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

Zairyo-to-Kankyo Vol. 50 (2001), No. 4

  • Surface-Enhanced Raman Scattering Spectroscopy to Investigate Mechanism of Corrosion Inhibition

    pp. 138-143

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    Surface-enhanced Raman scattering (SERS) spectroscopy, which is a useful tool for providing an in-situ molecular-level information of corrosion inhibitors at the metal-solution interface, is reviewed. Examples of SERS studies are demonstrated on inhibition mechanisms of several inhibitors for copper corrosion in aqueous solutions.
  • Influences of Metal Ions on Formation of α-FeOOH Rust

    pp. 155-161

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    To explore functions of unti-corroding alloying elements in weathering steels, α-FeOOH rust particles prepared from aqueous Fe(NO3)3 solutions added with Cu(II), Ni(II) and Ti(IV) of metal/Fe=0-0.1 in atomic ratio at 30, 50 and 100°C were characterized by various means. Most of the added metal ions were contained in the formed rust particles. Influences of the metal ions on the structure of formed α-FeOOH depended on kind of added metal ions and preparation temperature. The crystallinity of the products at 30°C examined by XRD was reduced by addition of all the metal ions in the order of Cu(II)>Ni(II)≈Ti(IV) and the product at Cu/Fe=0.1 was noncrystalline. The results of specific surface area showed that less crystalline products comprise agglomerates of fine particles. The simultaneous addition of Cu(II), Ni(II) and Ti (IV) exhibited no specific multiple or compensating effect, which indicates that these metal ions independently affect α-FeOOH formation.
  • Rusting Conditions for Stainless Steels in Marine Atmosphere in Terms of Amount of Deposited Sea Salt and Relative Humidity

    pp. 162-169

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    Four kinds of stainless steel coupons and a Fe/Ag-coupled ACM (Atmospheric Corrosion Monitor) type corrosion sensor were exposed at six locations in marine atmosphere for seven periods each of which consisted of one to two months. At the end of each exposure the coupons were examined visually for rusting or no rusting. The outputs of ACM sensor and RH (relative humidity) sensor were measured by a data logger every ten minutes to determine the amount of deposited sea salt, Ws. By accumulating Ws and RH data for all the exposure periods and locations, the critical humidity, RH*, below which a steel rusts and the critical Ws value, Ws*, above which a steel rusts could be determined. The (Ws*/g·m-2, RH*/%) values are obtained as: (5×10-3, 45) for Type 410 steel, (5×10-1, 38) for 22Cr-0.8Mo steel, and (4×10-2, 43) for Type 430 steel and Type 304 steel.
  • Impedance Measurement for Slow Decline of Electrode Potential of Fe-doped TiO2 Coating

    pp. 170-176

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    It has been found that a TiO2 coating under illumination can cathodically protect substrate metals, in a non-sacrificial way, by its photoelectrochemical characteristics. It has also been found that, when TiO2 layer containing Fe is coated under a plain TiO2 layer, the effect continues even after stopping illumination. In this work, the impedance of TiO2 coatings on ITO glass substrate, plain TiO2 (TiO2/ITO), TiO2 containing xFe% Fe (xFe Fe-TiO2/ITO), and TiO2 coating with 2 layers, plain TiO2 for outer layer and xFe Fe-TiO2 for inner layer (TiO2/xFe Fe-TiO2/ITO), were measured in a phosphate buffer solution, pH6, to study this slow decline of electrode potential after stopping illumination. A capacitance, CIL, was detected in the xFe Fe-TiO2/ITO and it was considered to be associated to the oxidation/reduction of Fe(II)/Fe(III) along with that of space charge layer, CSP. The CIL values reached a maximum around V=-400- -300mV vs. SCE. The maximum value increased with increasung xFe up to 6000μF/cm2 for 20% Fe-TiO2/ITO, being significantly larger than CSP (10-100μF/cm2). It was confirmed that the behavior of CIL and the slow decline of electrode potential for xFe Fe-TiO2/ITO could be explained by the model which assumed that the ratio of Fe(II)/Fe(III) depends on electrode potential. A significantly large capacitance was also detected in the TiO2/xFe Fe-TiO2/ITO. The flatband potential, Efb, or onset potential, Eon, for TiO2/xFe Fe-TiO2/ITO decreased down to the similar values to those of TiO2/ITO, while those values of xFe Fe-TiO2/ITO were relatively noble. Based on these results, it was confirmed that the TiO2 coating with 2 layers (TiO2/xFe Fe-TiO2/ITO) combines both less noble photo potential for cathodic protection and good characteristic of slow decline of electrode potential after stopping illumination.
  • Alkaline Corrosion and its Prevention on the Inner Surface of Superheater Tubes in Municipal Refuse Incinerator

    pp. 177-183

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    Since around 1992, some accidents involving penetration from inner side of superheater tubes during operation have occurred at municipal refuse incinerators.
    This study presents that the investigation on corrosion and damage of the inner surface of superheater tubes of a municipal refuse incinerator. Pitting corrosion is observed at the lower part of surface of the inner superheater tube on horizontal type primary superheater tube. A relatively high concentration of potassium is detected in the scale which is condensed into the scale by potassium contained in the steam. It is suggested that the corrosion is classified to alkaline corrosion. When a part of boiler water contained an alkaline ion takes place carry-over owing to generating of large volume of steam exceeding separation capacity of steam water, the steam is introduced to superheater tube, so that the ion is condensed in water of the tube.
    Thin membranous scale cover over the inner surface of secondary superheater tubes, in which potassium is also detected in the scale. The case is also caused by alkaline corrosion. Since potassium ion contains in the original supplied water at the start of running, desuperheating water is contaminated by the ion. The potassium ion is concentrated in the scale during the operation.

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