Progress in Organic Coatings 53 (2005) 71–76 Accelerated tests for the evaluation of the corrosion performance of coil-coated steel sheet: EIS under cathodic polarisation R.M. Souto ∗ , M.L. Llorente, L. Fern´ andez-M´ erida Department of Physical Chemistry, University of La Laguna, E-38071 La Laguna, Tenerife, Spain Received 21 September 2004; accepted 28 January 2005 Abstract Cathodic polarisation has been used as an accelerated test method to assess the degradation of coil-coated architectural cladding. Data were recorded via electrochemical impedance spectroscopy (EIS) measurements. Coil-coated steel (CCS) sheet with three different intermediate zinc-based metallic coatings were investigated, namely galvanised, galfan and aluzinc. Corrosion tests were performed by polarizing the painted panels to two cathodic potentials corresponding to either aluminium or magnesium sacrificial anodes while immersed in 3% NaCl aqueous solution in a horizontal flat-cell. The cathodic potential values considered were -1.082 and -1.522 V versus saturated calomel electrode (SCE). Cathodic polarisation conditions were found for which differences in the corrosion resistance of the three systems were detected from the analysis of the EIS data at significantly shorter exposure times than in the case of EIS data measured at their corresponding open circuit potential, thus providing an accelerated test method for determining the quality of the CCS system. © 2005 Elsevier B.V. All rights reserved. Keywords: Coil coating; EIS; Corrosion; Cathodic polarisation; Zinc-based metallic coating; Failure modes 1. Introduction Coil-coated cladding is widely used for roof and wall el- ements in the building industry, casings for household ap- pliances and internal parts or bodywork structures for the automotive industry. Its interest originates mainly from its enhanced corrosion resistance compared to metallic and or- ganic coatings applied separately [1–3], which adds to lower cost and reduced environmental impact. It typically consists of galvanised steel, with additional protection provided by an organic coating and top coat. These materials are used as-received; thus, transportation and installation procedures are performed with the organic coating already applied to the metal surface, which may eventually result in mechanical damage and/or exposed cut edges. Major research is, there- fore, devoted to the design of coil-coated materials, which may more effectively outstand defects and cut edges. Recent investigations at our laboratories have demon- strated that the composition of the intermediate Zn-based ∗ Corresponding author. Tel.: +34 922 318030; fax: +34 922 318002. E-mail address: rsouto@ull.es (R.M. Souto). metallic layer greatly affects the corrosion resistance char- acteristics of the materials. In this way, the extent of un- dercoating delamination originating from defects operated in the coating [4] or from cut edges [5,6] greatly depended on whether pure Zn or Zn alloyed constituted the interme- diate metallic layer. Indeed, differences were also observed in non-defective samples investigated with electrochemical impedance spectroscopy (EIS) during natural exposure to 3% NaCl solution for exposures up to 1 year [7]. In all the cases, the superior characteristics of either galfan (Zn–5Al) or aluzinc (Zn–55Al)-containing coil-coated steels as com- pared to pure galvanised was established [4,7]. Significantly smaller were observed to be the differences between gal- fan and aluzinc-containing samples, and long exposure tests were needed for their discrimination [7]. The different cor- rosion protection characteristics of those materials were also observed, when samples with a holiday were connected to sacrificial anodes [4], though the behaviour of non-defective specimens was not attempted at that stage. The work reported herein represents the second stage in efforts to characterize the corrosion resistance of non- defective CCS panels with intermediate metallic layers of 0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.01.009