Corrosion monitoring of galvanised coatings through electrochemical impedance spectroscopy Y. Hamlaoui a , F. Pedraza b, * , L. Tifouti c a Institut des Sciences et Sciences de l’Inge ´nieur, Centre Universitaire de Souk-Ahras, BP 1553, 41000 Souk-Ahras, Algeria b Laboratoire d’Etudes des Mate ´riaux en Milieux Agressifs (LEMMA), Po ˆle Sciences et Technologie, Universite ´ de La Rochelle, Avenue Michel Crepeau, 17042 La Rochelle Cedex 1, France c Laboratoire de Ge ´nie de l’Environnement, Universite ´ Badji Mokhtar, BP 1223, 23020 El Hadjar-Annaba, Algeria Received 8 February 2007; accepted 1 February 2008 Available online 23 February 2008 Abstract In this work, corrosion of industrial galvanised coatings is monitored through potentiodynamic and electrochemical impedance spec- troscopy (EIS) methods and supported by real-time immersion tests. For such purpose, the corrosion behaviour is studied in different media (NaCl, NaOH and rain water), at different concentrations and varying immersion times. The results show that EIS allows to estab- lish the interfacial reactions and the dissolution mechanisms occurring in three corrosive media, hence to foresee the protection conferred by these coatings. The impedance diagrams of the coated steel do not provide information on the slowest reactions, which only occur in natural rain water. Finally, each Zn/medium interface is characterised by a specific equivalent circuit giving a similar impedance response. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Galvanised steel; B. Corrosion monitoring; B. Electrochemical impedance spectroscopy 1. Introduction Corrosion monitoring is typically carried out using elec- trochemical methods. Among these, AC impedance has been quoted to provide an upper estimate of the corrosion rate although the Tafel parameters relating corrosion rate and polarisation resistance need to be first evaluated [1]. However, electrochemical impedance spectroscopy (EIS) offers the advantage of providing enough insight on the formation and protection mechanisms of a given surface layer [2]. Using the EIS method, various sacrificial Zn-based coatings have been evaluated because of their industrial application. For instance, Deslouis et al. [3] determined the kinetics of corrosion of zinc in aerated Na 2 SO 4 solutions and described a dissolution model. Cor- rosion was shown to occur essentially at the base of the pores of the coatings and progress of anodic dissolution gave rise to four loops with decreasing frequency [4]. Indeed, the compactness of the corrosion products layer developed on zinc needs to be introduced to evaluate its barrier effect [5]. Based on this, further insight on the disso- lution of Zn in sulphate medium has been provided by Cachet et al. [6] who described a reaction model in which three paths associated to three adsorbed intermediates ðZn þ ad ; Zn 2þ ad and ZnOH ad Þ were identified depending on the initial surface condition. However, it has to be borne in mind that actual indus- trial coatings often show defects at the surface, pores and roughness, their extent determining the quality of the coat- ings. The presence of chlorides or soluble complex species like Zn +2– Cl  –OH  in such defects has been claimed to ini- tiate localised corrosion [7]. Likewise, differences in the sur- face potential have also been quoted to induce selective corrosion [8]. Therefore, the assessment of corrosion behaviour by EIS on a hot dip and electrolytic Zn-based 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.02.010 * Corresponding author. Tel.: +33(0)546458297; fax: +33(0)546457272. E-mail address: fpedraza@univ-lr.fr (F. Pedraza). www.elsevier.com/locate/corsci Available online at www.sciencedirect.com Corrosion Science 50 (2008) 1558–1566