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Journal Articles

Localized corrosion in crevice of SUS316 stainless steel in oxygenated high temperature and high purity water

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 288$$^{circ}$$C 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$$_{2}$$O$$_{4}$$ and island-shaped residual metals. The latter initiated on inclusions containing Ca and S and microcrystalline FeCr$$_{2}$$O$$_{4}$$ 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.

Oral presentation

In-situ measurement of solution electrical conductivity within crevice of stainless steels in high temperature water

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 288$$^{circ}$$C 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.

Oral presentation

Crevice corrosion in high temperature water

Soma, Yasutaka

no journal, , 

Crevice corrosion have been studied.

Oral presentation

Effect of crevice geometry on corrosion environment within crevice of stainless steel in high temperature water

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.

Oral presentation

Effect of impurities in steel and ppb level of chloride in bulk water on electrical conductivity of stainless steel's crevice solution in high temperature water

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.

Oral presentation

Mechanism of localized corrosion in crevice of stainless steel in high temperature and high purity water

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.

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