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

Zairyo-to-Kankyo Vol. 47 (1998), No. 1

  • Roles of Crevice Corrosion on Stress Corrosion Cracking for Austenitic Stainless Steels in Neutral Chloride Solutions

    pp. 2-14

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    Corrosion cracks initiated in the potential zones, zone I (active range) and zone III (pitting region), in 1N H2SO4 solution containing 0.5M NaCl at 25°C under pulsating load to promote crack initiation. Based on these facts the following conditions were found to be required for the cracks to initiate: a) cracks must occur at a dissolving surface with {100} faceting, and b) crack growth rate, C, is faster than the dissolution rate, Ih, at the surface. In practical neutral solutions, the dissolving surface specified in the conditon (a) consists of large growing pits or a corroding crevice, depending on the Cl- concentration. The conditions (a) and (b) were also confirmed to apply to SCC initiation from pits under static stress for type 304 steel in 25% MgCl2 solution at 80°C. A circumferential notch, introduced into a round type 310S steel bar as both corrosion crevice and stress intensifier, extended the SCC range to both less noble potential region and lower concentration range of MgCl2 solutions at 80°C when compared to SCC initiated from pits on a smooth specimen. By decreasing the bulk Cl- concentration, it becomes difficult for cracks to start at pits where Ih markedly increases. However, as Ih in crevice is lower than in pits and the condition (b) is readily satisfied with a lower dissolution rate Ih<C, a corroding crevice becomes a good initiation site. SCC initiations in lower Cl- solutions were demonstrated using c) a tapered double cantilever beam specimen of type 316 steel with an artificial slit crevice, and d) a spot-welded specimen with both crevice and residual stresses for improved SCC resistant 304 steels. By using the spot-welded specimen maintained slightly above the crevice repassivation potential, it was possible to evaluate the sensibilities of the relevant steels at a given solution/temperature with no additional applied stress.
  • Effects of Aluminizing on Tensile Property and High-Temperature Oxidation of TiAl-base Alloys

    pp. 29-35

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    The influence of aluminizing treatment on oxidation resistance at 1173K in the atmosphere and on room temperature ductility of TiAl based alloys with a good room temperature ductility has been examined. The quaternary alloys containing of 2mol%Nb, Ti-48 Al-2Nb-2Cr and Ti-48Al-2Nb-2Mn, show better oxidation resistance than Ti-50Al binary alloy and/or Ti-47Al-2Cr-0.2Si quaternary alloy. An aluminizing treatment of 1173K×18ks forms an coating layer of the TiAl3 phase of approximately 30μm in thickness. The coating layer facilitates the formation of the Al2O3 phase in the initial stage of oxidation. The aluminizing treatment improves the oxidation resistance of the binary and quaternary alloys, irrespective of their composition. The quaternary alloys with aluminizing treatment have better room temperature elongation than the Ti-50Al with or without the aluminizing treatment, though room temperature ductility of all the alloys used decreases with the aluminizing treatment.
    It has been concluded that the aluminizing treatment for quaternary alloys of Ti-48Al-2Nb-2Mn and Ti-47Al-2Cr-0.2Si is beneficial to application of these alloys under the oxidizing atmospherical circumstances at high temperature of 1173K.
  • The Protective Effectiveness of Cathodic Protection for Sand Erosion in Waves Sea

    pp. 36-41

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    The catholic protection have been applied to steel structures located in submerged zone of sea wave environments. The purpose of this study is to examine the protective effectiveness of catholic protection and steel potential behavior of steel affected by sand erosion at sea bottom. The study described in this paper was carried out by using existing steel pipe pile structure and test pieces. From the test results, it was found that protective effectiveness of cathodic protection was recoginized even for the steel under sand erosion. Also, it was found that the behavior of steel potential was affected by the sand drift at sea bottom caused by (significant) wave.
  • Cyclic Oxidation Behavior of Fe-20Cr-4Al Alloys with Small Amounts of Sulfur at 1473K

