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

Zairyo-to-Kankyo Vol. 49 (2000), No. 10

  • Corrosion Protective Surface Treatment for Aerospace Materials-Traditional Process and New Development

    pp. 579-584

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    Corrosion Protection for Aerospace Products is quite important from the point of flight safety and long term service. This document describes the corrosion protective surface treatment technologies such as Anodizing, Alodizing, Cadmium plating, Painting traditinally used in the Aerospace Industry. New trend of environmentally compliant coating technologies for surface treatment is also mentioned about.
  • An Overview of Research on Functionally Graded Materials for Reusable Launch Vehicle Applications

    pp. 585-590

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    Functionally Graded Materials (FGMs) are considered to have important applications for future reusable space transportation vehicles, which cyclically encounter extremely high heat loads in their engines and airframes. Under the actual heat loading conditions affecting such FGMs, it is necessary to evaluate the thermal fatigue properties of FGMs induced by macroscopic and microscopic thermal stress due to high temperature differences. The thermal fatigue characteristics of the FGMs with a diameter of 30mm by exposing them under the large-temperature-difference conditions have been studied in vacuum and hot gas environments.
    Several studies on FGMs are under way for practical applications. Thrusters made of Ni-ZiO2 FGM and C/C composite covered with SiC-C FGM layer have been studied. The durability was confirmed from the tests.
  • Current Oscillation and the Dissolutions of Fe (II) and Fe (III) on Fe in Sulfuric Acid Solution Containing Chloride Ions Investigated by Channel Flow Electrode

    pp. 600-606

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    A relation between the current oscillation of iron and the Fe (II) and Fe (III) dissolutions during the active-passive transition in sulfuric acid solution (pH1) was investigated by channel flow double electrode (CFDE). Firstly, potentiodynamic polarization curves of iron in the solutions containing various concentrations of chloride ions were measured in order to find experimental conditions where the current oscillation appeared. Secondly, the time-series of the current oscillation in the active-passive transition potential region were measured, and Fe (II) and Fe (III) dissolved from the iron electrode were detected simultaneously. It was found that not only the working electrode current but also the dissolution rates of Fe (II) and Fe (III) oscillated. In the solutions without chloride ion and containing low concentration of chloride ion, Fe (III) dissolves previous to the current oscillation, contrary to this, the dissolution of Fe (II) shows a delay from the current oscillation. On the other hand, in the solution containing chloride ions more than 0.02mol/dm3, passivation was not observed on the polarization curve. In the high chloride concentration, the dissolution rates of both Fe (II) and Fe (III) show the delay from the current oscillation. On the basis of the above mentioned results, the current oscillation mechanism of the iron and the influence of chloride ion on the current oscillation were discussed.
  • Property of Pitting Corrosion of Steels Coated by Layered Oxides with Sputtering Method

    pp. 607-611

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    Double-layer coated films consisting of oxides, which possess different pHpzc, were deposited on stainless steels using RF magnetron sputtering method. To investigate the effect of the coating order of these oxides on pitting property, anodic polarization measurements in buffer solutions containing 1.0kmol/m3 NaCl and SIMS analysis were performed. The solution pH was adjusted between the values of pHpzc of both oxides so that one has positive surface charge and the other has negative surface charge.
    In the case of the specimens composed of Al2O3 and TiO2 as well as SiO2 and Al2O3, higher pitting resistance is obtained for the specimens whose upper layers possess lower value of pHpzc than lower layers. In the double-layer coated films, the distribution of chloride ions is assumed to depend on the coating order by the effect of surface charge. However, chloride ions as impurities were found to concentrate in the films originally. Therefore, the accurate change of the distribution of chloride ions that immigrate into defects from the solution was not observed in the cause of the background formed by impurities.
  • Corrosion Characteristics of Carbon Steel in Simulated Boiler Feedwater under Oxygenated Treatment Conditions

    pp. 612-618

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    In the simulated once-through type boiler feedwater under oxygenated treatment (OT) conditions, the fundamental corrosion characteristics of carbon steel for boiler tubes in fossil power plants were experimentally investigated, comparing with those under the all volatile treatment (AVT). The corrosion weight loss under the OT conditions was less than about 1/2 of that under the AVT condition. The dissolution ratio of the AVT scale (Fe3O4) into the high temperature AVT water showed the very large value of approximately 30%. On the contrary, the dissolution ratio of the OT scale (Fe3O4+α-Fe2O3) into the OT water was almost zero. The cross sectional morphology of OT oxide films showed the duplex structures consisting of the outer layer with the grain size of 1-2μm and of the inner layer with the grain size less than 0.1μm and also with thickness less than 1μm. These results indicate that the OT oxide film was very thin and tight in comparison with the AVT film.
    From the view point of the above fundamental corrosion characteristics, it is suggested that the OT for the feedwater is superior to the AVT concerning the corrosion resistance of the carbon steel.
  • Effects of Microstructural and Mechanical Factors on Crack Propagation Rate of Hydrogen Embrittlement in Ti-10V-2Fe-3Al Alloys

    pp. 619-624

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    The crack propagation rate VII in the region II of the relation between crack propagation rate and stress intensity factor has been formulated on the base of a theory of stress-induced hydrogen diffusion proposed by Gerberich. In present study, VII has been evaluated from microstructural factors in Ti-10V-2Fe-3Al alloy. Two different temperatures, i.e. higher and lower temperatures than both β transition temperature and martensite transformation starting points, were used for solution treatment. The difference of VII is about one order between two types of materials. The microstructure of the materials solution-treated at this high temperature consists of very fine α precipitates on the matrix β, while the other materials show relatively coarse α precipitate and primary α phases in the β matrix. VII has been understood to be mainly dependent on hydrogen diffusion coefficient D from the formulation. This study concludes that D is reduced and as a result VII is decreased by the many hydrogen trapping sites on the fine boundaries created through the martensite transformation in the solution treatment.
  • Photoeffect on Corrosion Behavior of SrTiO3-Coated Galvanized Steel

    pp. 625-631

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    The n-type semiconductor SrTiO3 was coated on the hot-dip zinc coated steel (Z18). Different from the sacrificial zinc coating, the coating acted as a stable photo-anode to offer non-sacrificial catholic protection to substrate steel. With ZnO formed on the galvanized steel by anodizing in 0.1M NaOH aqueous solution, a good photoeffect was found. With the sol-gel derived SrTiO3 additionally coated on ZnO, the photoeffect was further improved. The open-circuit potential was maintained less noble even after stopping illumination for the coating with intermediate Fe-doped SrTiO3 layer, SrTiO3/Fe-SrTiO3/ZnO/Z18. Moreover, photodecomposition that appeared for ZnO-coated galvanized steel could be prevented by the SrTiO3 coating. The couples of SrTiO3//Fe (Fe coupled with SrTiO3/Fe-SrTiO3/ZnO/Z18) and Zn//Fe (Fe coupled with Z18) were developed as ACM (atmospheric corrosion monitor) sensors to evaluate the corrosion behavior of galvanized steel with and without SrTiO3 coating in atmosphere. For the SrTiO3//Fe sensor, an anodic output was detected in the dark to show dissolution of Fe electrode, while it changed to cathodic one upon illumination. The cathodic value was larger than that of Zn//Fe sensor under certain conditions of lower amount of deposited sea salt and lower relative humidities, where substrate steel could be protected from rusting with the non-sacrificial coating.

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