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Takahashi, Rieko*; Taniguchi, Naoki
Zairyo To Kankyo, 73(6), p.153 - 163, 2024/06
Carbon steel is one of the candidate materials for overpacks in geological disposal of high-level radioactive waste, and is known to susceptible to stress corrosion cracking(SCC) depending on the condition in carbonate environment. In order to understand the influence of temperature on the SCC susceptibility of carbon steel, slow strain rate test (SSRT) of rolled steel were performed in NaHCO
aqueous solution with varying temperature in the range of 303-393K for conditions of 0.1-0.5 mol/dm
, which is assumed to be the upper limit of carbonate concentration in groundwater in a geological disposal environment. As the results, no obvious influence of temperature on mechanical properties such as fracture strain ratio and reduction area ratio were observed, but SCC susceptibility based on SCC fracture ratio increased at relatively low temperatures of 303K and 323K. It was suggested that the reason for the higher SCC sensitivity at lower temperatures was due to slower repassivation at lower temperatures. Regarding the type of SCC, intergranular SCC was dominant at low temperatures and tended to transition to intergranular SCC at higher temperatures. Transgranular SCC tended to be observed at lower potentials than those at which intergranular SCC was observed.
Ogawa, Yusuke*; Suzuki, Satoru*; Taniguchi, Naoki; Kawasaki, Manabu*; Suzuki, Hiroyuki*; Takahashi, Rieko*
Materials and Corrosion, 72(1-2), p.52 - 66, 2021/01
Times Cited Count：2 Percentile：9.66(Materials Science, Multidisciplinary)Cast steel is one of the promising alternative to forged steel that is the current reference material for carbon steel overpack. In this study, the full-scale cast steel overpack was produced experimentally and the distribution of casting defects were investigated. The corrosion test regarding corrosion rate and stress corrosion cracking (SCC) susceptibility were also conducted using samples taken from the full-scale cast steel overpack and the corrosion resistance of cast steel was compared with that of forged steel. From above two corrosion tests, it can be said that the corrosion resistance of cast steel is mostly the same as that of forged steel.
Kobayashi, Masato*; Yokoyama, Yutaka*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
Corrosion Engineering, Science and Technology, 46(2), p.212 - 216, 2011/04
Times Cited Count：4 Percentile：27.90(Materials Science, Multidisciplinary)The corrosion behaviour of a carbon steel weld joint under anaerobic conditions was investigated to estimate the long-term integrity of the carbon steel overpack. The weld specimens in this study were produced using three welding methods: GTAW, GMAW and EBW. General corrosion was observed for each immersion specimen and the weld joint corrosion rate was the same as or less than that of the base metal. The hydrogen concentration absorbed during immersion testing was less than 2.48
10
mol kg[Fe]
(0.05 ppm) after three years, a value regarded as having little influence on hydrogen embrittlement. The susceptibility to hydrogen embrittlement was highest in the base metal, suggesting that there was little adverse effect on the weld joint from welding. The welded carbon steel overpack is assumed to maintain its resistance to corrosion as a disposal container for the expected lifetime under anaerobic underground conditions.
Taniguchi, Naoki; Suzuki, Hiroyuki; Kawasaki, Manabu; Naito, Morimasa; Kobayashi, Masato*; Takahashi, Rieko*; Asano, Hidekazu*
Corrosion Engineering, Science and Technology, 46(2), p.117 - 123, 2011/04
Times Cited Count：10 Percentile：48.33(Materials Science, Multidisciplinary)Carbon steel has been selected as one of the candidate materials for overpack for geological disposal of high-level radioactive waste in Japan. Corrosion of carbon steel is divided into two types; general corrosion and localized corrosion. In this study, propagation behaviors of general and localized corrosions (pitting corrosion and crevice corrosion) were investigated by immersion tests of carbon steel under aerobic condition. The results of the immersion tests showed that the growth rate of corrosion was strongly dependent on the environmental condition and steel type, but the upper limit of pitting factor (the ratio of the maximum corrosion depth and the average corrosion depth) was approximately determined by only average corrosion depth. Based on these experimental data and literature data, an empirical model that predicts the maximum corrosion depth of an overpack from average corrosion depth was developed by applying the extreme value statistical analysis using the Gumbel distribution function.
