THE SORPTION CHARACTERISTICS OF EPOXY COATINGS ELECTRODEPOSITED ON STEEL DURING EXPOSURE TO DIFFERENT CORROSIVE AGENTS

Organic protective coatings on steel were formed by cathodic electrode position of an epoxy resin modified by an amine and isocyanate using a constant voltage method (resin concentration 10% wt, temperature 26 d egrees C, applied voltage 250 V). The corrosion behaviour of these coa tings 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 e xperiments and differential scanning calorimetry (DSC), with the aim o f explaining the mechanism of coating degradation. The time required t o saturate the coating with pure water, obtained from the sorption dat a, coincides with the initial increase in coating capacitance and init ial decrease in pore resistance, obtained from a.c. impedance measurem ents, denoting the entry of electrolyte into the coating. The first st ep of electrolyte penetration through the coating is related to water uptake, when molecules of pure water diffuse in the micropores of poly mer net according to Fick's law and it is independent of type and dime nsions of ions in the electrolyte. Similar values of enthalpy of vapor ization of different volatile electrolytes and the quantity of water i nside the coating, obtained from DSC measurements were in agreement wi th the proposed mechanism. From the calculated diffusion coefficient f or coatings where external layers were mechanically removed, it can be concluded that the micropores of the polymer net are homogeneously di stributed throughout the coating. Copyright (C) 1996 Elsevier Science Ltd