Potential and solution conductivity inside stainless steel crevices in a very dilute bulk solution

相馬 康孝; 小松 篤史; 五十嵐 誉廣

Corrosion Science, 265, p.113182_1 - 113182_13, 2026/06

https://doi.org/10.1016/j.corsci.2026.113812

This study investigates the ion enrichment behavior inside stainless steel crevices in a very dilute solution (water containing 10~ppb Cl), under conditions where no localized corrosion occurred. In~situ measurements of the crevice potential () and solution conductivity () were performed and analyzed using a finite element model. In crevices with sufficiently large depth-to-gap ratios (), an initial decrease in increased the potential difference between the crevice interior and the external surface (), thereby promoting an increase in due to chloride accumulation. At later stages, increased owing to a decrease in pH and a reduction in the IR drop, causing to peak and subsequently decrease. Larger ratios resulted in lower and higher . For the largest condition investigated (~mm, m), and reached 0.218~V and 195.3~Scm, respectively, after ~s, corresponding to an estimated chloride enrichment factor exceeding . These results indicate that a driving force for chloride enrichment can be sustained for long durations even in passive crevices.

Corrosion behavior of extra-high-purity Type 316 austenitic stainless steel in a liquid lead-bismuth eutectic with oxygen saturation or low oxygen concentrations

入澤 恵理子; 加藤 千明

Corrosion Science, 256, p.113173_1 - 113173_16, 2025/11

https://doi.org/10.1016/j.corsci.2025.113173

This study investigates the corrosion behavior of extra-high-purity Type 316 austenitic (316EHP) stainless steel with reduced impurity segregation at the grain boundaries in a liquid lead-bismuth eutectic (LBE) at 530C to evaluate (1) the resistance of the steel to intergranular oxidation in the LBE with oxygen saturation and (2) its dissolution corrosion resistance at lower oxygen concentrations than the equilibrium oxygen potential of magnetite. Under oxygen saturation conditions in the LBE, 316EHP generated protective uniform oxide layers without severe intragranular oxidation. Compared with the case of the conventional 316L stainless steel, enhanced Cr diffusion along the grain boundaries in 316EHP considerably improved the intergranular-oxidation resistance of the steel. However, in the LBE with a low oxygen concentration, 316EHP exhibited high susceptibility to dissolution corrosion, thus undergoing a rapid intergranular attack particularly for short exposure durations, and island-like ferritic particles were formed for long exposure durations. Future studies should explore the optimal oxygen concentrations for oxide scale formation and the long-term corrosion behavior of the steel in dynamic LBE systems.

Unique deformation behavior of ultrafine-grained 304 stainless steel at 20 K

Mao, W.*; Gong, W.; 川崎 卓郎; Gao, S.*; 伊東 達矢; 山下 享介*; Harjo, S.; Zhao, L.*; Wang, Q.*

Scripta Materialia, 264, p.116726_1 - 116726_6, 2025/07

https://doi.org/10.1016/j.scriptamat.2025.116726

An ultrafine-grained 304 austenitic stainless steel exhibited pronounced serrated Luders deformation at 20 K, with stress and temperature oscillations reaching 200 MPa and 20 K. neutron diffraction and digital image correlation revealed discontinuous Luders band propagation and burst martensite formation. During deformation, austenite phase stress remained lower than at upper yielding, indicating elastic behavior. Notably, martensite phase stress stayed lower than austenite until fracture, likely due to stress relaxation from burst martensitic transformation at 20 K. The low martensite stress delayed brittle fracture until austenite plastically yielded during uniform deformation.

Role of solute hydrogen on mechanical property enhancement in Fe-24Cr-19Ni austenitic steel; An neutron diffraction study

伊東 達矢; 小川 祐平*; Gong, W.; Mao, W.*; 川崎 卓郎; 岡田 和歩*; 柴田 曉伸*; Harjo, S.

