Papers by Author: Yutaka Watanabe

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Authors: Yutaka Watanabe, W. Gao, Hideki Shimizu, S. Kiyono
Authors: Yutaka Watanabe, Yuzo Daigo
Abstract: Corrosion properties were investigated on stainless steels and Ni base alloys in supercritical water containing 0.01mol/kg-H2SO4 at 400°C/30MPa as a function of oxygen concentration ranging from 3ppb to 800ppm. Alloys with high Fe content showed an interesting contrast in corrosion property in sulfuric acid-containing supercritical water between at the high oxygen condition (800ppm) and at the lower oxygen conditions (3ppb and 8ppm). At 8ppm of oxygen concentration or lower, corrosion rate was a unique function of Cr content of the alloys including both Ni base alloys and stainless steels. However, corrosion resistance of the iron-based alloys (316 stainless steels) was remarkably improved when oxygen concentration was increased up to 800ppm. Corrosion rate of alloy C-276, which contains 5.7% of iron, was also somewhat reduced under the high oxygen condition, while corrosion rate of the other “iron-less” Ni base alloys was accelerated as oxygen level was increased. Characteristics of oxide scales, in terms of chemical composition and compound structure, have been examined in connection with the corrosion properties of the alloys.
Authors: Hiroyuki Sugai, Masao Sataka, Satoru Okayasu, Shin Ichi Ichikawa, Katsuhisa Nishio, Shinichi Mitsuoka, Takamitsu Nakanoya, Akihiko Osa, Tetsuya Sato, Takashi Hashimoto, Sun Chan Jeong, Ichiro Katayama, Hirokane Kawakami, Yutaka Watanabe, Hironobu Ishiyama, Nobuaki Imai, Yoshikazu Hirayama, Hiroari Miyatake, Takanori Hashimoto, Masahito Yahagi
Abstract: Non-destructive and on-line Li diffusion experiments in Li ionic conductors are conducted using the short-lived !-emitting radiotracer of 8Li. The radiotracers produced as an energetic and pulsed ion beam from TRIAC (Tokai Radioactive Ion Accelerator Complex) are implanted into a structural defect mediated Li ionic conductor of NaTl-type intermetallic compounds ("-LiGa and "-LiIn). The experimental time spectra of the yields of !-particles are compared with simulated results and Li diffusion coefficients in the intermetallic compounds are extracted with an accuracy of ±10%. The diffusion coefficients obtained for "-LiGa with Li content of 43-54 at.% are discussed in terms of the interaction between Li-ion and the structural defects in the specimen, compared with the cases of "-LiAl and "-LiIn. The nonlinear Li-content dependency of Li diffusion coefficients for "-LiGa suggests that the Li diffusion with the Li-deficient region is obstructed by the defect complex composed of vacancies at the Li sites.
Authors: Yutaka Watanabe, H. Abe, Yuzo Daigo, R. Fujisawa, M. Sakaihara
Abstract: It was previously demonstrated by the authors that density of water and the density-related physical properties of water are ones of the major governing factors of corrosion of metals in supercritical aqueous solutions. The water density is expected to have significant effects also on stress corrosion cracking (SCC) of metals in supercritical water. In this study, we have looked into cracking behavior of sensitized and non-sensitized stainless steels in water under various pressures and at fixed temperatures above and below the critical point by using SSRT technique, and discussed its correlation with dielectric constant of water. The experimental results have suggested two different cracking mechanisms of 316(L) stainless steels as follows; For sensitized 316 SS - pure water system; (1) Effects of phase state of water and applied pressure, more essentially, physical property of water, were clearly observed. (2) SCC did not occur in the oxygenated 'gas-like', supercritical water at 400°C/25MPa. (3) Cracking occurred at 400°C/30MPa and the cracking severity was more pronounced as applied pressure was increased up to 60MPa at the same temperature. (4) This variation in cracking susceptibility being dependent on pressure was understood from dielectric constant of water. (5) The results give a strong evidence of the dissolution mechanism. For non-sensitized 316L SS - sulfuric acid water system; (1) Non-sensitized 316L SS severely cracked with IG even in a 'gas-like' supercritical water. (2) Dielectric constant did not affect cracking severity. (3) Cracking was more enhanced at higher temperature. (4) The results suggested oxidation cracking.
