Comparing EIS Methods for Estimating the Degree of Delamination of Organic Coatings on Steel

Electrochemical Impedance Spectroscopy (EIS) was used to evaluate the degree of delamination of the epoxy primer Uniprime ED-4* electrocoated on phosphate treated cold-rolled steel in 3.5% NaCl exposed to air. Specimens with coating thicknesses of 20.3 and 20.8 mm and surface areas of 78.5 cm2 were used. Coating parameters were obtained using a circuit-analog model. The area of delamination was estimated using several EIS parameters, including the pore resistance and the break point frequency. These values were compared to the corroded area determined visually. All methods considered exhibited a strong correlation over some or all of the range of exposure times.