Characterization of protective coatings by electrochemical impedance spectroscopy and a thermostimulated current method: influence of the polymer binder

Abstract Electrochemical impedance spectroscopy (EIS) was coupled with a thermostimulated current (TSC) method to investigate the role of the polymer binder on the flexibility and on the barrier properties of films applied on galvanized steel. The systems studied were polyurethane based with or without epoxy. The TSC method allows the analysis of dielectric relaxation in insulating materials on a nanometric scale. With organic coatings on metal, specific effects on the nature of the binder can be investigated. Electrochemical impedance measurements were carried out for various exposure times of the systems in an aerated 0.5 M NaCl solution. TSC measurements showed that the presence of epoxy favoured the texturation of the primer and stabilized the structure of the coating. This result was correlated to the values of the electrolytic resistance through the pores in the coating (barrier properties) which were higher for the primer containing epoxy. TSC was also used to analyze the evolution of the microstructure of the systems after immersion in the sodium chloride solution. This method revealed the influence of chromate lixiviation which leads to a modification of the microstructure of the coating. This result was correlated to the barrier properties of the organic coating, characterized by EIS measurements. Impedance results have clearly evidenced the beneficial effect of chromates for the corrosion resistance of galvanized steel. They impede the corrosion of the substrate and the delamination process.