Yokoyama, Yutaka*; Mitsui, Hiroyuki*; Takahashi, Rieko; Taniguchi, Naoki; Asano, Hidekazu*; Naito, Morimasa; Yui, Mikazu
JAEA-Research 2008-072, 232 Pages, 2008/10
JAEA-Research-2008-072.pdf:45.2MB
It is possible that the corrosion resistance at the overpack welds is different from that at base metal due to the differences of material properties. In this study, corrosion behavior of welded joint for carbon steel was compared with base metal using the specimens taken from welded joint model fabricated by TIG (Tungsten Arc Welding, GTAW), MAG (Gas Metal Arc Welding, GMAW) and EBW (Electron Beam Welding) respectively. The results of these corrosion tests indicated that the corrosion resistance to general corrosion, pitting corrosion and crevice corrosion at welded metal of TIG and MAG was inferior to base metal. No deterioration of corrosion resistance was observed in any corrosion modes for EBW, which does not need filler material. Neither the base metals nor the welds is not susceptible to SCC under the carbonate concentration near that of the disposal environment.
Mitsui, Hiroyuki*; Takahashi, Rieko*; Taniguchi, Naoki; Otsuki, Akiyoshi*; Asano, Hidekazu*; Yui, Mikazu
JAEA-Research 2006-080, 322 Pages, 2006/12
JAEA-Research-2006-080.pdf:90.52MB
There is some possibility that the corrosion resistance of overpack welds is different from that of base metal due to the differences of material properties. In this study, corrosion behavior of welded joint for carbon steel was compared with base metal using the specimens taken from welded joint model fabricated by TIG, MAG and EBW respectively. The corrosion tests were performed for following four items. (1) Passivation behavior and corrosion type, (2) Propagation of general corrosion, pitting corrosion and crevice corrosion under aerobic condition, (3) Stress corrosion cracking susceptibility, (4) Propagation of general corrosion and hydrogen embrittlement under anaerobic condition. The results of these corrosion tests indicated that the corrosion resistance of welded metal by TIG and MAG was inferior to base metal for general corrosion, pitting corrosion and crevice corrosion. It was implied that the filler materials used for welding affected the corrosion resistance. No deterioration of corrosion resistance was observed in any corrosion modes for EBW, which does not need filler material. The susceptibility to stress corrosion cracking of welded metal and heat affected zone was lower than that of base metal.
Iimoto, Takeshi*; Kakefu, Tomohisa*; Takagi, Rieko*; Takahashi, Itaru*; Nakamura, Takashi*; Kito, Keiko*; Watanabe, Yoko; Yamashita, Kiyonobu
no journal, ,
no abstracts in English
Mitsui, Hiroyuki*; Takahashi, Rieko*; Otsuki, Akiyoshi*; Asano, Hidekazu*; Taniguchi, Naoki; Yui, Mikazu
no journal, ,
The slow strain rate tests(SSRT) of carbon steel were performed in sodium carbonate-sodium-bicarbonate aqueous solution to acquire knowlwdges regarding susceptibility to stress corrosion cracking of welded joint of carbon steel overpack for high-level radioiactive waste disposal. The specimens were taken from thick cylindrical sample welded by TIG and EBW, and then SSRT tests were applied to the each of specimens of base metal, heat affected zone and welded metal. Based on the results, the susceptibility to stress corrosion cracking of welded joint of carbon steel overpack was evaluated.
