Yukio Hemmi

Electrochemical Considerations Regarding General Corrosion of Materials in a BWR Primary Circuit

Electrochemical calculations of 3d-transition metal (Fe, Ni, Co and Cr)-water systems at 558 K were performed to evaluated the general corrosion behavior of materials in a BWR primary circuit. Thermodynamics expressed by Pourbaix diagrams, the solubilities of oxides and the equilibrium activities of H2, O2 and H2O2 in oxide formations, and kinetics determining the corrosion potential and corrosion rate of the materials were evaluated. The results assist recognition of the following corrosion behaviors: 1. Good corrosion resistance of type 304 SS under various water chemistry conditions is due to the properties of NiFe2O4 and (Fe, Ni)Cr2O4; namely wide stable regions and low diffusion coefficients of metal ions and the ability of these oxides to compensate for each others weakness. 2. A typical increase of the corrosion rate of Alloy X750 and Stellite #6 in an oxygenated aqueous environment containing H2O2 is due to the properties of NiO and CoO; namely high solubilities and high diffusion coefficients of ...

Precise evaluation of corrosion environments of structural materials under complex water flow condition, (I). Estimation of corrosion potentials in reactor pressure vessel bottom of BWRs

To estimate the corrosion potentials of structural materials under complex water flow condition, such as reactor pressure vessel (RPV) bottom region of Boiling Water Reactors (BWRs), a method was newly developed. Three-dimensional water flow analysis was performed and corrosion potentials of structural materials were calculated on the basis of the flow analysis results. Water flow analysis showed the velocities in the RPV bottom region varied from about 0.1 ms-1 to 4.5 ms-1. From the corrosion potential estimation under hydrogen water chemistry (HWC) condition, at the jet pump outlet and shroud support leg, a rather large amount of hydrogen had to be added to reduce the potential because of high flow velocity condition. Conversely, a small amount of hydrogen was sufficient in the case of the stub tube of the control rod drive (CRD) guide tubing located in the center of the bottom region.

Sliding wear tests of stainless steel couples in water at high temperature, high sliding speed and high load

Abstract Sliding wear tests were conducted to study the consequences of extremely severe wear of stainless steel component in high-temperature, high-purity water. The tests were performed in a normal load range of 1.88 × 10 3 − 1.18 × 10 4 N at a sliding speed of 50 m s −1 , using stationary and rotating disc specimens on an industrial scale. Investigations were focused on the effects of load and water temperature on the wear rate, the mechanical behavior of the stainless steel surface and the water chemistry changes caused by continuous severe wear at elevated temperature. The wear rate increased exponentially with increasing load, and was two times greater at 260°C than at room temperature. The ratio of wear rate of the rotating specimen to the stationary specimen decreased exponentially with increasing load. The wear particle sizes were distributed widely up to 4 mm, and most wear debris consisted of thin flakes with scratch marks on the surface. A transformation layer 50 μm deep was observed in cross-sections of the debris, within which oxide incorporation and selective dissolution of metal components took place in high- temperature water. The bare surface of the stainless steel, created continuously by severe wear, was oxidized in high-temperature water and caused the water chemistry change such as the consumption of dissolved oxygen and the evolution of dissolved hydrogen.