    pp. 42-48

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    Cyclic oxidation behavior of Fe-20Cr-4Al alloys with 3, 35, 53, 104 and 171ppm of sulfur was studied in oxygen at 1473K. Cyclic oxidation consisted of a 18.0ks exposure and was performed by cycling between 1473K and room temperature. This oxidation-cooling cycle was repeated up to five times. Mass changes of the alloys with 3, 104 and 171ppm of sulfur increased roughly with increasing cycle and showed positive values after each cyclic oxidation. However, mass changes of the alloys with 35 and 53ppm of sulfur showed negative values after each cyclic oxidation, and was consistent with spallation of the oxide. Amounts of spalled oxide of the alloys with 35 and 53ppm of sulfur increased after 1 and 2 oxidation cycles, and then decreased after 3 or more oxidation cycles. Observation by a scanning electron microscope of both sides of the spalled oxide indicated that spallation maybe related to morphology of the surface oxide and to the morphology and volume of cavities present in the oxide/alloy interface. Many particles of chromium sulfide were observed at the oxide/alloy interface of the alloys with 104 and 171ppm of sulfur after 5 oxidation cycles. It is postulated that the existence of many particles of chromium sulfide probably plays a role in improvement of the oxide adherence.
  • The Critical Depth for Pits on Carbon Steel in the Concentrated Aqueous Solution of Lithium Bromide Containing Chromate at Elevated Temperatures

    pp. 49-57

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    Pitting corrosion behaviors for carbon steels in the working fluid for absorption heating and refrigerating systems were investigated. The fluid was an aqueous solution of 60% LiBr+0.2%LiOH+0.2%Li2CrO4. The testing temperature was 150°C which was 4°C below the boiling point. The critical depth concept which was developed for stainless steels in neutral chloride solutions was applied. The Vc, pit, Ez and ER, pit were measured to be -490, -540 and -550mV, respectively. For potentials above Vc, pit, pits initiated and continued to grow. For potentials between Vc, pit and Ez, the passive film broke down but the resulting depressions were found to repassivate before exceeding the critical depth r* which was determined to be 20μm. Nevertheless, pits deeper than 30μm continued to grow for potentials above ER, pit. When a carbon steel merely immersed in the solution, micro-pits which were shallower than r* generated intermittently at potentials above Ez. The dissolution current prevented the Ecorr from ennobling above the Vc, pit, and consequently the initiation of a growing macro-pit was inhibited. However, when carbon steel was held at -800mV for more than 10h in the solution prior to the open-circuit immersion, one growing macro-pit initiated during the initial period of the immersion. Because the resistance to film breakdown and cathodic reaction on the steel surface were increased with the pretreatment, the open-circuit potential for the initial period was maintained more noble than Vc, pit and one pit grew beyond r* before the Ecorr fell below Vc, pit. The pit was found to continue growing at the following Ecorr which was maintained above ER, pit.
  • Material Factors Affecting the Susceptibility to Intergranular Corrosion of Alloy 600 in the Transpassive Corrosion Potential Region

    pp. 58-65

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    In the present study, it has been reported on metallurgical factors like C, P, Cr contents and sensitization temperatures which affect the susceptibility to intergranular corrosion (IGC) of Alloy 600 in the transpassive corrosion potential region above 1200mVSHE. The Cr depleted zone at grain boundaries (GB) in Alloy 600 was found to cause no IGC, as was in stainless steels. On the other hand, Ni-P compound precipitated at GB caused a remarkable increase of IGC susceptibility in Alloy 600, as was in stainless steels.
    Moreover, Cr carbide precipitated at GB was found to increase IGC susceptibility due to the Ni-P compound in Alloy 600. On the contrary, it was found to decrease that in stainless steels.
  • The Site Tests for Protective Effectiveness of under Adhesion Marine Organisms for Cathodic Protection

    pp. 66-69

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    The catholic protection have been applied to submerged part of marine steel structures. However, pitting corrosion was recognized on the steel surface under adhesive marine organisms even under Cathodic protection. It was not clear whether such pitting corrosion was occurred due to insufficient protective current or before the application of catholic protection. In order to clarify the reason, the site tests was carried out. From the test results, it was found that the catholic protection can stop the corrosion generated during of no protection. Also, it was found that adhesion of marine organisms on steel surface were harmless to protective effectiveness of catholic protection.
  • The Influence of Tin Coating Weight and Reflow Treatment on the Filiform Corrosion of Lightly Tin Coated Steel Sheet

    pp. 70-72

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    Lightly tin coated steel (LTS) is widely used for the welded can, the filiform corrosion is the important performance of LTS.
    The tin coating weight and reflow treatment to improve the filiform corrosion resistance has been studied, and the following facts are revealed
    The filiform corrosion is greatly improved by the increase of the tin coating weight and the reflow treatment. These factors increase the cathodic polarization, so that the reaction of oxygen reduction is restrained. This is considered to be the reason why the process of filiform corrosion is impeded.

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