Influence of Curing Agent Content on the Performance of Epoxy Coatings on Mild Steels

The water transport behavior of epoxy coatings on mild steels in sodium chloride (NaCl) solution was studied using the electrochemical impedance spectroscopy (EIS) technique. It was found that, with the increase of the curing agent (polyamide) content, the water diffusion coefficient (D) decreased and then increased, with the minimum at 2.2 molar ratio of epoxy resin to polyamide. The differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) measurements were used to interpret the variations of D. FTIR tests showed that the chemical structure of epoxy coatings changed with the variation of the curing agent content. It was shown that the lower the polyamide content, the more the free glycidyl groups remained in epoxy paints, leading to the incomplete curing reactions and thereby the larger values of D. When the amount of curing agent was too high, the excess hydrophilic–NH₂ group was included in the coatings, resulting in them being more sensitive to water permeation in coatings, and thereby also resulting in the larger values of D. DSC results indicated that the glass transition temperature (T_g) of coatings increased and then decreased with the increase in the curing agent content, with the maximum also at 2.2 molar ratio of epoxy resin to polyamide. The analysis of impedance data of epoxy-coated metal systems was also carried out during the whole immersion period. After intensive immersion, three different kinds of diffusional impedance behavior appeared in EIS spectra at different immersion stages, i.e., Warburg diffusion, the open boundary finite length diffusion, and the blocked finite length diffusion, respectively.