The sorption characteristics of epoxy coatings electrodeposited on steel during exposure to different corrosive agents

Abstract

Organic protective coatings on steel were formed by cathodic electrodeposition of an epoxy resin modified by an amine and isocyanate using a constant voltage method (resin concentration 10% wt, temperature 26 degrees C, applied voltage 250 V). The corrosion behaviour of these coatings was investigated during exposure to different corrosive agents (H2O, 3% NaCl, 3% Na2SO4, 3% sodium salt of 2-naphthol-3,6 disulphonic acid) using a.c. impedance measurements, gravimetric liquid sorption experiments and differential scanning calorimetry (DSC), with the aim of explaining the mechanism of coating degradation. The time required to saturate the coating with pure water, obtained from the sorption data, coincides with the initial increase in coating capacitance and initial decrease in pore resistance, obtained from a.c. impedance measurements, denoting the entry of electrolyte into the coating. The first step of electrolyte penetration through the coating is related to water uptake, when molecules of pure water diffuse in the micropores of polymer net according to Fick's law and it is independent of type and dimensions of ions in the electrolyte. Similar values of enthalpy of vaporization of different volatile electrolytes and the quantity of water inside the coating, obtained from DSC measurements were in agreement with the proposed mechanism. From the calculated diffusion coefficient for coatings where external layers were mechanically removed, it can be concluded that the micropores of the polymer net are homogeneously distributed throughout the coating.