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Soma, Yasutaka; Komatsu, Atsushi; Ueno, Fumiyoshi
Corrosion, 78(6), p.503 - 515, 2022/06
Times Cited Count：0 Percentile：0(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.
generation; The Role of NO
in the crevice corrosion repassivation of type 316L stainless steelAoyama, Takahito; Sugawara, Yu*; Muto, Izumi*; Hara, Nobuyoshi*
Journal of the Electrochemical Society, 166(10), p.C250 - C260, 2019/01
Times Cited Count：5 Percentile：16.76(Electrochemistry)The role of NO in the repassivation of crevice corrosion of Type 316L stainless steel was investigated. In crevice corrosion tests, the solution was changed from 1 M NaCl to NaCl-NaNO. NO led to complete repassivation. Repassivation of the crevice corrosion was found to take place in two steps. In the first step, the estimated current density inside the crevice gradually decreased from ca. 5 mA cm
to ca. 5 A cm. After that, the current density suddenly decreased to less than 0.1 A cm. From the potentiodynamic polarization in acidic solutions simulated inside the crevice (pH 0.2) and in situ observations of the crevice corrosion morphology, the first step was thought to be generated by the suppression of active dissolution by NO. It would appear that the generation of NH
results in a pH increase and the further suppression of active dissolution, and then repassivation occurs.
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
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.
Soma, Yasutaka
no journal, ,
Crevice corrosion have been studied.
Soma, Yasutaka; Ueno, Fumiyoshi; Inagaki, Hiromitsu*
no journal, ,
Effect of crevice geometry on corrosion environment within crevice of stainless steel in high temperature water was studied.
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
no journal, ,
Commemorative speech for "The Award of JSCE for young researcher" of Japan Society of Corrosion Engineering on May 21st 2020, entitled "Characterization of the mechanism of localized corrosion in the crevices of stainless steel in high-temperature, high-purity water" will be made. In this study, we conducted followings: (i) Corrosion test of Type 316L stainless steel to analyze susceptibility to localized corrosion within a crevice in 561K high purity water, and (ii) Develop a sensor system to measure the solution conductivity in a crevice and study relationship between crevice water chemistry and the localized corrosion. These studies were done for the purpose of clarifying the mechanism of stress corrosion cracking (SCC). It was shown that Type-316L stainless steel is susceptible to intergranular corrosion inside the crevice. The developed sensors detected very high solution conductivity in the vicinity of the intergranular corroded area indicate highly corrosive environments were formed in crevice with small gaps. This system can be applied to clarify the mechanism of corrosion related failure, such as SCC, and is expected to contribute to the safety improvement of nuclear reactors.
on crevice corrosion of Type 316L stainless steelAoyama, Takahito; Kato, Chiaki
no journal, ,
Crevice corrosion tests were performed on SUS 316L stainless steel using a flow cell that allows in-situ observation of the inside of the crevice. The inside of the crevice was filled with 0.1 M NaCl, and 0.1 M NaCl and 0.1 M NaCl-10 mM [Cu(EDTA)]Na
were used for the outside solution. The results showed that the time required for crevice corrosion to occur in 0.1 M NaCl-10 mM [Cu(EDTA)]Na was longer than that in 0.1 M NaCl. The propagation behavior of crevice corrosion was also different. These results suggest that Cu(EDTA) suppressed the initiation and propagation of crevice corrosion.
Soma, Yasutaka; Komatsu, Atsushi; Kato, Chiaki
no journal, ,
Electrochemical environment within a crevice which is referred to as crevice (or crack) water chemistry is an important factor for understanding stress corrosion cracking (SCC) of stainless steels in high temperature water. There have been many studies and well-organized reviews on the crevice water chemistry. However, the measurement of the real-time behavior of crevice water chemistry as a function of potential (electrochemical potential, ECP) remains a major challenge. In this study, the in situ measurement of the electrical conductivity of the solution inside crevices, was carried out to analyzed -ECP relationship.
