Zairyo-to-Kankyo
Online ISSN : 1881-9664
Print ISSN : 0917-0480
ISSN-L : 0917-0480
Volume 67 , Issue 9
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Research Paper
  • Takumi Haruna, Yuki Miyataki, Youhei Hirohata, Toshio Shibata, Naoki T ...
    2018 Volume 67 Issue 9 Pages 375-380
    Published: September 15, 2018
    Released: June 22, 2019
    JOURNALS RESTRICTED ACCESS

    This research aimed to confirm the formation of Fe3O4 film on Fe immersed in aqueous 45 mass% NaOH solution containing some oxidants at the boiling temperature, to recognize the optimum immersion time for the formation of thick and protective film, and to reveal the absorption behavior of D2O in the Fe3O4 film at room temperature. The results were obtained as follows. It was confirmed that Fe3O4 film was formed on Fe immersed in the NaOH solution for a time more than 0.6 ks, and the film thickness increased parabolically with an increase in the immersion time. D2O absorption test was carried out to the films formed in the NaOH solution for immersion times of 1.2 and 3.6 ks. An amount of D2O absorbed into the film increased with an increase in an absorption time up to 1000 ks, and an absorption time more than 1000 ks made an amount of D2O constant. The constant amount of D2O was larger for the film formed on Fe immersed in the NaOH solution for 3.6 ks than that for 1.2 ks. The transient of the amount of D2O absorbed into the film was analyzed on the basis of Fick’s law for diffusion, and diffusion coefficients of D2O were obtained to be 5.1×10-15 and 9.9×10-15 cm2・s-1 for the films formed for 1.2 and 3.6 ks, respectively. Therefore it was estimated that the diffusion coefficient of the Fe3O4 film was in the region from 5.1×10-15 to 9.9×10-15 cm2・s-1.

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  • Yasutaka Soma, Atsushi Komatsu, Fumiyoshi Ueno
    2018 Volume 67 Issue 9 Pages 381-385
    Published: September 15, 2018
    Released: June 22, 2019
    JOURNALS RESTRICTED ACCESS

    In-situ measurement of electrical conductivity of solution within crevice of SUS316L stainless steel in 288℃ water has been conducted with newly developed electrochemical sensor system. The sensor consists of electrode (φ≈ 250μm stainless steel) and insulator (high purity alumina) directory imbedded into crevice former plate. The sensor measures local electrical conductivity of crevice solution beneath the electrode (κcrev) 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 288℃, 8 MPa, pure oxygen saturated high purity water for 100 h. κcrev at a position with crevice gap(g) of ≈ 59.3μm was 8~11μS・cm-1, least deviate from conductivity of 288℃ pure water (3.7μS・cm-1) and no localized corrosion occurred. On the contrary, κcrev at a position with g ≈4.4μm increased with time and showed maximum value of ≈1600μS・cm-1 at 70 h. Localized corrosion occurred in the vicinity of this position. Thermodynamic equilibrium calculation based on oxide in the crevice and the maximum κcrev(1600μS・cm-1) showed that pH of crevice solution can be reached to 3.53. It can be concluded that acidification occurred in tight crevice even under high purity bulk water and resulted in localized corrosion.

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