Water diffusion mapping in organic coatings on metal substrates: A quasi-optical mm-wave technique Vs. Odd Random Phase Electrochemical Impedance Spectroscopy
Host Publication: EUROCORR
Authors: Z. Manoli, A. Pourkazemi, G. Pandey, J. Stiens and H. Terryn
Publication Date: Jan. 2018
Organic coatings deposited on metals act like protective barriers between the metal and the environment. They provide mechanical and chemical resistance. It is a fact that the corrosion on coated metals depends mainly on the water diffusion. Along with the water, other corrosive species penetrate the organic coating causing volumetric and chemical changes, producing mechanical forces hence gradually, the polymer decreases its mechanical performance and loss of adhesion at the interface between the coating and the metal is unavoidable. In this paper, we report on the experimental results of two complementary characterization techniques for the quantification of the water uptake process in organic coatings: the contact based low frequency Odd Random Phase Electrical Impedance Spectroscopy (ORP-EIS) technique versus the contact-free quasi-optical high-frequency dielectric permittivity sensing technique - typically in one or more bands of the millimeter wave range (30 to 300 GHz) - applied to organic coatings with thicknesses varying between a few tens of micrometer and several millimeter on metal substrates.Single-sine Electrochemical Impedance Spectroscopy (EIS) is a technique widely used to detect the degradation of the coating and the corrosion of the metal substrate. The complete information is extracted from the electrical response of the system subdue to a small perturbation that is induced over short periods of time. In this work, we study the water uptake evolution in thin organic coatings on aluminium substrate by utilizing Odd Random Phase Electrochemical Impedance spectroscopy (ORP-EIS) method [1,2]. Comparing to the standard single-sine EIS, the ORP-EIS is a multi-sine technique, allowing to evaluate more rapidly the coating performance. With this technique, we not only record the impedance, but also we record the experimental noise, the eventual non-linear distortions and the non-stationary behavior of the system, from which we extract valuable information about the moment that the corrosion starts. We also employed a novel contactless quasi-optical method based on the millimetre (mm) wave illumination of the coated metals. These waves together with the even higher frequency bands of the sub-THz range yield sufficient penetration in opaque non-conductive coatings . In order to substantially increase the sensitivity of this technique for water penetration in deep-sub wavelength thin layers, an advanced permittivity matching technique  has been demonstrated in the 61.5 GHz to 65 GHz band. With the mm-wave technique, we focus on the diffusion mapping of water in thick coatings by scanning the object in much shorter time periods (a few tens of seconds). Preliminary results show that mm-wave sensing is a powerful technique that can contribute in understanding the water uptake phenomena on coated metal due to the specific dielectric properties of water. This work is an evaluation of those different experimental techniques for studying the water diffusion process based on the water content change in an organic coating on a metal substrate. We underline the main differences in terms of experimental set-up and coating thickness selection as well as the advantages or each method compared to the conventional single-sine EIS.