Evaluation of corrosion behaviour of organic coatings with electrochemical noise and electrochemical impedance spectroscopy

Electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) measurements were used to evaluate protective properties and corrosion behaviour of two different organic coating systems. The coatings of commercial grade (zinc rich ethyl silicate) were applied to steel substrates and immersed for up to 3 months in 0·5M NaCl at 25°C. An analysis of the EN data was performed during this time, including the measurements of the mean potential E_mean, the mean current I_mean and the noise resistance R_n. The time series noise patterns were transformed into frequency domain by fast Fourier transformation. The power spectral densities (PSDs) of the potential and of the current and roll off slope coatings were then compared with each other. These coated plates were assessed by EIS. Electrolytic penetration through the coating, changes in the dynamic characteristics of the conduction and resistance of the coating film of the protected system as a function of time. Equivalent circuits were used to interpret the physical and electrochemical properties. The mechanism and kinetics of the electrochemical systems were assessed by measuring the fluctuations in capacitance and percentage of water uptake. The coatings were evaluated in salt spray and humidity tests according to ASTM B117 and D2247.