Thermal activation of impedance measurements on an epoxy coating for the corrosion protection: 2. electrochemical impedance spectroscopy study

In this series, an epoxy varnish for the corrosion protection of carbon steel was analysed in the dry state by broadband dielectric spectroscopy (part 1) to describe the molecular mobility of the epoxy network. Electrochemical impedance spectroscopy (EIS) measurements were then performed in a NaCl solution (part 2), with the intent to detect the signature of the molecular mobility in the wet state. The dielectric manifestation of the glass transition (α-mode), previously characterized in part 1 of this series for the dry varnish, was evidenced from EIS measurements through the use of the dielectric permittivity formalism. This α-mode showed the characteristic Vogel-Fulcher-Tamman dependence, shifted towards higher frequencies when compared to that of the dry varnish, consistently with the plasticization of the epoxy network due to the water uptake. Moreover, it was shown that taking the α-mode into account when fitting the EIS impedance data with equivalent circuits led to a much better fit in the capacitive-resistive transition region. It was shown that the dc charge transport processes are not only triggered but also governed by the molecular mobility of the epoxy network. In the case of thick coatings (several hundreds of micrometers), this dependence demonstrated the absence of through-pores, as it is often discussed in the literature. Indeed, under the assumption of current flowing through free electrolyte via open pores, the temperature dependence of the electrolyte's dc conductivity (Arrhenius law) should be found instead of the temperature dependence of molecular mobility.