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

Zairyo-to-Kankyo Vol. 40 (1991), No. 4

  • Influence of Mo Content Variation within Standard on Localized Corrosion of Type 316 Stainless Steel in Chloride Solution

    pp. 235-238

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    The influence of Mo content variation approved in the standard in Type 316 stainless steel was investigated electrochemically to the localized corrosion characteristics in chloride solution. The shift in pitting potential Vc′ 10, which corresponds to film breakdown, was changed a little by decreasing content of Mo in the regulated range, but magnificently in Vc′ 100, corresponding to a primary dissolution stage of the pitting. Anodic polarization characteristics of active pit anode, measured by using an artificial pit, were affected remarkably by the difference of content of Mo in 2% to 3%. Dissolution current of the active pit anode at a same potential increased in the magnitude of about 5 times in 2% Mo larger than in 3% Mo. These facts suggest that the Mo content variation approved in the standard do not affect to initiation stage of pitting so much, but strongly to propagation of pitting, which is similar to dissolution process of crevice corrosion and SCC. Recent 316, of which Mo content is controlled strictly at a lower limit in the standard, has a poor resistivity against the localized corrosions in chloride media.
  • Prediction of Intergranular Corrosion Sensitivity for SUS 304 Steel Having both Inter- and Intragranular Corrosion

    pp. 239-246

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    This paper deals with the method to separate the degree of sensitization at the grain boundaries from the sensitized specimen that has both inter- and intragranular corrosion. The specimens used in this experiment were type 304 stainless steel plates having 0, 5, 15 and 30% plastic strain. Isothermal heat treatments were used for the sensitizing heat treatments. The degree of sensitization was obtained from the EPR test. The degree of sensitization at the grain boundaries separated from the specimen that has both inter- and intragranular corrosion was the same value with the specimen having the same degree of sensitization caused only by intergranular corrosion without intragranular corrosion.
  • Corrosion Behavior of Stainless Steels in an Alternating Dry-Wet Environment with SO2 Gas

    pp. 247-253

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    In order to obtain the basic guideline for selecting materials used for smokestack exposed in a water dew point environment with SO2 gas (500, 2000ppm), corrosion tests of various kind of stainless steels were conducted by use of a laboratory made alternating dry-wet gas corrosion testing apparatus. Austenitic stainless steels, duplex stainless steels and superalloys showed very high corrosion resistance at a corrosion rate of less than 0.05g/(m2·h). While, low-alloy steel and ferritic stainless steels were less resistant against corrosion at corrosion rate of higher than 0.4g/(m2·h). In a water dew point environment with SO2 gas, the corrosion index (SO2/water dew point corrosion index), S.C.I=[Cr]+2.1[Mo]+1.2[Cu] was obtained. Stainless steels with S.C.I of higher than 20 were highly resistant to corrosion. Surface film formed on the austenitic stainless steels in the thickness range of hundreds to thousands Å were mainly consisted of oxides. It was observed that austenitic stainless steels became higher resistant to corrosion, the thinner the surface film was.
  • Cathodic Reduction of Passive Films on Cobalt in a Neutral Solution

    pp. 254-260

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    Cathodic reduction behaviors of passive films on cobalt have been studied in sodium borate-boric acid buffer solution of pH 8.39. The amount of dissolved Co2+ ion during the catholic reduction has been measured by channel flow double electrodes. On the anodic polarization curve of cobalt there are two passive regions, one of which is passive region I from -0.3 to 0.2V and the other the passive region II from 0.3 to 0.8V. During the cathodic reduction of the passive film formed in region I, which probably consists of a hydrated CoO, no dissolved Co2+ ion is observed. During the cathodic reduction of the passive film formed in region II, which mainly consists of Co3O4, Co2+ ion dissolves from the film. However, the passive film formed at the higher potential of passive region shows an induction period in its cathodic dissolution as Co2+, attributing to the reduction of Co2O3 to Co3O4 in solid phase. The current efficiency for the reductive dissolution from Co3O4 to Co2+ is independent of the applied cathodic current density and increases with increase of electrolyte flow rate. The cathodic reduction mechanism has been estimated from an assumption that a partial diffusion of Co2+ to bulk solution and precipitation of Co(OH)2 onto cobalt simultaneously take place.
  • Serious Effects of Chlorofluorocarbons on Global Environment and Contermeasure Technologies

    pp. 261-270

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    Chlorofluorocarbons (CFCs) and chlorinated hydrocarbons are considered to contribute to depletion of stratospheric ozone layer and global warming.
    The following scientific knowledges concerning the global environmental problems and countermeasure technologies are reviewed. 1) Formation of ozone layer, especially some key chemical reactions, 2) Mechanisms of ozone layer depletion by CFCs and chlorinated hydrocarbons, 3) Trends of ozone concentration and Antarctic ozone hole phenomena, 4) Phase out of CFCs and chlorinated hydrocarbons, 5) Technologies to meet the CFCs phase out situation.
  • Corrosion of Iron and Zinc in Concrete

    pp. 271-278

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    Corrosion characteristics of iron and zinc in imitated cement solution or concrete are explain with example. Corrosion behavior and its protection are consider on the basis of electrochemical potential theory. Spread paint to deposited zinc surface is more effective for corrosion prevention of reinforced concrete.
  • The Effect of Cutting Conditions on the Corrosion of Stainless Steels

    pp. 289-290

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