Zairyo-to-Kankyo
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ONLINE ISSN: 1881-9664
PRINT ISSN: 0917-0480

Zairyo-to-Kankyo Vol. 49 (2000), No. 7

  • Corrosion Loss in the Chemical Plants of Chugoku-Shikoku District of Japan

    pp. 394-403

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    An investigation on the corrosion loss has been carried out from the view of industrial economy during the period of 1996 and 1997 fiscal year. Two different approaches, initiated by T. P. Hoar and by H. H. Uhlig respectively, were both adopted for this investigation. Questionnaires based on the Hoar approach were mailed to 111 chemical factories of which 43 factories have responded. The questionnaires composed of 13 independent questions to collect information on several items such as size of total production, investment in plant and equipment and expense for corrosion protection. In the Uhlig approach, which analyzes the production data in the industries related to the corrosion, the survey proceeded for the cost analyses of 8 basic topics. The Uhlig approach underestimated the corrosion loss as compared with that in the Hoar approach. The ratio of indirect over direct corrosion loss was estimated to be in the order of 0.1 irrespective of the character of each individual plant.
  • Photoelectrochemical Property, of Rutile (Ti, M) O2 Oxides (M: Nb, Mo, Ta, W) Formd on Ti-M Binary Alloys

    pp. 412-419

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    The effects of 4d and 5d transition metals (Nb, Mo, Ta, W) on photosensitization property of the rutile titanium oxide were investigated by a series of experiments and a molecular orbital method using a cluster model of the rutile.
    The cluster model used in this study is MTi10O22, in which the central Ti atom in the cluster is replaced by each transition element, M. The electronic structure of the rutile is simulated by the DV-Xα method using the cluster. Photoelectrode characteristics of the rutile titanium oxide doped with each transition element are evaluated by a series of experiments. The doped specimens are prepared by oxidation of Ti-5 mol%M alloys. The results obtained from the experiments and the calculation are as follows; It is confirmed by a conventional X-ray diffraction method that we can make the pure rutile titanium-oxide as well as the oxides doped with the transition elements can be made by means of the oxidation of single phase of Ti-M alloys. It is suggested by the calculation that the donor levels of the additional transition elements appear in the energy gap of the rutile and that the donor levels of the additional transition elements lower in order of Nb, Mo, Ta, W, and their levels are 0.17eV, 0.58eV, 0.79eV, 0.89eV, respectively, below the conduction band of the rutile. The photocurrents density in the anodic polarization measurements starts to increases at about 0V, and it saturates at about 1200mV vs. SCE. The saturation photocurrent-density of the (Ti, M)O2 specimens depends on the additional element, M, and it lowers in order of M: Nb, Ta, Mo, W. It is found that the increase of photocurrent density at 620, 730nm in the (Ti, Ta)O2 and (Ti, W)O2 rutile specimens is caused by the existence of the donor levels of the additional transition elements.
  • Effect of Ti, Mo and Al Addition on the Sulfidation Behavior of an Ni-20Cr-13.5Co Alloy

    pp. 420-425

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    Effect of Ti, Mo and/or Al addition on the sulfidation behavior of an Ni-20Cr-13.5Co alloy was investigated in H2-H2S mixtures with sulfur partial pressures of 10-7, 10-5.5 and 10-4Pa at 873K. A four-layers scale was formed on the surface of the alloys at the sulfur partial pressure of 10-4Pa, while at the 10-5.5Pa-sulfur pressure a triplex scale was formed. The sulfidation resistance of the Ni-20Cr-13.5Co alloy was improved by adding of Mo or Al, especially Al, in those atmospheres. Outer sulfidation layer of Cr3S4, thin inner sulfidation layer and internal sulfidation were observed at the sulfur partial pressure of 10-7Pa. As the atomic ratio of Al to Ti in the alloy was larger than 1 on the alloys added Ti, Mo and Al all together, the thickness of the internal sulfidation layer greatly got thinner in this atmosphere.
  • High Temperature Corrosion Mechanism of SCH2 Heat Resistant Steel in Waste Incineration Furnace

