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Soma, Yasutaka; Igarashi, Takahiro
Dai-70-Kai Zairyo To Kankyo Toronkai Koenshu (CD-ROM), p.199 - 202, 2023/10
Since an acidic corrosive environment (crevice environment) is formed inside the stress corrosion cracking (SCC) of stainless steel in high temperature water, it is important to understand the corrosion behavior in the crevice environment for the better understanding of crack growth behavior. In the previous study, the authors measured the electrical conductivity inside the crevice and obtained values of 380 
S/cm and 1600 S/cm for the crevice with and without intergranular corrosion, respectively. In this study, we defined the crevice environment I (pH
=4.41) and II (pH=3.13) corresponding the above conductivity values, and the corrosion behavior of Fe-xCr-20Ni (x=16.9, 19.8, 22.9, 24.3, 25.9) in each crevice environment was investigated. In the simulated crevice environment-I, the all alloys showed passive behavior, while in the environment-II, severe corrosion with intergranular cracking was observed for x = 16.9 and 19.8, and a thick oxide film was formed. On the other hand, above x=22.9, oxide film growth was suppressed and a clear passive region appeared on the polarization curve.
Soma, Yasutaka; Komatsu, Atsushi; Ueno, Fumiyoshi
Corrosion, 78(6), p.503 - 515, 2022/06
Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)The effects of electrochemical potential (ECP) on water chemistry within a crevice are of critical importance for understanding stress corrosion cracking (SCC) of Fe-Cr-Ni alloys in high temperature water. In this study, the effects of ECP on the electrical conductivity of a solution within a Type-316L stainless steel crevice (
) have been studied in 288
C and 8 MPa water containing 10 ppb Cl
as major anionic species. In situ measurements of in a rectangular crevice with a gap of 15 m and a depth of 23 mm have been conducted using small sensors installed at different crevice depths. An increase in ECP from -0.49 V (vs. standard hydrogen electrode) to -0.12 V resulted in an increase in from 12 Scm
to 160 Scm at a distance of 21 mm from the crevice mouth. The increase in reached a maximum at about 0.15 V (about 300 Scm) and then tended to decrease with increasing potential. Finite element model analysis taking into account the electrochemical reaction quantitatively reproduced this behavior. It is considered that Cl is the major anionic species transported into the crevice at relatively low potentials, and that increases monotonically with increasing ECP. On the other hand, when ECP exceeds around 0 V, a sufficient amount of HCrO
generated by transpassive dissolution also transported into the gap. Since this chemical species is highly oxidizing, unlike Cl, it is assumed that it reacts with metal cations to oxidize and precipitate them, thereby lowering conductivity.
Soma, Yasutaka; Kato, Chiaki
Zairyo To Kankyo 2022 Koenshu (CD-ROM), p.219 - 220, 2022/05
It is important to understand the electrochemical properties of stainless steel in environment created within crevice of stainless steel in high temperature water (crevice environment). This is because acidification and concentration of impurity ions occur in the crevice environment and this is common inside the stress corrosion crack. In this study, we reproduced the crevice environment in bulk scale and investigated mainly the effect of Cr concentration on the electrochemical properties of Fe-Cr-Ni alloys. Polarization curves of Fe-20Ni-xCr (x=16.4, 23, 26) were measured in water with a temperature of 288C, a Cl concentration of 2
10
mol/dm
, a pH value of about 4.5, and a dissolved hydrogen concentration of 10 ppb. The peak currents of active dissolution (at -400 mV) and passive current density (at -50 mV) for specimens with Cr concentrations x = 16.4, 23, and 26% were approximately 13.8, 15.9, 10.0 Acm
, and 18.4, 8.5, 8.5 Acm, respectively. Although the current values of x=26 were slightly lower in both cases, it was concluded that there was no clear dependence of the polarization curve on Cr concentration in this environment.
