Correlation of Impedance Parameters with Long'Term Coating Deterioration

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 sys-tems. Coating thicknesses ranged from 20 to 185 ~m. Specimens were exposed under freely corroding conditions and at two cathodic polarization levels (-850 and-1250 mV vs. SCE) in ASTM artificial ocean water. The objective was to iden-tify impedance parameters which measure subcritical coated-metal system property changes at early exposure times that predict significant long-term coating deterioration. Impedance data developed at early times, including coating resistance