Thermal activation of impedance measurements on an epoxy coating for the corrosion protection: 1. Dielectric spectroscopy response in the dry state

In this series, an epoxy varnish for the corrosion protection of carbon steel was analyzed in the dry state by broadband dielectric spectroscopy (part 1) to describe the molecular mobility of the epoxy network. Electrochemical impedance spectroscopy measurements were then performed during immersion in a 0.5M NaCl solution (part 2), with the intent to detect the signature of the molecular mobility in the wet state. The present part 1 focuses on the analysis of the dipolar relaxation times associated with the dielectric manifestation of the glass transition in the dry varnish (a-mode). They displayed the characteristic Vogel-Fulcher-Tammann dependence on temperature, which is the typical signature of the amode. The study of the relaxation times showed an anti-plasticizing process upon heating the sample, attributed to the outgassing of absorbed ambient humidity and plasticizers from the formulation. When these elements were completely removed, through an annealing step, the epoxy network became stabilized. The low-frequency electrical conductivity of the varnish presented a very similar temperature dependence to the a-mode, meaning that the molecular mobility governs the electrical charge transport processes in this system. Absorbed ambient humidity and plasticizers also had a great influence on the electrical conductivity, resulting in three decades higher values compared to the annealed sample without these elements. The strong influence of plasticizers on both the molecular mobility (through plasticization) and the electrical conductivity should be taken into account in view of the barrier properties of the final coating. Part 1 of this series provides a basis for a molecular mobility analysis of the electrochemical impedance measurements performed in part 2