases. These results have revealed differences in the barrier characteristics of the various CCS, namely the galvanised system exhibited the shortest time to breakdown, whereas both galfan and aluzinc maintained the system protected for significantly longer immersion periods.