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 SCe.
In many cases of stress corrosion cracking (Scq, a very long initiation stage is followed by a rather fast propagation of the cracks. Lives of structural components subject to SCC have a large scatter even in the same conditions, since initiation of SCC is dominated by local conditions and has a stochastic characteristic. Development of an in-situ monitoring method which can detect SCC in its initiation stage can make a considerable contribution to life management of actual plant structures. Development of experimental methodologies which enable real-time monitoring of both initiation and early stage propagation of SCC is highly desired also from a mechanistic research point of view as well, since SCC phenomena in the initiation stages are not well understood yet.
Since electrochemical noise can give valuable information especially about the onset of localized corrosion 1. 2, a number of research efforts have been made to find useful correlation between electrochemical noise and SCCbehavior aiming to utilize it for detection and evaluation of cracking events. Those research efforts can be classified into the following three categories from the viewpoint of types of signal being measured; (1) fluctuation of corrosion potential 3-12, (2) fluctuation of applied current under controlled potential 13, and (3) fluctuation of short-circuit current between a working electrode and a
counter electrode under free corrosion 14. IS. Basically, fluctuation of current directly indicates real-time variation in reaction rate, while fluctuation of electrochemical potential may be recognized to reflect variation in driving force of the reaction. All the studies above suggested potential of electrochemical noise analysis as an indicator of see events. Hov.t:ver, further study and discussion are still needed both for optimization of measurement techniques and for data analysis and its interpretation.
In the present study, electrochemical noise behavior of austenitic stainless steels with various degrees of sensitization was investigated under slow strain rate tensile (SSRT) condition in a dilute sodium thiosulfate solution. Characteristics of fluctuations both in short-circUit current and in corrosion potential in see initiation processes have been investigated
