ABSTRACT
Epoxy coatings on steel were formed by cathodic electrodeposition of an epoxy resin modified by an amine and isocyanate using the constant voltage method (resin concentration 10 wt%, temperature 26°C, applied voltage 250 V). The corrosion behaviour of these protective coatings was investigated during exposure to different corrosive agents (H2O, 3% NaCl, 3% Na2SO4, 3% sodium salt of 2-naphthol-3,6 disulfonic acid) using a.c. impedance measurements (EIS), gravimetric liquid sorption experiments, differential scanning calorimetry (DSC) and potential-time measurements.
The time required to saturate the coating with pure water, obtained from sorption data, coincided with the initial increase in coating capacitance and relative permittivity and the initial decrease in pore resistance, obtained from a.c. impedance measurements, denoting the entry of electrolyte into the coating. The closely similar values of diffusion coefficient for water across an epoxy coating and for the energy of activation of water diffusion, obtained in the different corrosive agents, suggest that the first step of electrolyte penetration through the coating is related to water uptake and that it is independent of the type and dimensions of ions in the electrolyte. The closely similar values of enthalpy of vaporisation of different volatile electrolytes and the quantity of water inside the coating, obtained from DSC measurements, provide additional confirmation for the proposed mechanism. The time dependence of potential is in accordance with results obtained from EIS.
