Application of electrochemical impedance spectroscopy (EIS) for monitoring of SCC-CRACKS

It was investigated if Electrochemical Impedance Spectroscopy (EIS) could be used for the monitoring of Stress Corrosion Cracking (SCC). First a model was developed that described the impedance response of a cracked surface, i.e. the model could distinguish between a flat electrode surface and a surface with cracks. Experiments with stainless steel electrodes with artificial "cracks" (electrically drilled small holes) in 0.1 M Na₂SO₄ showed that the influence of the artificial "cracks" on the impedance response was mainly reflected in the phase shift at frequencies between 100 and 1 Hz. To generate real stress corrosion cracks, Slow Strain Rate Tests (SSRT) were performed with sensitised SS 304 specimen in a 5 N H₂SO₄ + 0.1 M NaCl solution at room temperature. Analysis of the fracture surface clearly shows intergranular attack, suggesting that stress corrosion cracks were formed. Impedance measurements were performed twice an hour during experiments with stress (SSRT) and without stress. Phase shifts at frequencies 100 and 10 Hz showed a clear difference between the results with stress and without stress suggesting that stress corrosion cracks were detected by the impedance measurements.