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Soma, Yasutaka; Komatsu, Atsushi; Ueno, Fumiyoshi
Corrosion, 78(6), p.503 - 515, 2022/06
Times Cited Count：0 Percentile：0.01(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.
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, 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：46.83(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：8 Percentile：31.34(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：50.77(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.
C pure waterSoma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
no journal, ,
Surface oxide layers on stainless steel were formed in 288C 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.
Soma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
no journal, ,
Surface oxide layers on crevice of 316L stainless steels in high temperature and high pressure water containing 2ppm dissolved oxygen (DO) were investigated by using SEM, STEM, and Raman spectroscopy. Effects of distance from open-mouse of the crevices (Y) and corresponding DO concentration on surface oxide layer were studied. Depending on the Y value, four different regions with different surface oxide layer were observed if the crevice were sufficiently narrow. The oxide layers were consisted of inner and outer oxide layer at the all regions. Change in composition of outer oxide layer clearly showed that the DO concentration decreased with increasing Y value. However, thickness of the inner oxide layer showed maximum value at certain Y value and then, decreased with the Y value. This behavior considered to be brought by differences in the DO concentration, ionic concentration and pH.
Soma, Yasutaka; Kato, Chiaki; Yamamoto, Masahiro
no journal, ,
Surface oxide layers were formed within crevices of type 316L stainless steels in pure water at 561 K and 8 MPa. Cross-sectional structures of the surface oxides were analyzed using transmission electron microscopy. Under the condition of a dissolved oxygen concentration of 2 ppm, the properties of the surface oxide layer changed with the crevice position and dual or triplex layered oxides were formed, which had not been observed on a boldly exposed surface. The thickness of the inner oxide layer showed a local maximum at a certain distance from the crevice mouth. These oxide characteristics suggest the presence of locally varying electrochemical conditions. Potential-pH diagrams of the multilayered oxides suggest that the potential decreased at the early stage of exposure and then re-increased, accompanied by weak acidification.
Kato, Chiaki; Soma, Yasutaka; Ueno, Fumiyoshi; Okada, kiyotaka*; Ebina, Tetsunari*; Kano, Yoichi*; Nakayama, Jumpei
no journal, ,
no abstracts in English
Sugiyama, Hiroshi*; Okada, kiyotaka*; Ebina, Tetsunari*; Kano, Yoichi*; Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi; Nakayama, Jumpei*
no journal, ,
no abstracts in English
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
no journal, ,
Within crevice of austenitic stainless steel (solution treated, mirror poloshed) immersed in high-temperature water (561K, 8.5MPa, dissolved oxygen conc. 32ppm, conductivity ca.1 to 1.5e-6S/cm), new form of intergranular oxidation was occured. The intergranular oxidation occured in certain area within the crevice, that is, the area with relatively low crevice gap and distant from the crevice mouth. Cr was enriched in the oxide and the oxidation occured both grain boundary and grain matrix. After the intergranular oxidation, some grain was loosen and peered off. Maximum depth of the intergranular oxidation was 50e-6m/500h. Because almost no stress was applied on the specimen, relationship between this oxidation bahavior and stress corrosion cracking should be clarified by further experiment.
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
no journal, ,
To understand the effect of crevice solution chemistry on stress corrosion cracking of stainless steel in high temperature pure water, local solution electrical conductivity was measured using artificial crevice with small sensors. If the crevice gap was narrow enough, local solution electrical conductivity increased more than 100 times than that of bulk pure water and intergranular corrosion occurred.
Soma, Yasutaka; Kato, Chiaki; Ueno, Fumiyoshi
no journal, ,
Stress corrosion cracking (SCC) on stainless steels have been recognized as one of the most important corrosion-related failure in light water reactors. Many researches have been pointed out that the SCC advances under altered solution chemistry condition at the crack tip region compared to the bulk pure water. However, little works have been done to clarify degree of the alteration as function of bulk water condition, geometrical factor, and time. In this work, we carried out in-situ measurement of solution electrical conductivity within crevice of stainless steels. To create crevice specimen, a couple of stainless steel plate was fixed with bolts and nuts. Small sensors were imbedded into the crevice plate at three different positions with different crevice gaps. The crevice specimen with sensors was exposed to 288C water with pressure of 8 MPa, dissolved oxygen concentration of 32 ppm. The solution electrical conductivity at the crevice gap of 6e-5 m was almost same to that of bulk pure water. At the crevice position with 1e-5 m gap, the maximum conductivity value was nearly 1000 times higher than that of bulk water and that is equivalent to decrease in pH of 3 from the neutral value. This indicates, if the crevice gap was narrow enough, local acidification occurred at the tip of the crevice.