Soma, Yasutaka; Kato, Chiaki
Dai-68-Kai Zairyo To Kankyo Toronkai Koenshu (CD-ROM), p.205 - 206, 2021/10
This study investigates the effect of temperature on dissipation behavior of Cl ion within the crevice of stainless steel. Concentration of Cl ion was evaluated by conductivity measured by using sensors installed at crevice specimen. At 50 and 80 C, Cl ions within the crevice of PEEK and Pt dissipated in accordance with concentration diffusion. On the contrary, dissipation speed of Cl ions inside the Type-304L stainless steel were much lower than those anticipated by simple concentration diffusion. This behavior attribute to the anodic dissolution of stainless steel inside the crevice, therefore, to quantitatively understand the effect of temperature on the dissipation behavior, it is necessary to know the anodic dissolution rate and occurrence of localized corrosion. Numerical analysis taking the effect of concentration diffusion and migration into account is also needed.
Tsukada, Takashi; Soma, Yasutaka
Hozengaku, 19(4), p.37 - 44, 2021/01
Corrosion Cracking phenomena in JPDR (Japan Power Demonstration Reactor) the first Japanese Light Water Reactor is reviewed. This review describes two major cracking failure. The first was found during inspection in 1966 as the cracking failure on weld-overlay cladding at the inner wall of the top head. A series of analysis showed that some of the cracks reached the base metal across the weld boundaries and further penetrated into the vessel wall. Significant depletion of ferrite content was detected in manually welded part considered to assisted the cracking. These inspection result in improvement of the welding procedure and no similar failures have been reported in Japanese reactor. This mode of failure gave rise to a new research field studying the corrosion fatigue behavior of low alloy steel because of importance to assess pressure boundary of the reactor. The experiment of JPDR also contributed to the establishment of international cooperation for studying EAC (environmentally assisted cracking). The second failure was found in 1972 near the welded part between stainless piping and safe end. The extensive research concluded that this failure was caused by Stress Corrosion Cracking.
Yamamoto, Masahiro; Soma, Yasutaka; Igarashi, Takahiro; Ueno, Fumiyoshi
Proceedings of Annual Congress of the European Federation of Corrosion (EUROCORR 2018) (USB Flash Drive), 7 Pages, 2018/09
In order to clarify the SCC behavior of SUS316L under BWR environment, mass transfer inside crevice of SUS316L in high temperature water using various crevice gap samples was investigated. The samples were prepared by put together two SUS316L sheets. Crevice gap differs from 0.005 mm to 0.1 mm. Corrosion tests were conducted in 8 ppm dissolved oxygen (DO) conditions. Surface oxide film was analysed by laser Raman spectroscopy (LRS) after immersion. Numerical simulations were also conducted by using COMSOL Maltiphysics. Diffusion process of DO and the other chemical species were calculated with connected to electrochemical process. Electrical conductivities inside the crevice were 100 times larger than these of outer water. The reason of high conductivity is existence of Fe
ions at the DO depletion crevice.
Yamamoto, Masahiro; Sato, Tomonori; Igarashi, Takahiro; Ueno, Fumiyoshi; Soma, Yasutaka
Proceedings of European Corrosion Congress 2017 (EUROCORR 2017) and 20th ICC & Process Safety Congress 2017 (USB Flash Drive), 6 Pages, 2018/09
The authors have studied the differences between outer surface and the crevice-like portion of SUS316L in high pressurized and high temperature water containing dissolved oxygen. We have already introduced that changes in the characteristics of corrosion products along the crevice directions and gap width. It is suggested that the environmental conditions are different with the features of crevice from these results. In this report, we introduce the changes in oxide films with crevice gaps and comparison with the numerical simulation data utilizing of FEM calculation.
Soma, Yasutaka; Komatsu, Atsushi; Ueno, Fumiyoshi
Zairyo To Kankyo, 67(9), p.381 - 385, 2018/09
In-situ measurement of electrical conductivity of solution within crevice of SUS316L stainless steel in 288C water has been conducted with newly developed electrochemical sensor system. The sensor measures local electrical conductivity of crevice solution beneath the electrode (
) with electrochemical impedance method. The sensors were installed at different positions within tapered crevice of SUS316L stainless steel. The crevice specimen with the sensors were immerged into 288C, 8 MPa, pure oxygen saturated high purity water for 100 h. at a position with crevice gap of 
59.3m was 8-11S/cm, least deviate from conductivity of 288C pure water (4.4S/cm) and no localized corrosion occurred. On the contrary, at a position with crevice gap of 4.4m increased with time and showed maximum value of 1600S/cm at 70 h. Localized corrosion occurred in the vicinity of this position. Thermodynamic equilibrium calculation showed of 1600S/cm being equivalent to pH of 3 to 3.7. It can be concluded that acidification occurred in tight crevice even under high purity bulk water and resulted in localized corrosion.
