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

Zairyo-to-Kankyo Vol. 47 (1998), No. 8

  • Application Studies on Corrosion Protection by Titanium Clad Steel Lining in Mega-Float Project

    pp. 494-500

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    Mega-Float is a ultra large floating structure, whose size can be several kilo-meters scale and planned to be used for city infrastructures, such as the airport, the heliport and the power plant, etc. and then expected to have ultra long durability over 100 years.
    Mega-Float Research Union was established in 1995 for the realization and the research project is now going to overcome some difficulties.
    In this project, titanium clad steel lining was adopted as the corrosion protection method for the splash zone of the around side wall, considering ultra long durability. Various techniques for the lining work, such as automatic titanium welder and the repair method for some type of damages of titanium clad steel lining, were developed and tested using the proto-type large-scale floating model (300m length, 60m width, and 2.0m height) which was constructed in a real sea area.
    It was, from these results, confirmed that the titanium clad steel lining was a very useful corrosion protection method for Mega-Float.
  • Accelerated Thickness Reduction of Water-Wall Tube in Municipal Solid Waste Incinerator due to Low Air-Ratio Combustion

    pp. 512-518

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    Abnormally accelerated thickness reduction in the water-wall tube made of plain carbon steel has taken place in the secondary combustion zone of many domestic municipal solid waste (MSW) incinerators since introducing the low air-ratio combustion in order to suppress the emission of high concentration NOx. In the present study, corrosive failure analyses were conducted for the water-wall tube operated for a long period under the low air-ratio condition in actual MSW incineration plants in Tokyo Metropolitan, with relation to the environmental analyses and with comparing with the case of high O2 combustion of 10% O2. Low air-ratio combustion was found to cause the significant temperature increment in water-wall tube at the secondary combustion zone together with an increased thermal fluctuation so as to enhance various modes of the scale degradation such as exfoliation, adhesion reduction, and/or cracking, which result in the much increased thinkness of the multiple-layered and non-protective scale. On the other hand, no correlation was observed between the much enhanced water-wall tube corrosion and concentration of the specific corrodants possible for forming the low-melting eutectics such as chlorides. Such an increased tube temperature also could bring about the promotion of the complicated corrosion by both gaseous species and molten salt of mainly chlorides as compared with the case of high O2 combustion. Possible corrosive failure mechanism of water-wall tube induced by the low air-ratio combustion was discussed to some detail.
  • Modeling of Atmospheric Corrosion Environments and Its Application to Constant Dew-Point Corrosion Test

    pp. 519-527

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    In order to develop an accelerated corrosion test which simulates the corrosion behavior of stainless steels in a marine atmosphere, modeling of atmospheric corrosion environments were tried. From the measurement of the night and day variation of meteorological data, it was found that the dew-point of outdoor air is approximately constant and humidity depends on an air temperature. The mole fractions of Na+, Mg2+, and Cl- ions in deposited salts on a stainless steel plate specimen exposed to a marine environment were the same as those in the seawater, and the Cl- ion concentration of NaCl-MgCl2 solution depends on the humidity of ambient air. Consequently, the Cl- ion concentration of electrolyte layer or droplet formed on a metal surface, which is the critical factor of atmospheric corrosion in a marine environment, can be determined as a function of the dew-point of moist air and the temperature of metallic specimen. Then, the amount of electrolyte layer or droplet is determined by the total amount of chloride deposition on a metallic specimen. The determinative condition of atmospheric corrosion environments, therefore, can be described by the dew-point of moist air, the amount of chloride deposition, and the night and day variation of specimen temperature. An environment model described by the determinative condition was applied to simulate the corrosion behavior of Type 304 stainless steel in a subtropical marine environment in Okinawa. The generation and growth behavior of rust on the steel in the environment was well reproduced by using the constant dew-point test, that is, the accelerated corrosion test based on the model.
  • Development of Solid Electrolyte Reference Electrode for High Temperature Water System

    pp. 528-533

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    A newly designed solid electrolyte internal reference electrode has been developed for electrochemical measurement in high temperature water systems. This reference electrode is composed of an yttria stabilized zirconia (YSZ) disk electrode with a sputter-deposited Ni·NiO composite film on the backside, an asbestos plug soaked with a neutral phosphate buffer solution as an internal solution and an oxidized zircaloy-4 body. The thermodynamic response of the YSZ reference electrode has been confirmed in phosphate buffer solutions (pH4.4-9.3) with an Ag/AgCl (0.1kmol·m-3KCl) internal reference electrode at temperatures up to 523K. It was found that the potential of the YSZ reference electrode has the following linear relationship with temperature:
    EYSZ/Ni·NiO=2395-5.37T/mVvs. SHE
    Using the YSZ reference electrode, the measurement of anodic polarization curves of Type 304 stainless steel in a 0.5kmol·m-3Na2SO4 solution at temperatures up to 563K has been performed. Anodic polarization curves measured with the YSZ reference electrode coincided with those with the Ag/AgCl internal reference electrode in the temperature range from 473K to 563K.
  • Investigation of Limiting Potential of Protection of Stainless Steel in Electrochemical Protection

    pp. 534-539

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    The appropriate cathodic protecting potential of the two-phase stainless steel (SUS 329J4L) was investigated in artificial sea water by the constant load method. In the constant load method, the rupturing time was used as an estimation parameter. And then, the constant tensile stress test was able to be carried out at a short time before rupturing by using the estimation parameters of mechanical characteristics. It was found that the hydrogen embrittlement of steel occurred at less noble than potential -0.85V (vs. Ag/AgCl) by the decreasing of elongation and the contraction percentage of area. After the test for 400h test, 0.2% proof strength and tensile strength of steel were increased by embrittling hardening at less noble potential than -0.85V. At the potential occuring hydrogen embrittlement, the fractured surface differed from that at the potential unoccuring embrittlement the observation using scanning electron microscope (SEM), and total length of fracture was correlated with the degree of hydrogen embrittlement.
  • Critical Impact Velocity in the Solid Particles Impact Erosion of Metallic Materials

    pp. 540-547

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    Solid particles impact erosion of metallic materials proceeds through two kinds of damage processes. One is the removal of material due to repeated plastic deformation, and the other is cutting. These processes occur simultaneously and the ratio of each contribution to the total damage depends not only on the impact angle (the predominant parameter) but also on the impact velocity. As the impact velocity goes down, a solid particle tends not to skid on the target material surface, and hence the cutting damage is reduced. At a certain lower velocity, the particle does not skid at all, resulting in no cutting damage but plastic deformation damage only. This velocity was defined as the “critical impact velocity”. In this study the methodology to determine the critical velocity through a measurement of the coefficient of friction was established, that is, the dynamic friction coefficient during skidding and the static friction coefficient during rolling without skidding. In order to measure the coefficient of friction at the moment of particle impact, a rotating target apparatus was developed. The critical impact velocity thus determined depended on the hardness of both materials and particles, as well as on the shape and size of the solid particles.

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