Study on Electrochemical Corrosion Potential of Stainless Steel in High Temperature Water under Irradiation of Ionizing Radiation

控制电化学腐蚀电位(ECP)对缓解核反应堆结构材料不锈钢的应力腐蚀开裂(SCC)具有重要意义。本研究旨在探讨影响ECP的各种因素。据报道，控制ECP的关键因素是反应器水辐射分解产生的氧化物种，主要是H_2O_2和O_2。利用目前建立的与沸腾堆反应堆水相关的均相体系的输入参数，对辐解反应动力学进行了模拟。结果表明，氧化剂的主要氧化物种为H_2O_2和O_2，但在某些特定条件下，OH自由基的贡献是不可忽略的。在基于混合电位法的ECP计算中，评价水与金属相间的界面反应是必不可少的。发展了一个新的近似模型，该模型不仅考虑了扩散…内部的扩散，而且考虑了辐射分解在界面处形成了更多的层，在以前的模型中忽略了后者。利用该模型计算了不同条件下的化学物种浓度分布。结果表明，如果考虑扩散层内的辐解作用，在某些条件下，阴极反应的极限电流密度比按先前模型计算的电流密度增加一个数量级以上。这些曲线被认为在很大程度上受形成的表面氧化层的性质影响，但细节尚不清楚。对经过表面氧化层处理的金属样品进行了阳极极化曲线的测量。结果表明，与磁铁矿(Fe_3O_4)和铁氧体(MFe_2O_4)组成的氧化层相比，主要由赤铁矿(Fe_2O_3)组成的氧化层在极化曲线钝化区的电流密度显著降低，导致ECP.值增加。为了研究辐射对表面氧化层的影响，以赤铁矿为表面氧化物的金属样品在光照射下观察了其极化行为。实验结果表明，光照使阳极极化的ECP降低，钝化电流密度增大。根据光照射产生的激子在赤铁矿层中作为n型半导体产生的激子行为，定性地解释了这些结果。针对贵金属添加作为控制ECP的有效方法，研究了进入氧化膜的铂的性质。较少

Control of electrochemical corrosion potential (ECP) is very important for mitigating stress corrosion cracking (SCC) of stainless steels used as structural materials in nuclear reactors. This study aims at investigating various factors affecting ECP.It has been reported that key factors controlling ECP are oxidizing species, mainly H_2O_2 and O_2, which are produced by radiolysis of reactor water. Simulation on reaction kinetics of radiolysis was carried out using currently established input parameters for homogeneous systems relating to reactor water of BWR. It was confirmed that the major oxidizing species are H_2O_2 and O_2. However, it was found that the contribution of OH radical is not negligible under some specific conditions.Evaluation of the interfacial reactions between water and metal phases is essential for the calculation of ECP based on mixed potential method. A new approximation model was developed to consider not only diffusion but also radiolysis within the diffusion … More layer formed at the interface, the latter being neglected in the previous model. Using this model the concentration profiles of chemical species were calculated under the various conditions. It was clarified that if the radiolysis within the diffusion layer is taken into account the limiting current densities for the cathodic reactions increase under some conditions by more than one order of magnitude compared to those calculated according to the previous model.Anodic polarization curves derived from experiments are used for the ECP calculation. These curves are presumed to be largely affected by the property of surface oxide layers formed, but the details are not yet clear. Measurements of anodic polarization curves were carried out with metal specimens previously treated to give surface oxide layers. It was found that oxide layers comprising mainly hematite (Fe_2O_3) reduces significantly the current density in the passive region of the polarization curve in comparison with those composed of magnetite (Fe_3O_4) and ferrites (MFe_2O_4), and results in the increase in ECP.In order to investigate the effect of radiation on surface oxide layers, the polarization behaviors are examined under the light irradiation with the metal specimens provided with hematite as surface oxide. The experimental results show that the light irradiation induces the decrease in ECP and increase in the passive current densities for the anodic polarization. These results were qualitatively explained on the basis of the behavior of excitons produced by the light irradiation in the hematite layer as n-type semi-conductor.In relation to noble metal addition proposed as an effective method to control ECP a study was carried out to clarity the properties of Pt taken into oxide films. Less