Soma, Yasutaka; Ueno, Fumiyoshi
Zairyo To Kankyo, 67(5), p.222 - 228, 2018/05
Localized corrosion in crevice of SUS316 stainless steel after immersion in 288C high purity water with dissolved oxygen concentration of 32 ppm for 100 h was analyzed. Two different types of localized corrosion initiated on grain boundary and inclusions. The former initiated on grain boundary and oxide grown into grain matrix. The oxidized area showed duplex structure composed of microcrystalline FeCr
O and island-shaped residual metals. The latter initiated on inclusions containing Ca and S and microcrystalline FeCrO grown into metal matrix. These localized corrosion occurred selectively in oxygen depleted area indicated formation of macroscopic corrosion cell with the corroded area as anode and surrounding oxygenated area as cathode.
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Vol.2, p.509 - 521, 2018/00
In-situ electrochemical measurement within crevice of stainless steel in 288C water has been conducted to analyze crevice water chemistry. Small sensors (
250m) measured local solution electrical conductivity, 
, polarization resistance, and electrochemical corrosion potential. Real-time response of the as functions of bulk water conductivity, dissolved oxygen (DO) concentration has been quantitatively analyzed. The effect of geometrical factors on the crevice environment was also studied. The differ more than an order of magnitude depending on the oxygen potential inside the crevice. The increased by small amount of bulk DO (e.g. 30 ppb). Maximum was observed with DO of 32000 ppb and became more than 100 times higher than that of bulk water. Crevice geometry affected significantly on the water chemistry inside.
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
Fushoku Boshoku Kyokai Dai-63-Kai Zairyo To Kankyo Toronkai Koenshu (CD-ROM), p.253 - 256, 2016/10
Contribution of corrosion to advance of stress corrosion cracking (SCC) of stainless steel in high temperature water must be assessed because serious corrosion can be found within SCC of light water reactors. The corrosion took the form of both intergranular and grain-matrix attack indicate aggressive corrosion condition was formed in the crevice of the SCC. We have investigated the crevice environment electrochemically and found that local electrical conductivity of the crevice solution at satisfactory narrow crevice gap having more than 100 times higher than that of bulk solution. In this research we assessed effect of cyclic deaerated and aerated bulk solution to the crevice environment. The result showed that electrical conductivity of the crevice solution under the deaerated bulk solution increased more than 10times by injection of pure oxygen suggest that the dissolved oxygen caused aggressive corrosion condition within the crevice.
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
Proceedings of 23rd International Conference on Nuclear Engineering (ICONE-23) (DVD-ROM), 8 Pages, 2015/05
Intergranular oxidation (corrosion) occurred within crevice of austenitic low-carbon stainless steel (solution treated, almost no applied stress) after immersion in high temperature water (288C, 8.5 MPa, dissolved oxygen conc. 32 ppm, electrical conductivity: 1.2
0.2S (measured value at 25C)) for 500 h. The intergranular oxidation occurred at specific position within the crevice that is relatively distant from the crevice mouth with relatively low crevice gap. Both the grain boundary and grain matrix were oxidized. In the oxidized area, Fe and Ni were depleted and Cr was enriched compared to the matrix. Maximum penetration depth of the oxidation was approximately 50 m after 500 h. In order to understand potential-pH condition within the crevice, surface oxide layer was microscopically and thermodynamically investigated. Thermodynamic properties of the surface oxides near the intergranular oxidized area indicated lowered pH of approximately 3.2 to 3.4. In-situ measurement of local solution electrical conductivity was carried out using small electrodes (dia. 800 m) imbedded into the crevice former plate. The solution pH was estimated using theoretically calculated pH vs. electrical conductivity relationship. In the area where the intergranular oxidation occurred, the solution electrical conductivity was nearly 100 times higher than that of bulk water and which indicated lowered pH of approximately 3.5. The above results suggested that, in the high temperature and relatively high purity water, acidification occurs within crevice of stainless steels and such aggressive corrosion condition result in the intergranular oxidation.
