Studies of water transport behavior and impedance models of epoxy-coated metals in NaCl solution by EIS

Abstract

Using electrochemical impedance spectroscopy (EIS) technique, the analyses of impedance models and the water uptake processes of epoxy-coated mild steels and LY12 aluminum alloys immersed in 3.5% (weight fraction) NaCl solution were studied. The fitting results using suitable electrical equivalent circuits (EEC) suggested that there were four kinds of impedance models in the course of immersion. In the initial period of immersion, an EEC containing the coating capacitance and the coating resistance was used to fit the EIS data. Along with the immersion time, the number of time-constant of impedance spectra for metal/coating systems increased, indicating that corrosion reaction took place. After a certain time's attacking of metal substrates, the diffusion process towards solution of corrosion products forming at the metal/coating interface might be restrained due to the presence of coating barrier, resulting in the appearance of diffusion elements. This diffusion did not act as an ideal Warburg behavior, leading to a deviation of dispersive number, n, from 0.5. After longer immersion times, the Cl − ions reached the surface of metal through the coating and participated in chemical reactions with corrosion products to form salt film on aluminum alloy surface. The formation of chloride ion-incorporation salt film resulted in the change of impedance spectrum characteristics. The studies revealed that the water transport in epoxy varnish coatings followed the Fickian diffusion law during the initial time of exposure to the testing solution. After immersion for a certain time, the water uptake reached saturation and the coating capacitance tended to a relatively steady value. According to the variation of coating capacitance with time, the water diffusion coefficient was deduced. The experimental results indicated that the water diffusion coefficients in epoxy coatings were slightly dependent on coating thickness and were to some extent influenced by substrate type. The similar surface chemical structures of epoxy coatings on different metals were found from the Fourier transform infrared spectroscopy (FT-IR)–attenuated total reflection (ATR) measurements. The glass transition temperature ( T g) of epoxy coatings prepared on different metal substrates before and after immersion was measured by using differential scanning calorimetry (DSC). Lower diffusion rate of water in Al alloy based epoxy films than that in steel based ones was interpreted by the higher absolute value and smaller decrease of T g of organic coatings painted on the former substrate.