Electrochemical Impedance of Organic‐Coated Steel: Correlation of Impedance Parameters with Long‐Term Coating Deterioration

Abstract : Electrochemical impedance results are presented for 550 day exposures of organic coated carbon steel samples. Coatings consisted of translucent pigmented and unpigmented epoxy and conventional opaque epoxy polyamide systems. Coating thicknesses ranged from 20 to 185 microns. Specimens were exposed under freely corroding conditions and at two cathodic polarization levels (-805 mV and -1250 mV versus SCE) in ASTM artificial ocean water. The objective was to identify impedance parameters which measure subcritical coated-metal system property changes at early exposure times that are indicators of significant long term coating deterioration. Impedance data developed at early times, including coating resistance, coating capacitance, the increase in frequency associated with the coating's resistive-capacitive (RC) 45 degree phase angle, and low frequency impedance data, are compared to the coating system's performance evaluated after 550 days exposure. Keywords: Electrochemical impedance, Organic coating, Corrosion, Alternating current impedance, Anti-corrosion coating.