Impedance Characteristics of Organic-Coated Stainless Steel Sheet with an Artificial Scratch

The AC impedance method has been applied to the study of degradation processes of organic-coated SUS304 stainless steel with a macroscopic line defect. A corrosion test of the coatings was conducted by 0∼7 day exposures to 0.5 kmol/m3 NaCl solutions containing 3 vol%H2O2. The impedance was measured in 0.5 kmol/m3 NaCl solution over the frequency range of 10 mHz to 100 kHz. For the comparison with the impedance results the delaminated area of the coating from the substrate was determined by a delamination test with an adhesive tape after the corrosion test.The delamination propagates in the direction normal to the macroscopic line defect during the corrosion test and a thin water layer is formed under the delaminated coatings. The equivalent circuit can be constructed by a parallel combination of the circuits for a macroscopic defective part and for delaminated part. The later can be expressed by one dimensional distributed constant circuit. The parameters in the equivalent circuit were determined by the fitting of the theoretically calculated curve to the impedance data. The calculated double layer capacitance well correlates with the actual delaminated area.