The sorption characteristics and thermal stability of epoxy coatings electrodeposited on steel and steel electrochemically modified by Fe-P alloys

The corrosion behaviour and thermal stability of epoxy coatings electrodeposited on steel and steel electrochemically modified by Fe-P alloys of different phase structures have been investigated during exposure to 3% NaCl. Electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments and thermogravimetric analysis (TGA) were used. Epoxy coatings were formed by cathodic electrodeposition of epoxy resin on steel and steel modified by Fe-P alloys at various current densities, using a constant voltage method. From the results obtained from EIS (pore resistance, coating capacitance), gravimetric liquid sorption experiments (diffusion coefficient) and TGA (water content inside the coating and thermal resistance), it can be concluded that the electrochemical, transport and thermal properties of epoxy coatings are strongly affected by surface modification of the substrate. The reduced electrochemical and transport properties of epoxy coatings on Fe-P sublayers can be: explained by the catalytic activity of Fe-P alloys for the Hz evolution reaction, while the improved thermal stability of these coatings is probably connected with an increasing number of hydrogen bonds inside the polymer net. The mechanism of electrolyte penetration through an organic coating is also given. (C) 1998 Elsevier Science S.A