Electrochemical techniques for monitoring stress corrosion cracking of Type 40Cr steel in acidified chloride solution

Electrochemical impedance spectroscopy （EIS） and electrochemical noise （EN） techniques were used to detect stress corrosion cracking （SCC） on 40Cr steel specimens exposed to the acidified chloride solution at ambient. To test these two techniques, slow strain rate tensile （SSRT） tests were performed with 40Cr specimen in the identical corrosive solution at room temperature. In impedance measurements, phase shifts in frequency range from 1 to 1 000 Hz show a clear difference between the stressed and non-stressed specimens, suggesting that stress corrosion cracks are detected by the impedance measurements. EN signals in the process of SCC were recorded and then analyzed by standard deviation （STD）. On the other hand, the mechanical properties, such as maximum tensile strength （MTS） and fracture strain （FS） measured by the SSRT, decrease significantly when the specimens are exposed to the corrosive solution relative to that in an inert medium. The SSRT results are consistent with fractography of the tested specimens by scanning electron microscopy （SEM）. Analysis of the fracture surface clearly shows intergranular attack, suggesting that stress corrosion cracks are formed.