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

Zairyo-to-Kankyo Vol. 42 (1993), No. 8

  • Deoxygenation of Boiler Water by Malto-oligosaccharides -(I)-

    pp. 494-500

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    Seven kinds of malto-oligosaccharides (MO) with different number of glucose units, NG, were synthesized and their deoxygenation behavior in boiler water was examined by using a test boiler system simulated actual high pressure boilers. All the MOs were used in forms of phosphate adducts. In order to perfectly remove the oxygen, the mole ratio of glucose units against oxygen is necessary to take greater value than k=2.0, regardless of NG value of MO.
    In this condition, the removal of oxygen was about 1 mole of oxygen per 1 mole of glucose unit constituting MO. The main reaction products of MO were lactic acid (C=3), glycolic acid (C=2), gluco-iso-saccharinic acid (C=6), and dideoxypentonic acid (C=5) in a mole ratio of 2.6:2.1:1.0:1.2. A small amount of diacethyl was detected in vapor. These resultss did not agree with the deoxygenation mechanism that the MO was assumed to decompose by the cleavage of glucoside bonds between glucose units. New reaction mechanism was proposed that the oxidative decomposition of MO occurs only at a terminal glucose unit with reducing hydrogen, through two parallel reaction steps called ‘direct oxidation’ and ‘hydrolysis-oxidation’ processes. The reaction mole ratio of glucose unit with oxygen, and the amounts of organic acids formed are quantitatively explained.
  • Deoxygenation of Boiler Water by Malto-oligosaccharides -(II)-

    pp. 501-507

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    The deoxygenation of boiler water by malto-oligosaccharide (MO) was studied in the range of NG values from 1 to 13.9 as a function of the number of glucose units NG, constituting MO. Phosphate was always added to MO as an indicator. The amount of P-alkalinity of boiler water decreased with NG, reached a minimum at NG=5.9, and then increased with NG. It corresponds to the fact that the amount of mono-carboxylic acids produced by the reaction reach a maximum at NG=5.9. The amount of (free) phosphate inos released from MO, rp, decreased with an increase in NG, reached a minimum value of 72% at NG=5.9, and then increased up to nearly 100% at NG=13.9. Diacethyl (CH3COCOCH3) having odor was produced in the reaction. The evolution amount of the diacethyl increased with NG, and had the maximum value at NG=2. This amount decreased with the NG over NG=2, and then reached a constant small value at NG=5.9.
    From these results, it was found that the MO with NG=13.9 is superior for the oxygen scavenger.
  • Corrosion of CrN Coating in High-Temperature Demineralized Water

    pp. 508-513

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    Corrosion behavior of CrN coating by an arc ion plating on type 304 stainless steel in demineralized water at 286°C was studied using the circulating type experimental loop. Exposure tests were carried out under three different dissolved O2 concentrations of 150μg/l, 500μg/l and 1, 000μg/l, respectively. Corroded surface was observed by SEM, XPS and GDLS. The weight loss of specimen after exposure decreased with exposure time linearly. The corrosion weight increased proportionally with an increase of dissolved O2 concentration. It was assumed that the dissolved O2 became a dominant factor for corrosion of CrN in high-temperature demineralized water.
  • High Temperature Oxidation and Hot Corrosion of a Yttria ODS Alloy

    pp. 514-520

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    Addition of yttrium to nickel-base alloys is known to improve their oxidation resistance. It is not clear, however, if a comparable effect is expected by addition of yttria or not. This question was investigated by comparing a hot corrosion behavior in a crucible and cyclic oxidation behavior in oxygen gas of a yttria added alloy with those of its yttria-free control of the same composition. As for the hot-corrosion, addition of yttria caused no significant difference for the solution-treated specimens, while it increased corrosion susceptibility for the as-extruded ones. The increase was considered to be due to a smaller crystal-grain size for the yttria-added specimen. Addition of yttria increased corrosion susceptibility to the dry oxidation as well. The reason was less clear. A possible one was a fine scale structure caused by yttria addition, which might have prevented formation of protective alumina layer over the alloy surface.
  • Relationship between the Apparent Activation Energy of Corrosion Reaction and Alloy Contents of Stainless Steels

    pp. 521-528

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    In order to confirm the general applicability of the previously proposed GI (General Corrosion Resistance Index) value1), influence of temperature upon the corrosion rates of three series of stainless steels (Fe-Cr, Fe-Cr-Ni, Fe-Cr-Ni-Mo alloys) in 10.2mol·kg-1 H2SO4 aqueous solution was studied. Though measured partial molar activation energy for each alloying element was different depending on the series of the alloy, the relationships between corrosion rates and GI values were commonly expressed as:
    logC.R.=A′-(B′/T)-(C/T)·GI
    where C.R. represents corrosion rate, T absolute temperature, and A′, B′, C are coefficients characteristic to the alloy series. GI is given by the first order polynominal.
  • UV-Visible Spectroscopy

    pp. 529-538

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    UV-visible spectroscopy combined with differential and potential-modulated reflectance methods is one of the most versatile of currently available spectroelectrochemical techniques, because it provides sensitive detection of very thin films on electrode surfaces in the bulk solution and deals information on the formation process, the structure and properties of the films. The theory and experimental methods of this technique were explained and following typical examples were reviewed: anodic dissolution products, submonolayer metal deposits, organic adsorbates, and passive films on metal electrodes.
  • Hot Vacuum and Materials

    pp. 539-546

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    Vacuum chambers of electron storage rings are generally made of aluminum alloys. Beam life time is limited by dynamic outgassing from the chambers due to irradiation of synchrotron radiation. Such condition of vacuum is namely “Hot Vacuum”. Surface contamination of the aluminum alloy was removed by ion beam etching. Therefore dynamic outgassing of the aluminum alloy material was reduced three order. Organic material and air were damaged and decomposed synchrotron radiation. In the mixture of organic material, air and water, the strong acids such as HCl, HF, HNO3, and H2SO4 are made and attack on the metal components. These processes are radiation corrosions. Ceramic coating such as SiO2 is one of anticorrosive treatments for the metal components.

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