Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat EIS study of epoxy resin applied on carbon steel using double-cylinder electrolyte cell B. Bíaz, X.R. Nóvoa, C. Pérez ⁎ , A. Pintos ENCOMAT Group, University of Vigo, E.E.I. Campus Universitario, 36310 Vigo, Spain ARTICLE INFO Keywords: Double-cylinder electrolyte cell Parallel and normal conduction Porous film Interfacial properties ABSTRACT The assessment of the protection properties of an organic film applied on metallic substrate is still a challenge. Barrier features are currently studied by electrochemical impedance spectroscopy (EIS) technique, using a classical three electrode arrangement, where current flow normal to the metallic substrate is measured, nevertheless, any information about parallel ionic conductivity is not acquired. The present paper proposes the use of a double cylinder electrochemical cell, in which the combination of three and four electrode arrangements allows the measurement not only the current flow normal to the metallic substrate but also parallel to the metal- coating interface. The EIS experiments are performed in a thick epoxy resin film applied on carbon steel, during 60 days of immersion in a 0.1 M Na 2 SO 4 + NaOH solution. The impedance values measured using the three electrode configuration are much higher than those obtained by the four electrode arrangement. This result can be explained considering that a current fraction flows parallel to the metal-coating interface. The impedance evolution is explained considering the presence of diverse pore families, which evolve in different ways depending on the ionic motion direction, normal or parallel. Changings are more remarkable in the parallel direction, reflecting the anisotropic character of the film. 1. Introduction Organic coatings represent the most common way to protect me- tallic structures against corrosion. It is generally accepted that the protective action is based on two main properties: barrier (for oxygen, water and aggressive species), and blocking of the ionic paths between anodic and cathodic areas along the metal/polymer interface [1]. The barrier properties deal with the transport of ions and aggressives through the film, in the direction normal to the metallic substrate. The Electrochemical Impedance Spectroscopy (EIS) technique has been used widespread for this study, mainly using the three electrode arrange- ment, where the reference electrode is place in the test solution. In such setup, the measurements are related to the sorption characteristics of the whole film in this direction [2–6]. This approach does not dis- criminate between the characteristics of the film at bulk film and at the metal-coating interface region, where chemical and physical interac- tions are expected. Kittel et al. employed an additional electrode em- bedded in the coating, which allows the discrimination between the impedance of the inner part in contact with the metal surface and that of the outer part in contact with the electrolyte [7,8]. However, only information about ionic conductivity in normal direction is acquired. On the other hand, parallel conductivity has strong influence on the initiation and propagation of corrosion. It is well-known that, for a given coating, the protection effectiveness against corrosion depends on the surface treatment that could lead to distinct adhesive forces be- tween film and metal [9,10]. Luo et al. stated that in the cathodic disbonding process the ionic transport along the coating/metal inter- face is more important than through the film [11]. Thus, the interest of this aspect is undoubted, nevertheless, the studies found in the litera- ture are scarce, maybe due to the more complex experimental config- uration needed. Some researchers use the Fourier Transform Infrared- Multiple Internal Reflection (FTIR-MIR) technique for in situ quanti- fying the water layer at the coating/substrate interface [12]. The main restriction is that it is limited to substrates that produce total internal reflection. Iron is a poorly-reflective metal, which constrains the use of this technology in the corrosion study. An interesting approach is the measurement of impedance between electrodes located at coating/ substrate interface [13–15] or embedded into the polymer [16–18]. However, placing the electrodes is not a simple task, mainly at the in- terface level, even though the main constrain is that current flow that crosses the coating in a parallel way is minimum in most of the situa- tions, thus much of the information in this direction is lost [17]. https://doi.org/10.1016/j.porgcoat.2018.02.002 Received 28 August 2017; Received in revised form 13 December 2017; Accepted 6 February 2018 ⁎ Corresponding author. E-mail address: cperez@uvigo.es (C. Pérez). Progress in Organic Coatings xxx (xxxx) xxx–xxx 0300-9440/ © 2018 Elsevier B.V. All rights reserved. Please cite this article as: Bíaz, B., Progress in Organic Coatings (2018), https://doi.org/10.1016/j.porgcoat.2018.02.002