Evaluation of the corrosion performance of coil-coated steel sheet as studied by scanning electrochemical microscopy

This work focuses on the investigation of the very early stages of the degradation of coil-coated galvanized steel sheet through swelling and blistering during exposure to chloride-containing aqueous solutions and the possible effect of the zinc-based metallic coating on the degradation at these very early stages. Three types of coil-coated steel whose difference was in the zinc-based metallic coating, namely galvanized, galfan and aluzinc, were considered. Scanning electrochemical microscopy (SECM) operating in the feedback mode was employed to image topographic changes when the samples were left at their spontaneous open circuit potential. Swelling of the coating and nucleation of blisters were observed for all the samples when they were exposed to naturally aerated 0.1 M KCl solution within 24 h exposure. Conversely, featureless and flat surfaces were found when the samples were either exposed to 0.1 M K2SO4 or to 10 mM KCl. For chloride concentrations of 0.1 M and above the chloride ions were observed to promote coating degradation nearly immediately upon immersion in the electrolyte.