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

1.开发测试设施是为了监测模拟轻水堆冷却剂环境中合金应力腐蚀开裂（SCC）过程中产生的电化学噪音。特别是解决了接地回路问题和压力波动引起的电化学噪声问题，成功实现了高灵敏度的电化学噪声监测.三电极法，它包括一个工作电极（试样），由相同的材料制成的对电极的试样，和参比电极，采用同时监测耦合电流和腐蚀电位的波动。实验结果表明，该方法可以高灵敏度地检测高温高压水中应力腐蚀开裂的发生.基于电流噪声分析，支持敏化不锈钢在模拟沸水堆环境中应力腐蚀开裂的滑移溶解机制.压水堆一回路水中镍基合金600的应力腐蚀开裂（SCC）表现为阴极电流尖峰叠加在阳极电流尖峰上，表明机械破膜产生的裸金属表面金属阳极溶解和阴极反应（制氢）均得到加强。这一事实支持滑移溶解机制或氢开裂。结果表明，金属阳极溶解产生的过量电子在金属-水界面的双电层电容处暂时积累。这一事实意味着，即使在氧化剂浓度很低的一次水环境中，如果是瞬态事件，也可能发生相对较大的阳极事件。提出了一种多对电极法监测电站构件应力腐蚀开裂的概念，并通过模型试验验证了该概念的有效性。

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.