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 sol utions such as tap wafer and 3% sodium chloride (NaCl). The method use d was simple and quick and did not utilize electrochemical (potentiost atic) control. Previously immersed specimens of three types of coated steel were sandwiched between metallic foil electrodes and connected t o the impedance measurement instrumentation. Typical Nyquist and Bode plots were obtained showing progressive losses in coating resistance w ith exposure time. The advantage of the method was that immersion in t he electrolyte solution was not necessary during measurements for atmo spherically exposed coatings, as required in electrochemical impedance spectroscopy (EIS). As a result, weathered and otherwise degraded coa tings in previous atmospheric exposures could be evaluated without the additional effects present during electrolyte immersion in EIS. The m ethod successfully discriminated bet ween low- and intermediate-resist ance alkyd and high-resistance epoxy coatings.