J. Adhesion Sci. Technol. , Vol. 18, No. 4, pp. 455– 464 (2004) Ó VSP 2004. Also available online - www.vsppub.com Laboratory evaluation of corrosion resistance at metallic substrates by an organic coating: delamination effects S. GONZÁLEZ, V. FOX and R. M. SOUTO ¤ Department of Physical Chemistry, University of La Laguna, Campus de Ciencias Experimentales, E-38071 La Laguna, Canary Islands, Spain Received in nal form 8 February 2004 Abstract —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 lm 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. Keywords: Electrochemical impedance spectroscopy; organic coatings; epoxy-polyamide polymers; corrosion; galvanized steel; adhesion. 1. INTRODUCTION Organic coatings (paint lms) are the most widely used method of protection against atmospheric corrosion of metals. A paint lm reduces the corrosion of a metal substrate in two ways [1–5]: rst, it acts as a barrier between the aggressive environment and the metallic material, though an intact coating is permeable to aggressive species such as water, oxygen and ions. Second, the organic coating contains additives and pigments which may act as inhibitors of the anodic reaction responsible for metal corrosion. Finally, the nature of the metallic substrate and of the intermediate (conversion) layers may determine the protection characteristics of the painted system. Recently, the corrosion protection characteristics of an epoxy-polyamide coating immersed in 3 wt% NaCl aqueous solution have been investigated by electrochem- ¤ To whom correspondence should be addressed. Tel.: (34-922) 318-030. Fax: (34-922) 318-002. E-mail: rsouto@ull.es