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

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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