Yokoyama, Yutaka*; Mitsui, Hiroyuki*; Takahashi, Rieko*; Otsuki, Akiyoshi*; Asano, Hidekazu*; Taniguchi, Naoki; Yui, Mikazu
no journal, ,
In Japan, carbon steel is one of the candidate materials for the disposal container (overpack) for high-level radioactive waste (HLW). Overpack seals vitrified waste and is required to isolate it from contact with groundwater for 1,000 years in Japan's waste management program. After overpack is placed in a deep underground repository, it is presumed that it will be exposed to environmental conditions which include lithospheric pressure from bedrock, hydrostatic pressure from groundwater and swelling pressure of the buffer material as mechanical forces and contact with groundwater as chemical conditions that may induce corrosion. Therefore, the wall thickness of overpack is designed with consideration for mechanical strength and corrosion property so that it can maintain its integrity for 1000 years. On the other hand, the overpack is enclosed by welding. With regard to the assessment of the applicability of welding techniques to the overpack, it is important to confirm the long-term integrity of the weld joint from the aspects of mechanical strength and corrosion property, too. In this study, the corrosion behavior of weld joints, which were provided by TIG (GTAW), MAG (GMAW) and EBW (Electron Beam Welding), was investigated by immersion tests under the aerobic conditions considering environment at the early stage of the repository.
Kobayashi, Masato*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
no journal, ,
Overpack is one of the components of EBS (Engineering barrier system) and is required to contain the radioactive waste at least for 1000 years. It has been evaluated that carbon steel overpack can be achieved lifetime of over 1000 years in expected deep underground environment by securing 40mm corrosion allowance. However, since the welded zone have different characteristics from the base metal in chemical composition and microstructure, its corrosion behavior would possible to be different from that in base metal. Concerning such background, corrosion tests to evaluate the corrosion resistance of welded zone (including heat affected zone and welded metal) were carried out and the effects of welding on the corrosion properties were investigated.
Yokoyama, Yutaka*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
no journal, ,
In order to clarify the corrosion behavior of overpack weld at the initial stage of repository, immersion tests were carried out in oxidizing environment and following results were obtained;(1) Selective corrosion was prone to occur at welded metal of TIG and MAG welding. It is possible that the welded metal has affected by the composition in filler material used in welding and by the change of microstructure at welded zone. (2) According to the results of extreme value statistical analysis, estimated corrosion depths by the selective corrosion at welded metal did not exceed the maximum corrosion depth evaluated in the past lifetime assessment of overpack.
Kobayashi, Masato*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Fujita, Tomoo
no journal, ,
Susceptibility of hydrogen embrittlement of carbon steel weld joint was evaluated by slow strain rate testing (SSRT) in consideration of a diffusive hydrogen concentration and a presence of a fine flaw which is hard to detect by ultrasonic testing due to the dimension. In order to avoid an effect of metallurgical micro structure, normalized specimens were prepared to the SSRT. The results of SSRT showed the rupture elongation decreases with an increase in hydrogen concentration. And observation of the fracture surface and measurement of the hydrogen concentration in the specimens showed a generation of a crack initiated from fine flaw at up to a certain value of hydrogen concentration. Although decreasing of fracture strain was observed on the specimens fracture morphology, it was evaluated as no significant effect in comparison with the effect of the concentration of the diffusive hydrogen in the specimens. These results suggest that a presence of a fine flaw has a small impact on the hydrogen embrittlement of weld joint.
Kobayashi, Masato*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
no journal, ,
The susceptibility to hydrogen embrittlement of welded metal of carbon steel was evaluated by slow strain rate test (SSRT) at constant current density. In SEM observation, some circle quasi-cleavage fractures with center at microdefects (blowhole, BH) were observed. Such fractures were initiated only in the case of charging hydrogen by cathodic polarization. The area of the quasi-cleavage increased with the radius of BH. As the area of quasi-cleavage fractures increased, the fracture strain became small.
Kobayashi, Masato*; Yokoyama, Yutaka*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
no journal, ,
no abstracts in English
Yokoyama, Yutaka*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki; Naito, Morimasa
no journal, ,
no abstracts in English
