Anodic and cathodic polarization curve measurements for BWR structural materials in high temperature - high purity water

In order to evaluate the health of a BWR plant, it is necessary to improve the accuracy of electrochemical corrosion potential (ECP) calculations. A polarization curve is necessary as an input for ECP calculations. The conventional ECP calculation method has been used to make estimates for type 304 stainless steel (SS) as a typical core structural material. However, since electrochemical measurements in high temperature-high purity water are difficult, knowledge about the polarization curves of other materials is insufficient. So, in order to confirm the effects exerted on polarization curves by various materials, the polarization curves of type 316L SS and Alloy 182 weld metal were measured. The shape of the anodic polarization curve of type 316L SS was found to be similar to that of the type 304 SS. However, the current densities at low potential were about 1/2 those of the type 304 SS. The anodic polarization curve of Alloy 182 indicated completely different behavior compared with stainless steel at lower potential than for a transpassive state. No active state was observed and it seemed that the passive state spread. The oxygen concentration dependences of ECPs were calculated from the intersections of the measured anodic polarization curves and the cathodic polarization curves. The oxygen concentration dependence of ECPs indicated similar behaviors for the two materials. (author)