Development of monitoring method for SCC initiotion in LWR components by means of electrochemical noise analysis

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1. Testing facilities were developed to monitor electrochemical noise generating during stress corrosion cracking(SCC)processes of alloys in simulated LWR coolant environments. In particular, ground loop problems and fluctuation in pressure, which can cause electrochemical noise, were solved and successful monitoring of electrochemical noise with high sensitivity was achieved.2. The three electrodes method, which consists of a working electrode(specimen), a counter electrode made of the identical material to the specimen, and a reference electrode, was adopted to simultaneously monitor fluctuations of coupling current and of corrosion potential. It was successfully demonstrated that SCC initiation in pressurized high-temperature water can be detected with high sensitivity by this method.3. Based on current noise analysis, the slip dissolution mechanism was supported for SCC of sensitized stainless steels in a simulated boiling water reactor(BWR)environment.4. As for SCC of Ni-base Alloy 600 in primary water of pressurized water reactor(PWR), cathodic current spikes were superimposed on anodic current spikes, indicating that both anodic dissolution of metal and cathodic reaction(hydrogen generation)were enhanced on bare metal surface produced by mechanical film rupture. This fact supports either the slip dissolution mechanism or hydrogen cracking.5. It was found that excess electrons produced by transient anodic dissolution of metal were temporarily accumulated at double layer capacitance of metal-water interface. This fact implies that relatively large anodic event can take place, if that is a transient event, even in the primary water environment, where concentration of oxidizer is very low.6. A concept of multiple counter electrodes method was devised for monitoring of SCC initiation in plant components and validity of the concept was demonstrated by a model experiment.