Current and potential fluctuation characteristics in IGSCC processes of stainless steels

Electrochemical noise behavior of austenitic stainless steels with various degrees of sensitization was investigated under slow-strain-tensile (SSRT) conditions in a dilute sodium thiosulfate solution. Fluctuations both 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 degree of sensitization. No significant fluctuation was detected from a slightly sensitized sample where no SCC was found and only permanent anodic current was observed from a highly sensitized material after yielding point without any transition behavior. Transition behavior with current pulses and corresponding potential fluctuations was detected from an intermediately sensitized sample. Most pulses were detected after yielding point and the frequency of pulse generation gradually increased as strain was increased and finally changed into permanent current mode. A one to one comparison was made between current pulses and surface damages for an interrupted SSRT specimen and a direct correlation was found between a pulse and a crack initiation event. Current pulses had one to one consistency to a differential of the corresponding potential signals, dE/dt. This indicated that double-layer capacitance took the dominant role for a cathode in the initiation stage of SCC.