Electrochemical Impedance Spectroscopy for the detection of stress corrosion cracks in aqueous corrosion systems at ambient and high temperature

Electrochemical Impedance Spectroscopy (EIS) has been used as a tool to detect stress corrosion cracking in a stainless steel sample exposed to an aqueous environment at ambient and high temperature. A model has been developed to describe the impedance of a cracked surface. This model could distinguish between a flat electrode surface and an electrode surface with cracks. To test this technique, three case studies were considered: (1) Slow Strain Rate Tests (SSRT) on sensitised Type 304 SS specimen in a 5 N H2SO4 + 0.1 M NaCl solution at room temperature, (2) Constant load tests on Type 304 SS in a boiling („b 110 ¢XC) acidified sodium chloride solution, and (3) Slow Strain Rate Tests (SSRT) on Type 304 SS in an oxygen containing solution of 0.01 M Na2SO4 at 300¢XC. EIS measurements were always performed simultaneously on two identical samples: one stressed with a SSRT or a constant load test and one free of stress. Kramers Kronig Transformations have been used to validate the experimental data obtained with the EIS measurements. It was shown that the phase shift between the two samples could be related to the stress corrosion cracking process. Analysis of the fracture surfaces confirmed that stress corrosion cracks were formed in all three cases.