Laboratory evaluation of corrosion resistance at metallic substrates by an organic coating:: delamination effects

The corrosion behaviour of an epoxy-polyamide primer applied on galvanized steel specimens during immersion in 3% NaCl aqueous Solution was examined by electrochemical impedance spectroscopy (EIS). The investigation of both intact and defective coatings allowed for the comparison of their electrochemical behaviours in order to assess the anticorrosive characteristics of the system. The impedance response of the intact coating was found to correspond to a porous film presenting localised electrochemically active areas. in which the precipitation of zinc-containing corrosion products contributes to the sealing of the coating. Conversely, scribed defects cannot be spontaneously sealed and no effective protection of the metal can be achieved.