Determination of Electrochemical Corrosion Potential Along the JMTR In-Pile Loop - I: Evaluation of ECP of Stainless Steel in High-Temperature Water as a Function of Oxidant Concentrations and Exposure Time

In-pile loop experiments are one of the key technologies that can provide an understanding of corrosion behaviors of structural materials in nuclear power plants (NPPs). The experiments should be supported not only by reliable measurement tools to confirm corrosive conditions under neutron and gamma-ray irradiations but also by theoretical models for extrapolating the measured data to predict corrosion behaviors in NPPs.

The relationships among electrochemical corrosion potential (ECP), metal surface conditions, exposure time, and other environmental conditions have been determined from in situ measurements of corrosion behaviors of stainless steel specimens exposed to H₂O₂ and O₂ in high-temperature water. Based on the relationships, a model to evaluate the ECP of stainless steel was developed by coupling an electrochemical model and a double-oxide layer model.

Major conclusions obtained from the evaluation model are as follows: (a) The difference in ECP behaviors of the specimens exposed to H₂O₂ and O₂ were mainly from the thickness and developing rate of the inner oxide layers. (b) Calculated ECP behaviors, e.g., the different responses to H₂O₂ and O₂ and hysteresis and memory effects, agreed with the measured ones. (c) Neutron exposure might decrease ECP due to radiation-induced diffusion in the oxide layer.

The ECP evaluation model will be applied to evaluation of corrosive conditions in the JMTR in-pile loop.