    pp. 426-430

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    The high temperature intergranular corrosion behavior of SCH2 heat resistant steel in a waste incinerator was investigated by scanning electron microscopy, electron probe micro-analysis, and X-ray analysis. Specimens were exposed in a waste incinerator where plastics and used tires were burned under thermal cycling conditions of heating at 1423K for 6.2ks and 1150K for 17ks and then cooling to room temperature.
    A Cr carbide network structure was identified, but there was no intergranular corrosion near the surface of SCH2, SiO2 down to about 20μm and 2μm diameter Mn sulfide were observed at the top of the alloy. There was no Cr carbide network about 100μm from the scale/alloy interface. Cr diffused into the substrate during the exposure and then formed a Cr2O3 scale at the alloy surface by rapid diffusion in ferrite. It is suggested that intergranular corrosion was not the cause because of the disappearance of Cr carbide near the surface. The EPMA analysis showed that Cr was depleted while Ni was concentrated at the top of the alloy. The Fe-Cr-Ni phase diagram shows that the γ-phase appears at 1423K and it is assumed that this area was changed from ferrite to austenite.
    It is concluded that SiO2 formed due to diffusion of oxygen (O) through the austenitized area, while Cr and Mn sulfides were formed under it.
  • Corrosion of Carbon Steel in Flowing Pure Water under High Temperature and High Pressure Conditions

    pp. 431-436

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    The all volatile treatment (AVT) has been currently adopted for once-through boilers in Japan. Recently, the introduction of oxygenated treatments, namely combined water treatment (CWT) and neutral water treatment (NWT) are being examined with the intention of reducing the pressure loss in the water line. In this study, corrosion tests were conducted on carbon steel in a jet-in-slit testing apparatus under high pressure at elevated temperatures. The mass loss of specimen and damage depth were used as the index of corrosion damage. It was revealed that AVT is superior to others in that the mass loss as well as the damage depth of specimen was smallest in this water. This is because the corrosion product film generated on the specimen surface is of the magnetite. It is vested with enough mechanical strength not to be removed from the metal surface even at those place where the intensity of turbulence and the shearing stress of the fluid flow is locally raised. Thus, the occurrence of so-called erosion-corrosion is almost prevented. In CWT environment, the erosion-corrosion did not occur but radial deep ditches were formed on the surface near the periphery of the specimen where the flow velocity of water was lower than it was at the central part. This might be attributed to the enhanced oxygen supply in the central part of specimen which brought about the generation of sound passivation film there. On the surrounding surface, in contrast, the oxygen supply was poor so that the passivation film was not complete, which must be linked with the formation of the ditches apparently under the influence of fluid flow. In the NWT environment, the most inferior result was obtained in the both aspect of mass loss and damage depth.
  • Investigation of Irradiation Induced Inter-granular Stress Corrosion Cracking Susceptibility on Austenitic Stainless Steels for PWR by Simulated Radiation Induced Segregation Materials

    pp. 437-444

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    An Irradiation Assisted Stress Corrosion Cracking (IASCC) has not been found in Pressurized Water Reactors (PWRs). However, the authors have investigated on the possibility of IASCC so as to be able to estimate the degradation of PWR plants up to the end of their lifetime.
    In this study, the authors melted the test alloys whose bulk compositions simulated the grain boundary compositions of irradiated Type 304 and Type 316CW stainless steels. Low chromium, high nickel and silicon (12%Cr-28%Ni-3%Si) steel showed high susceptibility to PWSCC (Primary Water Stress Corrosion Cracking) by SSRT (Slow Strain Rate Tensile) test in simulated PWR primary water. PWSCC susceptibility of the test steels increases with a decrease of chromium content and a increase of nickel and silicon contents. The aged test steel included coherent M23C6 carbides with matrices at the grain boundaries showed low PWSCC susceptibility. This tendency is in very good agreement with that of the PWSCC susceptibility of nickel based alloys X-750 and 690.
    From these results, if there is the possibility of IASCC for austenitic stainless steels in PWRs, in the future, the IASCC shall be caused by the PWSCC as a result of irradiation induced grain boundary segregation.

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