Evaluation and investigation of surface treatment of industrial coatings by impedance spectroscopy

Three organic coatings of commercial grade (zinc rich epoxy-polyamide) were applied to steel substrates and immersed for up to 2160 h in 0.5M NaCl at 25°C. These coated plates were assessed by ac impedance in the frequency range 100 kHz to 1 mHz. The coatings were evaluated in salt spray and humidity tests according to ASTM B117 and D-2247. X-ray diffraction and X-ray fluorescence techniques were applied to analyse the coated surface. Electrolytic penetration through the coating changes the dynamic characteristics of the conduction and resistance of the coating film of the protected system over time. Equivalent circuits were used to interpret the physical and electrochemical properties of the changes so, for each coating system, equivalent circuits were appointed using analysis of impedance curves. The numerical quantities of elements were estimated by the Boukamp method. The mechanism and kinetics of the electrochemical systems were assessed by studying the changes in electrochemical parameters v. time. The qualities of the coatings can be compared by measuring the fluctuations in capacitance, dielectric constant and percentage of water uptake. The results obtained from impedance and salt spray tests were compared. Results showed that the best quality coatings can be introduced by measuring the capacitance, water uptake and dielectric constant.