Soma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
Corrosion, 70(4), p.366 - 374, 2014/04
Times Cited Count：9 Percentile：40.71(Materials Science, Multidisciplinary)Surface oxide layers were formed within crevices of type 316L stainless steels in pure water at 288C and 8 MPa. Cross-sectional structures of the surface oxides were analyzed using transmission electron microscopy. In the condition of dissolved oxygen concentration of 2 ppm, the properties of the surface oxide layer changed with position and dual or triplex layered oxides were formed at a certain distance from the crevice mouth. The multilayered oxides were composed of Fe-based oxide in the core and a high-Cr content in the outer layer, which had not been observed on a boldly exposed surface. On the contrary, in deaerated condition, the surface oxide layers were composed of a Fe
O-based outer and a Cr-enriched inner oxide layer, regardless of the crevice position. Electrochemical condition within the crevice was identified by using E-pH diagram. It was suggested that, at 400m distance from the crevice mouth, the potential lowered at the early stage of exposure and then, shifted to noble direction with decrement of pH. Consequently, even within a narrow crevice with a gap size of a few m, the uniqueness of the crevice electrochemistry, characterized by the position and time dependence of both the potential and the pH, has been exhibited.
Soma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
Journal of the Electrochemical Society, 159(8), p.C334 - C340, 2012/07
Times Cited Count：9 Percentile：30.82(Electrochemistry)Surface oxide layers on stainless steel were formed in 561 K pure water at different potentials. To understand the oxide's properties in terms of their potential dependence, cross-sectional views of the oxide layer were analyzed using an electron microprobe technique and potential-solubility (equilibrium concentration of ionic species) diagram. In the potential range investigated, duplex oxide layers composed of mono- and bimetallic oxide were formed. Both the structure and composition of the oxide layer were affected by solubility of oxides.
C waterSoma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
Materials Transactions, 53(1), p.195 - 200, 2012/01
Times Cited Count：9 Percentile：46.37(Materials Science, Multidisciplinary)Surface oxide layer on SUS316L stainless steels exposed to 288C pure water with 2ppm dissolved oxygen (DO) for 1100h were analyzed using Focused Ion Beam and Scanning Transmission Electron Microscope equipped with EDS to understand the early stage of surface oxide layer formation. At 1h exposure, double oxide layer which is composed of compact inner oxide layer and outer oxide layer with Fe-rich and Ni-rich oxide particles was formed. At the outermost region of the SUS316L substrate, Ni and Cr were enriched. At 100h exposure, growth of the inner oxide layer was suppressed and the Ni and Cr enriched region at the alloy substrate was preserved underneath the Ni-rich outer oxide particles. At 1h exposure, most of the outer oxide particles were composed of Fe-rich ones, at 10h exposure, another Ni-rich outer oxide particles were nucleated and grew faster than Fe-rich ones. Consequently, a part of pre-formed Fe-rich outer oxide particles were covered with Ni-rich ones.
Soma, Yasutaka; Yamashita, Shinichiro; Hasegawa, Akira*; Kondo, Sosuke*
no journal, ,
Cr-coated cladding (Cr-Zry) is subjected to degradation due to corrosion and other factors in the operating environment, but published data is insufficient. Currently, in-pile tests for the performance evaluation of Cr-Zry depend on overseas reactors, which requires large amount of cost and time despite small number of output data. Therefore, an alternative evaluation technique under irradiation that can be performed more flexibly in Japan is strongly needed. In response to this need, JAEA has been developing a device that can evaluate the behavior of Cr-Zry under the effect of simultaneous irradiation and corrosion. In this presentation, we report on the development status of the apparatus equipped with a system for the in-situ measurement of corrosion behavior by electrochemical measurement, which is a feature not found in previous studies.
