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

Abstract

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 (a 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 a 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 a 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.

Correlation between the physical structure of a commercially formulated epoxy paint and its electrochemical impedance response

“…As detailed previously [14], in the case of EIS spectra, this peak is deeply buried in the contribution of water uptake to the charge transport phenomena, as can be seen in Fig. 8b.…”

“…Because the molecular mobility of polymers highly depends on temperature, the relaxation times are very sensitive to small temperature changes. As an example, with that particular polymer network an approximately 40% decrease in the relaxation times was reported when raising the temperature from 20 to 25°C [14]. This is a matter that does not attract a lot of attention in EIS studies of polymer-based coatings, but it can significantly influence the outcome of room temperature studies over long immersion times.…”

“…In the previous two sections, the effect of immersion on the coating's physical structure was established by referring to the plasticization phenomenon in polymers. From that perspective, the EIS results were analyzed, with an approach previously described [13,14]. This methodology consists in converting the raw impedance data to intensive, material intrinsic properties: namely the dielectric permittivity (Eqs.…”

“…However, the coating physical structure modifications induced by the water absorption are rarely discussed in the literature, with regard to the electrochemical impedance response. Recently, by performing EIS at different temperatures, Roggero et al [13,14] have evidenced the signature of the molecular mobility on the impedance response of an epoxy varnish. They showed that water uptake was responsible for both an important shift of the manifestation of the molecular mobility towards lower temperature and an increase of the dc conductivity by three orders of magnitude compared to the dry varnish.…”

“…The aim of the present work was to investigate the effect of the water absorption on the physical structure of a commercial high solids epoxy-based paint deposited on a carbon steel substrate using EIS. This paint corresponds to the complete formulation of the epoxy varnish previously studied [13,14]. First, the evolution of the impedance https://doi.org/10.1016/j.porgcoat.2020.105729 (1)…”