Progress in Organic Coatings 59 (2007) 197–205 EIS characterisation of new zinc-rich powder coatings A. Meroufel, S. Touzain ∗ Laboratoire d’Etudes des Mat´ eriaux en Milieux Agressifs EA3167, Bˆ atiment Marie Curie, Pˆ ole Sciences et Technologies, Universit´ e de La Rochelle, Avenue Michel Cr´ epeau, 17042 La Rochelle Cedex 01, France Abstract A zinc-rich powder coating, applied onto steel substrate, was studied using electrochemical impedance spectroscopy (EIS). Before immersion when coating is dry, EIS spectra revealed that the percolation threshold was reached. Then, an equivalent circuit including a transmission line was applied to model electrochemical response. In this model, which considers isolated and semi-isolated zinc particles within the binder, it was found that both phases were equally distributed in the coating. When specimens were immersed in a 3% NaCl solution, free corrosion potential E corr and EIS measurements were regularly performed. The electrochemical response was different from that observed with liquid or other zinc-rich powder formulations. It was proposed that the low porosity of the coating was the main factor contributing to a non-homogenous electrolyte penetration. Therefore, the previous model was modified to take into account all these contributions. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder coating; Zinc-rich paint; Electrochemical impedance spectroscopy; Transmission line model; Porosity 1. Introduction Zinc-rich paints (ZRP) have been successfully used since the 1940s for protecting steel from corrosion. The zinc dust (spher- ical or lamellar shape, or a combination of both) is dispersed in an inorganic (usually orthosilicates) or organic binder (usu- ally epoxies) [1]. These particles must be in electrical contact between themselves and the metallic substrate in order to ensure a well-established electrical conduction within the coating. In such conditions of percolation, a galvanic coupling is created between zinc and the substrate (steel) which is more noble as the zinc. Then, zinc can preferentially dissolve, acting as a sacrifi- cial pigment, and allowing a cathodic protection of the substrate. Many studies [2–14] exist in literature and relate the protection mechanisms and degradation processes of such coatings. According to Morcillo et al. [4], a network of capillaries or ionic conduction paths between the anodic (zinc particles) and cathodic zones (steel) is a second basic condition for the cathodic protection mechanism in the ZRPs. However, the metallic zinc content in the dry film is a very important parameter in the techni- cal specifications of zinc-rich paints. In the case of solvent-based (liquid) ZRPs, Abreu et al. [11] report that a high zinc concen- ∗ Corresponding author. Tel.: +33 5 46 45 87 67; fax: +33 5 46 45 72 72. E-mail address: stouzain@univ-lr.fr (S. Touzain). tration (typically >60 vol.%, i.e. 90 wt.%) is necessary to ensure a good electrical contact. The coating is highly porous: water can easily reach zinc particles and simultaneously the metallic substrate, which is then placed under cathodic protection (active protection). Finally, zinc corrosion products form and fill up the pores, and the coating behaves like a barrier-type paint (pas- sive protection). However, these solvent-based coatings present some problems of volatile organic compounds (VOC) emission and powder zinc-rich paints have been developed since they respect the environmental standards (no VOC). In powder for- mulations, the zinc concentration is limited to 70 wt.% which is insufficient to ensure a good cathodic protection [15]. In order to increase the percolation within the coating, conductive pig- ments (e.g. carbon black) are added. In recent papers [15–18], powder ZRP systems containing 0, 2 and 5 wt.% of carbon black were studied, using mainly electrochemical techniques such as electrochemical impedance spectroscopy (EIS). It was found that the percolation threshold may be not be reached, essentially because of the good wetting properties of the binder. In the case where percolation was effective, an equivalent circuit including a de Levie’s type transmission line [19] was used to model the electrochemical response. In particular, this model allowed to take into account the distribution of the zinc particles within the coating. In the present work, the electrochemical behaviour of a zinc- rich powder coating with a low carbon black content (2 wt.%) 0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.09.005