Mohamad, A. B.; Soma, Yasutaka; Nemoto, Yoshiyuki; Abe, Yosuke; Ioka, Ikuo; Sato, Tomonori; Ishijima, Yasuhiro; Miwa, Shuhei; Nakajima, Kunihisa; Kaji, Yoshiyuki; et al.
no journal, ,
Japan Atomic Energy Agency (JAEA) has launched fundamental researches on zircalloy with accident tolerance since 2019. The main purposes of the fundamental researches are to deepen the understanding of the zircalloy behavior under long-term normal operation or Loss of Coolant Accident (LOCA), beyond design basis accident (B-DBA) and severe accident (SA) conditions, and to support the implementation of Cr-coated zircalloy which is being developed by Japanese vendor. JAEA has also been conducted basic technology developments which is necessary for the understanding of the behavior of accident tolerant coated-zircalloy under normal operation, LOCA, B-DBA and SA conditions. For example, the ion irradiation technique combined with light water reactor (LWR) coolant conditions is being developed to simulate the normal operation condition. In addition, to understand LOCA phenomena, the results obtained from the LOCA test are implemented in the machine learning to understand in more detail the cladding fracture and ballooning. Furthermore, a separate effect test, such as the high temperature oxidation test, is also carried out. The fission product release during the B-DBA and SA are also included in the research program. The research results obtained by using these basic technologies will be integrated and implemented into the fuel performance analysis code to predict the fuel performance under reactor operating conditions.
Soma, Yasutaka; Komatsu, Atsushi; Igarashi, Takahiro; Yamamoto, Masahiro*
no journal, ,
Inside the stainless steel crevices including Stress Corrosion Cracking (SCC) exposed to oxygenated high-temperature water, an acidic, corrosive crevice environment is formed. Dissolved oxygen (DO) is supplied near the crevice mouth but depletes further inside, leading to a formation of the differential aeration cell and crevice environment. The precise location of the boundary between the bulk and crevice environments, known as the Electrochemical Crevice Mouth (ECM), is critical to provide a crevice corrosion criteria but not clearly defined. This study experimentally determined the ECM in Type-316L stainless steel by analyzing surface oxides formed in the crevice and the measurement of crevice solution's electrical conductivity at 288C. The ECM location was identified by a change in surface oxide composition and a significant increase in solution conductivity. Over time, the ECM shifted deeper into the crevice, influenced by factors like DO levels and oxide layer growth. Numerical simulations supported these findings, suggesting that as the surface oxide layer inside the crevice grew, it decreased cathodic DO consumption, causing the ECM to move deeper into the crevice.
Soma, Yasutaka; Komatsu, Atsushi; Kato, Chiaki
no journal, ,
In this study, the effect of impurities in steel and ppb level of chloride in bulk water on electrical conductivity of stainless steel's crevice solution (K) has been studied. Crevice specimens were made of as-polished Type-316L stainless steel (standard-SS), standard-SS exposed to 60% nitric acid to dissolve sulfur containing inclusions (acid-picked SS), and 316EHP steel in which sulfur and phosphorous concentration was decreased compared to standard-SS (EHP-SS). These crevice specimens were immersed into 561 K, 8 MPa water the K values were measured as a function of time with stepwise increase in dissolved oxygen levels. In addition, effect of 50 ppb Cl added to bulk water was investigated using standard-SS crevice. The all of the standard-SS, acid-picked SS, and 316EHP showed similar K vs time curves. It can be concluded that impurities dissolved from the steel itself do not significantly contribute to the increase of K. The effect of 50 ppb Cl on K vs time curve was obvious because maximum K value became more that 2 times larger than the solution without Cl addition. This indicate that small concentration of impurities can be migrated into the crevice.
Soma, Yasutaka; Komatsu, Atsushi; Kato, Chiaki
no journal, ,
This study investigates the effects of potential (electrochemical corrosion potential (ECP)) on the water chemistry within a crevice of stainless steel in 288C water containing Clmajor anionic impurities. In situ measurements of the electrical conductivity of a solution within a 15 m gap crevice () were conducted using small sensors installed at different crevice depths. The ECP at the external surface of the crevice specimen (Eext) was controlled by the dissolved oxygen concentration in the bulk water. An increase in Eext from approximately -0.49 V (versus a standard hydrogen electrode at 288C) to -0.12 V resulted in an increase in from 12 to 160.3S/cm at 21 mm from the crevice mouth. FEM analysis taking into account the electrochemical reaction quantitatively reproduced this behavior. Cl was considered to be the major anionic species transported into the crevice in this potential range. A further increase in Eext up to 0.3 V led to a decline in . An increase in the flux of oxidizing (O and hydrochromate ion) and alkalizing species (OH) into the crevice was considered the cause of this behavior.
