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

Zairyo-to-Kankyo Vol. 45 (1996), No. 2

  • Cathodic Corrosion of Chromium in Molten Na2SO4

    pp. 60-66

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    The corrosion behavior of chromium under catholic polarization in molten Na2SO4 at 1173K has been investigated. The loss of chromium under catholic polarization in the molten salt increased with an increase in the quantity of electricity for the cathodic reaction. In order to examine the mechanism of catholic corrosion of chromium in the molten salt, direct observation of the chromium electrode under catholic polarization and identification of corrosion products were conducted. A black turbidity near the chromium electrode was observed in the melt during catholic polarization, suggesting a generation of corrosion product from the electrode. It was found that the corrosion product consisted of Cr2O3. Investigation of the cathodic polarization reaction showed that a two-electron reduction reaction of SO42- occurred under catholic polarization in molten Na2SO4. Consequently, it was proposed that catholic corrosion for chromium in molten Na2SO4 occurred by a chemical reaction of the chromium with SO32- and O2- which appeared as catholic products.
  • Effect of Dissolved Oxygen on the Dissolution Rate of Magnetite in EDTA Solutions

    pp. 67-74

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    The dissolution rate of sintered Fe3O4 pellets in 3mol/m3 EDTA (H4Y) solutions with and without dissolved oxygen (DO) was measured at different pHs (1.1-4.6) at 80°C. The dissolved Fe concentration increases with time in a parabolic manner, exceeding the EDTA concentration in the absence of DO but not exceeding the EDTA concentration in the presence of DO. The rate of dissolution changes with pH, with and without DO. The dissolved Fe concentration vs. pH curves for different dissolution times show maxima at pH 2.4, and the rates without DO are always higher than those with DO. It is assumed that the dissolution reaction involves two coupled processes: (a) the transfer of Fe ions by reaction with EDTA to form FeIIIY- and FeIIY2-; and (b) the transfer of lattice O2- ions by reaction with H+ ions to form H2O. In the absence of DO, the formed FeIIY2- reacts with Fe3+ to dissolve further Fe3O4, and can be regarded as equal to free EDTA. The rate equation explaining the dissolution without DO is derived as follows:
    d[Fe]/dt=k′k″[H+]8/3α([Y]T-2[Fe]/3)/(22/3/3)k′[Fe]+k″[H+]8/3 (1)
    where [Fe]=[FeIIY2-]+[FeIIIY-], k′ and k″ are composite rate constants, α is the fraction of Y4- in free EDTA, and [Y]T is the total concentration of added EDTA. In the presence of DO, the formed FeIIY2- is instantly oxidized to FeIIIY-, and the rate equation for this condition was reported elsewhere. The rate equations for conditions with and without DO reproduce the time-courses of [Fe] well, and the calculated [Fe] vs. pH curves show peaks at pH 2.4. The rate constants k′ and k″ are not affected by DO, and the effect of DO on the dissolution rate is due to different “effective” EDTA concentrations: with DO, the dissolved Fe is FeIIIY-, which does not take part in further dissolution, without DO, one third of the dissolved Fe is FeIIY2-, this can dissolve further Fe3O4 to form FeIIIY- and Fe2+.
  • Hydrogen Permeation Behavior of Pure Nickel Implanted with P, S, B and Bi Elements

    pp. 75-82

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    The entry and transport of hydrogen in phos-phorous-, boron-, sulfur- and bismuth-implanted pure nickel specimens with a fluence range of 1×1015 to 1×1017/cm2 have been investigated using an electrochemical permeation technique. The steady state permeation current density increases with increasing fluence for the implanted specimens except for sulfur-implanted specimens. The break-through time of the transient curves is found to depend largely upon fluence and implanting elements. In addition, a removal method for dissolving implanted layers using 0.2% HF solution is applied to investigating an effect of implanted elements themselves on hydrogen permeation transients. The results are explained by considering shallow and deep hydrogen traps, compressive stress, the change in phase structure of implanted layer and so on.
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    1. Influence of the Alloying Elements of Fe-Cr-Ni-Al alloys on the Corrosion Behavior in Molten Carbonate Zairyo-to-Kankyo Vol.45(1996), No.4
  • Effect of Sigma Phase on Corrosion Resistance of Duplex Stainless Steels

    pp. 83-89

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    Austenitic/ferritic duplex stainless steels, which contain 22-25%Cr-3%Mo-5-7%Ni-0.15%N, show excellent corrosion resistance in many corrosive environments. When duplex stainless steels were heat-treated, an intermetalic precipitate such as sigma phase was formed. In this paper, effect of sigma phase on corrosion resistance of duplex stainless steels was studied. Precipitates of sigma phase did not affect intergranular corrosion resistance of duplex stainless steels in the Huey and the Streicher tests. In the Strauss test, cracking was observed in test specimens in which sigma phase precipitates, when bending even before corrosion test was carried out. On the other hand, sigma phase greatly affected pitting corrosion resistance. Corrosion rate in Cl- ion including solution increases with area fraction of sigma phase. General corrosion resistance must be determined by Cr content in the fresh austenite phase which precipitated with sigma phase spontaneously. As a result, pitting corrosion test and bending test were the most sensitive methods in detecting sigma phase precipitate. High ferrite phase content of duplex stainless steels leads to prevention of sigma phase formation and improves pitting corrosion resistance of heat-treated materials.
  • Effect of Cr Carbo-nitride on Corrosion Resistance of Duplex Stainless Steels

    pp. 90-96

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    Austenitic/ferritic duplex stainless steels, which contain 22-25% Cr-3%Mo-5-7%Ni-0.15%N, show excellent corrosion resistance in many corrosive environments. When duplex stainless steels were heat-treated, Cr carbo-nitride which was Cr nitride including C was formed. In this paper, effect of Cr carbo-nitride on corrosion resistance of duplex stainless steels was studied. Cr carbo-nitride affected intergranular corrosion resistance of duplex stainless steels in the Huey and the Streicher tests. Cr depleted zone formed with Cr carbo-nitride will lead to intergranular corrosion in such tests. In the Huey test, sigma phase behave as protective even when a large quantity of Cr carbo-nitride precipitated. Cr carbo-nitride did not affect intergranular corrosion resistance in the Strauss test and general corrosion resistance. On the other hand, Cr carbo-nitride a little affected pitting corrosion resistance. As a result, the Streicher test was the most sensitive method in detecting Cr carbo-nitride. Reducing C, Si, Mn, P and S of duplex stainless steels lead to prevention of Cr carbo-nitride precipitation and improve intergranular corrosion resistance of heat-treated materials.
  • Corrosion Behavior of Aluminum Overlay Weld Alloys with Dispersed NbC Particles

    pp. 97-103

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    The aluminum overlay weld alloys with dispersed niobium carbide particles (NbCp/Al) were prepared by a plasma transferred arc welding process. The corrosion behavior of the NbCp/Al alloys was studied in a sodium chloride solution by means of immersion test, electrochemical techniques and scanning electron microscopy. The aluminum alloys used were pure Al, Al-Mg, Al-Mg-Si and Al-Cu. The addition of NbC particles shifted corrosion potentials of these alloys to positive direction. However, the pitting potentials were almost similar to that of overlay weld alloy without NbC particle. On immersion test in quiescent 0.5mol/l NaCl open to the air, the preferential localized corrosion for was observed all NbCp/Al alloys at the matrix-NbC particles and crystalline material interfaces.

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