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

Zairyo-to-Kankyo Vol. 53 (2004), No. 12

  • States of Hydrogen in Materials I

    pp. 548-553

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    Fundamentals of hydrogen behaviors in materials are reviewed with respect to degradation of mechanical properties. The present article is the first part that deals with the states of hydrogen in solid solution. The temperature dependence of the solubility, permeability and technical problems particularly of electrochemical permeation measurement have been explained. The interstitial site of hydrogen and the symmetry of lattice strain have been examined.
  • Corrosivity Evaluation of Freshwater Environment on Stainless Steels Using Mahalanobis-Taguchi Method Considering Interaction Effect of Water Quality Parameters

    pp. 560-567

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    The corrosivity of freshwater on stainless steels is evaluated using the Mahalanobis-Taguchi System (MTS). At first, water quality parameters, such as pH, hardness and concentration of SO42-, Cl- are collected from a group of non-corrosive water of various regions (reference water). In MTS analysis, any unknown water can be judged whether it belongs to the group of the reference water or not according to its Mahalanobis distance (MD). MD of unknown water, a positive scalar value, is calculated from its water quality parameters and the inverse matrix of correlation matrix made from the reference water data.
    In case that the MD of unknown water becomes less than about 3-4, the water is evaluated to belong the group of reference water, that is, non-corrosive. On the other hand, when the MD becomes larger than 3-4, the water is assumed not to belong to the reference water group and is corrosive.
    In this study, interaction effects of water quality parameters are taken into consideration in MTS analyses. Parameters used in corrosivity evaluations are selected from six analytical variables (pH, conductivity, SO42-, Cl-, M-alkalinity and total hardness) and their interaction parameters that are calculated as products of any two of them. Present conclusions are (1) a number of misjudgments in corrosivity evaluation can be reduced remarkably by considering the interaction parameters of the reference water, (2) the corrosivity of unknown freshwater data that has been obtained can be correctly judged by using the proper set of 11 parameters, and (3) the parameters selected according to the lager the better type SN ratio aren't always optimal but are practically effective to some extent.
  • SCC Behavior of Low-Pressure Steam Turbine Materials for Fossil Power Plants under Conditions Coexisted with Oxygen and Corrosive Chemicals

    pp. 568-575

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    The materials of the attachment near the last stage of fossil power plant low-pressure steam turbines will be exposed to the severe corrosion condition due to the concentration of corrosive chemicals by the alternating dry & wet environment, of which phenomenon will be caused by the frequent shutdown and the load change of power plants. The SCC behavior of low-pressure steam turbine materials is evaluated by four kinds of SCC tests and the electrochemical corrosion methods. For all materials “3.5NiCrMoV, 12Cr and 17-4PH steels”, the ion which induced the most severe SCC sensitivity was the chloride ion (Cl-), the next severe one was the sulfate ion (SO42-) and the sodium ion (Na+) showed almost no SCC sensitivity. Especially, the coexistence of the chloride ion and oxygen accelerated remarkably the SCC sensitivity of all materials tested. From the consideration between the SCC sensitivity and characteristics of the corrosion film, it was shown that the Cr content in the materials controls the SCC sensitivity and also the corrosion potential. This is due to the formation of the passive Cr2O3 film.
  • Fundamental Study on Monitoring of High Temperature Oxidation of Carbon Steel

    pp. 576-582

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    An attempt has been made as a fundamental study on monitoring of high temperature oxidation of carbon steel with an AC impedance method. A typical environment of hot air at 873K was preferred for acceleration tests of oxide film growth. Two specimens of carbon steel were faced with each other with a narrow gap, which was placed by thin mica sheets at 873K. Then impedance measurements were carried out in a frequency range from 60Hz to 1MHz. It was recognized that both electric resistance and capacitance not only of hot air within the narrow gap but also of the oxide films formed on the surfaces gradually changed with the oxide film growth. During the film growth, the dielectric constant of the hot air was nearly constant. On the other hand, the changes in the dielectric constant of the oxide films were observed, which suggested a time dependence of the oxide film growth. The impedance measurements also detected the film delamination caused by thermal stress. These results suggested a good feasibility of monitoring of high temperature oxidation by the AC impedance method.

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