Authors: Hiroshi Abe, Yutaka Watanabe
Abstract: A number of mechanisms have been proposed to understand stress corrosion cracking (SCC) of metals, e.g. (1) slip dissolution and active pass corrosion based on anodic dissolution of metals, (2) tarnish rupture and internal oxidation based on oxidation ahead the crack tip followed by cracking of the oxides, and (3) hydrogen cracking, etc. If dissolution of metals takes the essential role in the stress corrosion cracking concerned, cracking susceptibility is expected to be significantly affected by dielectric constant of water. Because dielectric constant represents a character of water as a solvent, which determines solubility of metal oxides, and therefore corrosion rate of metals is strongly dependent on dielectric constant of water. K-constant type SCC growth rate tests have been done as a function of physical property (dielectric constant) of water by either manipulating temperature under iso-pressure condition (15MPa) or manipulating pressure under iso-thermal condition (330oC). Intergranular cracking was more enhanced and the crack growth was significantly accelerated under the condition of higher dielectric constant, indicating that dissolution of metal plays important role in the cracking mechanism of 316L stainless steels under the present testing conditions.
Authors: Maribel L. Saucedo-Muñoz, Toshiyuki Hashida, Yutaka Watanabe, Tetsuo Shoji, Victor M. Lopez-Hirata
Abstract: Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 600 to 900°C for 10 to 1000 minutes in order to study the microstructural evolution and its effect on the fracture toughness at cryogenic temperatures. The Charpy V-Notch fracture energy at 77 K showed a significant decrease with aging time in JJ1 and JN1 steels because of their higher contents of C and N. In contrast, the fracture energy corresponding to the aged JK2 steel decreased gradually with aging time. The abundant intergranular precipitation of carbides and nitrides seems to be the responsible for the fracture toughness deterioration in the aged JJ1 and JN1 steels. On the other hand, the intergranular precipitation of carbides was less abundant in the aged JK2 steel. The scanning electron microscope fractographs of the CVN test specimens corresponding to the aged JJ1 and JN1 steels showed mainly an intergranular brittle fracture and its fraction increased with aging time and temperature. In general, the presence of a more abundant intergranular precipitation resulted in a more rapid decrease in toughness with aging time.
Authors: Vladimír Číhal, Rudolf Štefec, Tetsuo Shoji, Yutaka Watanabe, Vivenakand Kain
Abstract: The EPR test, designed to examine of the susceptibility to nonuniform, primarily intergranular corrosion, ranks among the more successful testing technique developments relating to stainless steels and alloys. One of its numerous advantages is that it lends itself to non-destructive, on-site examination. EPR enjoyed wide expansion over the years since first conceived by Čihal in 1969. Recent EPR measurements tend to focus on (1) double and/or single loop EPR as a modern technique used to establish the resistance of stainless steels and alloys to intergranular corrosion; (2) detecting integranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) susceptibility in alloy steels and nickel alloys for nuclear engineering applications; and (3) studies of grain boundary precipitation and other minute local changes to alloy composition and structure.
Authors: Yutaka Watanabe, Keisuke Sato, Tetsuya Uchimoto, Toshiyuki Takagi
Abstract: Although electrochemical transients can provide very sensitive indication about the onset of localized corrosion, including initiation of stress corrosion cracking (SCC), information on location of the initiation site is not available. In the present study, we proposed an estimation method of location and magnitude of SCC initiation events, where electrochemical transients associated with local anodic events are measured by multiple reference electrodes and location of the initiation sites are estimated based on relative magnitude of the electrochemical transients simultaneously measured at multiple locations.
Authors: S.K. Bidhar, Yutaka Watanabe, H. Tsukui, Tetsuya Uchimoto
Abstract: Series of slow strain rate tests (SSRT) were conducted on moderately sensitized austenitic stainless steel in diluted aqueous solution of sodium thiosulfate at ambient temperature and pressure. Only a small area of the test piece was exposed to test solution and electrochemical potential transients obtained during the straining was used to detect an initiation or a precursor event of stress corrosion crack (SCC). Tests were stopped intermittently after getting potential transients, so as not to allow crack to propagate and to observe the morphology of very initial stage of SCC initiation site. Visual observation by scanning electron microscope (SEM) before and after the test shows direct correlation between electrochemical transient of certain characteristic to the crack initiation. SEM observation shows δ- ferrite in the alloy as one of the preferential SCC initiation site for typical electrochemical transient. Attempt was made to calculate the charge associated with crack initiation event from potential transient using dummy anode test.
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