Acta Materialia, 287, p.120767_1 - 120767_16, 2025/04

https://doi.org/10.1016/j.actamat.2025.120767

Incorporating solute hydrogen into Fe-Cr-Ni-based austenitic stainless steels enhances both strength and ductility, providing a promising solution to hydrogen embrittlement by causing solid-solution strengthening and assisting deformation twinning. However, its impacts on the relevant lattice defects evolution (, dislocations, stacking faults, and twins) during deformation remains unclear. This study compared the tensile deformation behavior in an Fe-24Cr-19Ni (mass%) austenitic steel with 7600 atom ppm hydrogen-charged (H-charged) and without hydrogen-charged (non-charged) using neutron diffraction. Hydrogen effects on the lattice expansion, solid-solution strengthening, stacking fault probability, stacking fault energy, dislocation density, and strain/stress for twin evolution were quantitatively evaluated to link them with the macroscale mechanical properties. The H-charged sample showed improvements in yield stress, flow stress, and uniform elongation, consistent with earlier findings. However, solute hydrogen exhibited minimal influences on the evolution of dislocation and stacking fault. This fact contradicts the previous reports on hydrogen-enhanced dislocation and stacking fault evolutions, the latter of which can be responsible for the enhancement of twinning. The strain for twin evolution was smaller in the H-charged sample compared to the non-charged one. Nevertheless, when evaluated as the onset stress for twin evolution, there was minimal change between the two samples. These findings suggest that the increase in flow stress due to the solid-solution strengthening by hydrogen is a root cause of accelerated deformation twinning at a smaller strain, leading to an enhanced work-hardening rate and improved uniform elongation.

Elemental composition analysis of main structural materials of JMTR

永田 寛; 河内山 真美; 茅根 麻里奈; 菅谷 直人; 西村 嵐; 石川 譲二; 坂井 章浩; 井手 広史

JAEA-Data/Code 2024-016, 44 Pages, 2025/03

JAEA-Data-Code-2024-016.pdf:3.54MB

原子炉施設の構造材の元素組成は、廃止措置計画の策定などの際に評価を行う放射化計算において、重要なパラメータの一つとして使用されている。このうち、試験研究炉の構造材として使用されているアルミニウム合金などの元素組成については、主要成分以外の元素については十分なデータが得られていない。このことから、材料試験炉「JMTR」の主要な構造材として使用されてきたアルミニウム合金、ベリリウム、ハフニウムなどから試料を採取し、元素組成の分析を実施した。本報告書は、令和5年度に取得した78元素の元素組成データについてまとめたものである。

First freezing experiments with a molten mixture of boron carbide and stainless steel in core disruptive accidents of sodium-cooled fast reactors

江村 優軌; 松場 賢一; 菊地 晋; 山野 秀将

Proceedings of 13th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS13) (Internet), 8 Pages, 2024/11

Assuming the CDA of SFRs, the eutectic melting between BC as a control rod material and stainless steel (SS) as a structural material could occur below their melting points. After that, the mixture produced by eutectic melting between BC and SS (BC-SS mixture) would relocate inside or outside of the original core region. From the viewpoint of core reactivity changes, the relocation behavior of BC-SS mixture induced by its melting/freezing behavior, is one of the key elements to evaluate the CDA consequences. Many experimental studies on freezing behavior using core materials and its simulants, including molten UO, SS, tin, wood's metal have been reported in the past. Based on these experimental findings, the freezing/blockage model for the severe accident simulation code was established and discussed through analyses of freezing process. Specifically, it has been considered that the experimental correlation of melt-penetration length was a key indicator to quantitatively describe freezing behavior. However, there was no experimental data for the freezing behavior of actual BC-SS mixture. Therefore, the freezing experiments of BC-SS mixture were conducted to investigate the freezing and blockage behavior inside a flow path such as fuel pin bundle. In the freezing experiments, BC powder and SS block were heated up to around 1,750 K using a graphite heating furnace, then BC-SS mixture flowed down into an SS pipe for cooling below 750 K. The experimental results showed that the BC-SS mixture solidified and resulted in the blockage in the SS pipe with 4 mm or 6.7 mm in inner diameter, respectively. Furthermore, the observations for cross section of SS pipe suggested that the BC-SS mixture penetrated deeper than molten SS. This difference is considered to be influenced by decrease of the melting point.

