EIS study of epoxy resin applied on carbon steel using double-cylinder electrolyte cell

•A double-cylinder electrolyte cell allows the distinctionbetween normal and parallel flux in a film applied on steel.•The three electrode arrangement assesses the impedance normal to the metal substrate.•The four electrode configuration evaluates the impedance parallel to the metal-coat interface.•The equivalent circuit employed to study the behaviour takes account the porosity character of the film. 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 Na2SO4 + 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.