Evaluation of Corrosive Degradation in Coated Steel Using Alternating Current Impedance Spectroscopy

Alternating current (AC) impedance spectroscopy was applied to measure the degradation of steel coatings previously exposed to corrosive solutions such as tap water and 3% sodium chloride (NaCl). The method used was simple and quick and did not utilize electrochemical (potentiostatic) control. Previously immersed specimens of three types of coated steel were sandwiched between metallic foil electrodes and connected to the impedance measurement instrumentation. Typical Nyquist and Bode plots were obtained showing progressive losses in coating resistance with exposure time. The advantage of the method was that immersion in the electrolyte solution was not necessary during measurements for atmospherically exposed coatings, as required in electrochemical impedance spectroscopy (EIS). As a result, weathered and otherwise degraded coatings in previous atmospheric exposures could be evaluated without the additional effects present during electrolyte immersion in EIS. The method successfully discriminated between low- and intermediate-resistance alkyd and high-resistance epoxy coatings.