Study on eutectic melting behavior of control rod materials in severe accidents of sodium-cooled fast reactors, 3; Material analysis of boron carbide immersed in molten stainless steel

高井 俊秀; 江村 優軌; 山野 秀将

Proceedings of 14th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation, and Safety (NUTHOS-14) (Internet), 11 Pages, 2024/08

Interest in eutectic reaction between boron carbide, which used as control rod material and stainless steel, which used as cladding tubes, etc. is growing from a perspective to improve analysis accuracy of severe accidents analysis codes. Immersion experiment of boron carbide pellet into molten stainless steel were carried out in the temperature range between 1773 and 1973 K. The eutectic melting behavior of the pellet were investigated by observing the cross section of the pellet using an optical microscope, a scanning type electron microscope. And elemental distribution in there and crystal structure were analyzed to clarify the eutectic reaction behavior. Based on the thickness reduction of the pellet cross section, the reaction rate constants between boron carbide and stainless steel were evaluated under various conditions of contact temperature and contact time.

Thinning behavior of solid boron carbide immersed in molten stainless steel for core disruptive accident of sodium-cooled fast reactor

江村 優軌; 高井 俊秀; 菊地 晋; 神山 健司; 山野 秀将; 横山 博紀*; 坂本 寛*

Journal of Nuclear Science and Technology, 61(7), p.911 - 920, 2024/07

https://doi.org/10.1080/00223131.2023.2287109

Boron carbide (BC)- stainless steel (SS) eutectic reaction behavior is one of the most important issues in the core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs). In this study, the immersion experiments using BC pellets with molten SS were conducted to evaluate the CDA sequences such as contact event of solid BC with degraded core materials including SS at very high temperature. The immersion experiment aims at understanding the kinetic behavior of solid BC-liquid SS reaction based on the reduced thickness of BC pellet after the experiment in the temperature ranges from 1763 to 1943 K, which is higher than the temperature of solid BC-solid SS reaction. Based on the kinetic consideration of the reaction rate constants for solid BC-liquid SS reaction, it was found that similar temperature dependency was identified between solid BC-liquid SS and solid BC-solid SS. Besides, the reaction rate constants of solid BC-liquid SS were smaller than those of solid BC-solid SS extrapolated in higher temperature region by two or more orders of magnitude due to two different evaluation method for BC side/SS side. It was confirmed that this difference was reasonable through the consideration of previous reaction tests in solid-solid contact for BC side/SS side.

Proposal of negative stress ratio R for fatigue crack growth rates of austenitic stainless steels in air for ASME Code Section XI based on trend in experimental data

Negyesi, M.*; 山口 義仁; 長谷川 邦夫; Lacroix, V.*; Morley, A.*

Proceedings of the ASME 2024 Pressure Vessels & Piping Conference (PVP 2024) (Internet), 8 Pages, 2024/07

https://doi.org/10.1115/PVP2024-122614

Fatigue crack growth rates da/dN for stainless steels in air environment are provided by the ASME Code Section XI. The fatigue crack growth rates are given by da/dN = C(K), where C is the fatigue crack growth rate coefficient, n is the fatigue crack growth rate exponent and K is the stress intensity factor range. The coefficient C contains a temperature parameter S, and a scaling parameter, S, which is a function of the stress ratio R. When the stress ratio R is positive from 0 to 1, the parameter S increases with increasing the ratio R, and da/dN increases with increasing stress ratio R. When R is less than 0, the parameter is given by S = 1.0. Accordingly, fatigue crack growth rates under negative stress ratio are always constant, independent of stress ratios. This means that a cyclic stress state with a minimum that is compressive is considered to cause the same degree of crack growth as one with the same range but a zero minimum. However, from the results of literature survey, experimental data reveal that the fatigue crack growth rates decrease with decreasing R ratios below zero, i.e. negative stress ratios. The objective of this paper is to assess fatigue crack growth rates under such negative stress ratios for stainless steels in air environment. An equation determined from trends in experimental data is proposed for negative R ratios for calculating the parameter S for the ASME Code Section XI, Appendix Y, based on the literature surveyed in this study.

Mechanical stability of retained austenite and texture evolution in additively manufactured stainless steel

Chae, H.*; Huang, E.-W.*; Jain, J.*; Lee, D.-H.*; Harjo, S.; 川崎 卓郎; Lee, S. Y.*

Metals and Materials International, 30(5), p.1321 - 1330, 2024/05

https://doi.org/10.1007/s12540-023-01575-8

In situ neutron diffraction during tensile deformation was performed for the stainless steels prepared by the additive manufacturing (AM) processes with two strategies: vertically built and horizontally built. The AM steels were further aged without solid solution treatment. As the results, the retained austenite was found to be more stable because the chemical composition became homogeneous by aging, and the onset of deformation induced martensitic transformation was delayed.