A series of experiments was performed using the electrochemical noise technique to monitor the outside diameter stress corrosion cracking (ODSCC) of Alloy 600 MA (mill annealed) steam generator tubing in a simulation of an occluded tube support plate (TSP) crevice in the steam generator secondary side. The electrochemical noise data displayed distinct potential and current noise transients associated with the initiation and propagation of stress corrosion cracks, and the transients disappeared when final rupture of the tube allowed the internal high pressure helium to vent and removed the stress on the tube wall. A variety of analytical procedures was employed on the electrochemical noise data, including quantification of transient amplitudes and integration of current transients, under analytical conditions that presupposed that the signal arose from the stressed tube (i.e. due to ODSCC) and not the unstressed counter electrodes in the simulated TSP crevice. Integration of the electrochemical noise current transients, and the power noise parameter produced from EPN and ECN quantification showed that the total power in the system due to ODSCC, was probably independent of the time to failure, whether the alloy 600 tube failed within a few days of the start of the experiment or after several weeks exposure in a less aggressive environment. This supported the practicality of using such probes and the electrochemical noise technique to monitor the actual corrosion activity due to SCC in TSP crevices caused by chemical excursions in operational steam generators. Alternative methods such as electrochemical potential monitoring only provide an indication of the likelihood of pitting or stress corrosion cracking.
Keywords: alloy 600, autoclave testing, corrosion monitoring, electrochemical noise, electrochemical impedance spectroscopy, ODSCC, plant monitoring, power noise, PWR, steam generator, stress corrosion cracking, transient analysis.