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

In the present work, a commercial high solids epoxy-based paint used for the corrosion protection of steel was characterised by electrochemical impedance spectroscopy (EIS). The aim was to analyse the influence of water absorption on the physical structure of the paint, through the impedance data analysis. First, the water uptake was followed by gravimetric measurements on free-standing films as a function of the exposure time to a 0.5 M NaCl solution.

Acceptable agreement

Gravimetric measurements indicated a mass loss, linked to a release of a plasticiser from the formulation. This compound was dosed in the NaCl solution. The water volume fraction was also determined from the high-frequency part of the impedance diagrams, employing a linear rule of mixtures. The water fraction values obtained from the two methods were in acceptable agreement. The glass transition temperature (T_g) was determined by differential scanning calorimetry for the dry coating as well as for the coating after different immersion times in the NaCl solution.

The use of the dielectric permittivity formalism allowed the mean relaxation times to be obtained from the impedance data analysis. The real part of the electrical conductivity was also considered. The variation of these parameters values as a function of the exposure time to the NaCl solution, was correlated with the T_g variation. They accounted for plasticisation at the beginning of immersion (water uptake) and antiplasticisation for longer exposure times (plasticiser release).

For the studied system, it was possible to extract parameters representative of the molecular mobility from the impedance spectra analysis and to link them to the modification of the coating’s physical structure.

The study has been published in Progress in Organic Coatings, Volume 146, September 2020.