Current and Potential Fluctuation Characteristics in Intergranular Stress Corrosion Cracking Processes of Stainless Steels

Electrochemical noise behavior of Type 304 austenitic stainless steel (UNS S30400) with various degrees of sensitization was investigated under slow strain rate tensile (SSRT) conditions in a dilute sodium thiosulfate (Na₂S₂O₃) solution. Fluctuations in short-circuit current between a SSRT specimen and a counter electrode under freely corroding potential and in corrosion potential were simultaneously monitored. Current and potential fluctuation behavior was dependent on the degree of sensitization. No significant fluctuation was detected from a slightly sensitized sample where no stress corrosion cracking (SCC) was found, and only constant anodic current was observed from a highly sensitized material after the yield point without any transition behavior. Transition electrochemical noise behavior was observed for current and potential for a moderately sensitized material. Most pulses were detected after the yield point, and the frequency of pulse generation gradually increased as strain was increased and finally changed into constant current mode. A one-to-one comparison was made between current pulses and surface damage for an interrupted SSRT specimen, and a direct correlation was found between each pulse and a crack initiation event. Current pulses had a one-to-one consistency to the time differential of the corresponding potential signals, dE/dt. This indicated that double-layer capacitance exerted a dominant role on the cathodic reaction during the initiation stage of SCC.