Progress in Organic Coatings 52 (2005) 263–270 EIS evaluation of the filiform corrosion of aluminium coated by a cataphoretic paint M.-G. Olivier a , M. Poelman b,∗ , M. Demuynck a , J.-P. Petitjean a a Facult´ e Polytechnique de Mons, rue de l’Epargne 56, 7000 Mons, Belgium b Materia Nova ASBL, rue de l’Epargne 56, 7000 Mons, Belgium Received 17 November 2003; received in revised form 7 May 2004; accepted 7 May 2004 Abstract Filiform corrosion is mainly considered as a cosmetic attack and is undesirable in most applications. The initiation and propagation of the filaments are related to different parameters such as the presence of defects, the permeability of the coating to water and oxygen, the adherence of the paint system and the presence of salts. The aim of this work is to study the behaviour of painted aluminium samples towards filiform corrosion or delamination. The 6082 Aluminium alloy was selected and the samples were covered with a cataphoretic epoxy primer without lead (PPG Industries France). Prior to the application of the electrocoat, the samples were pre-treated by a commercial Zr/Ti or chromate conversion treatment or simply etched with a commercial acid etching product. Filiform corrosion was studied by the normalized test (ISO/DIS 4623): painted and scratched samples were inoculated in HCl and exposed in a constant humidity chamber at 40 ◦ C and 82% RH for 3 weeks. After exposure the samples were subjected to a visual and optical microscopic examination. Electrochemical impedance spectroscopy was used to study the sensitivity to filiform corrosion. The operating mode of this test is similar to the normalized one. The samples were scratched before inoculation for 1 h in HCl and then exposed to the humidity chamber for a maximum of 4 days. The samples were tested by EIS in an acidified 0.1 M Na 2 SO 4 electrolyte solution. The resulting impedance spectra were analyzed with an appropriate equivalent electrical circuit which allows the evaluation of the exposed metallic surface area, directly related to the extent of filiform corrosion or delamination. Different parameters were varied: the exposure time in the humidity chamber; the immersion time in the electrolyte sulphate solution as well as its pH and the coating thickness. © 2004 Elsevier B.V. All rights reserved. Keywords: Filiform corrosion; Electrochemical impedance spectroscopy; Aluminium alloy; Cataphoretic primer; Conversion treatment 1. Introduction Aluminium alloys are known to be particularly sensitive to filiform corrosion. This corrosion type is mainly a cosmetic attack because in most cases it does not weaken or destroy the metallic structure and concerns only the surface appearance. Filiform corrosion is a specific delamination which occurs only under atmospheric conditions with relative humidity be- tween approximately 50 and 90%. Small threadlike delami- nation tracks are formed which in general follow irregularities on the surface (either rolling or grinding marks). This specific ∗ Corresponding author. E-mail address: Mireille.Poelman@fpms.ac.be (M. Poelman). type of delamination is driven by a differential aeration cell. In most cases, it is related to anodic undermining [1]. Galvanic corrosion is created between the head front of the filament (anode) and the tail (cathode). This corrosion is controlled by many parameters: the metal substrate, the surface prepa- ration, the presence of defects, the permeability of the coating to water and oxygen, the adherence of the paint system and the presence of salts. As shown in different publications [2–4], the substrate preparation prior to coating (chemical etching, mechanical abrasion, or application of a pre-treatment) con- siderably affects the sensitivity to filiform corrosion. Numerous papers refer to the efficiency of electrochemical impedance spectroscopy (EIS) as a tool for the evaluation of adherence properties of organic coatings [5–9]. Moreover, a 0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.05.008