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

Zairyo-to-Kankyo Vol. 46 (1997), No. 2

  • Investigation of Corrosion Resisted Electroplating Films for Electronic Component's Materials

    pp. 66-75

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    It was introduced the 2 reseach results of electroplating films which have high corrosion resistance for electronic component's materials.
    To prevent the corrosion of palladium-plated ferro-alloy bases for leadframes, the effect of pulse plating on nickel deposits used as the underlayer for palladium-plated layers was investigated. The nickel underlayers deposited using pulse current at a duty cycle of 9% improved the corrosion resistance of the palladium-plated ferro-alloy beses.
    The deposition conditions and corrosion resistance of amorphous Ni-Mo-W ternary alloy films from a citric acid bath were investigated. The crystal structure of the films was influenced by the pH of the bath, and their corrosion resistance was excellent.
  • Hot Corrosion Behaviour of TiAl with Salt in Artificial Sea-Water

    pp. 83-89

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    Hot corrosion behaviour of a TiAl which was applied with salt in an artificial sea-water was examined. Specimens were immersed in an artificial sea-water, water was evaporated and then hot corrosion tests were performed at 973-1273K for 18-144ks.
    The depths of corrosion loss in the salt-applied specimens were higher than those of the salt-free specimens. A linear relationship between the reaction rate constant and reciprocal of absolute temperature was obtained in the test of salt-free specimens but it was not obtained in the test of salt-applied specimens. X-ray diffraction analysis showed that TiS2 was present in the scales in addition to TiO2 and Al2O3. High quantity of Al and high pH value were detected in the solution obtained by hot-water extraction of a specimen after hot corrosion. Therefore, it was considered that part of Al2O3 of the scale was converted to sodium aluminate and dissolved into the salt. And scanning electron micrograph showed that the Al2O3 layer in the scale was highly porous.
    It is thought that the hot crrosion of TiAl under application of the salt in the artificial sea-water is accelerated because the formed Al2O3 layer is porous.
  • Evaluation of Corrosion Degradation of Roof Materials Using Full Color Image Processing Technique

    pp. 90-94

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    Full color image processing technique was applied to the measurement of degree of degradation of a roof material. Utilizing a three dimensional geometric algorithm in a RGB color space, a corroded portion could be analyzed ignoring the effect of shadows of the structure. The results of the present method correspond to the results using rating number by manual procedure. It is possible to make an automatic process for evaluation of roof materials. It id also possible to make a remote sensing process in the sky, combining a technique to take aerial photographs.
  • Behavior and Mechanism of Corrosion Protection for Galvanized Steels by Self-healing Effect of Chromate Coatings

    pp. 95-102

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    Although the exact mechanism of self-healing effect of chromate coating is still not clearly understood, this mechanism is known to prevent not only the corrosion of metal substrate under the chromate coatings but also from the coating defects. This research focuses on investigating the corrosion behavior of chromated electro-galvanized steels by using both the a.c. impedance method and the scanning vibrating electrode technique (SVET). The corrosion protection mechanism of the self healing effect was estimated by using XPS analysis.
    The self-healing effect was clearly identified with the observed increase of charge transfer resistance when conducting the impedance measurement and the markedly decrease of both the corrosion area and corrosion current during the SVET experiments.
    The results of XPS analysis indicated that the chemical conversion type chromate coating contains both Cr(III) and Cr(VI). It was found that the water-soluble constituent consisted of mostly Cr(VI). Moreover, the chromate film was observed on non-coated metal surfaces in aqueous solution also. This was because when the Cr(VI) ions were reduced at corrosion catholic region, the corrosion current increased at anodic region as the consequences thus rendering the passivation of the metal surface. The Cr (VI), was reduced at cathode, became Cr(III) which then reforming the chromate, the passivity film. Based upon these observations, the self-healing effect was confirmed via the effect of anode-inhibitor and filming type inhibitor.
  • Alloying Effects on Dissolution Rate of Crevice Corrosion for Austenitic Stainless Steels in 3%NaCl Solution at 80°C

    pp. 103-112

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    Chloride stress corrosion cracking (SCC) has been a problem for austenitic stainless steel in aqueous environments containing chlorides. Studies have found that SCC initiates only from a dissolving surface and under the condition that the crack growth rate is higher than the dissolution rate of the dissolving surface. Research conducted to improve the resistance to SCC for Type 304 steels (UNS S30400) has revealed that while molybdenum and phosphorus are unfavored, the alloying of 3% aluminum combined with 2% copper can almost nullify their detrimental effect. Based on the mentioned criteria, this study was dedicated to clarify the mechanism behind these alloying effects by examining the relationship between the measured enhancements on SCC resistance and the dissolution rate observed by the moire technique. It was found that the addition of both molybdenum and phosphorus reduces the dissolution rate and therefore impaired SCC resistance; the addition of copper increases the dissolution rate of the steady growth stage where crevice corrosion proceeds at a constant rate. Moreover this dissolution rate could further be increased when combined with the alloying of aluminum. These observed results correspond well to that of the measured behavior of the SCC critical temperature, Tc, suggesting that the SCC susceptibility is influenced by anodic dissolution.
  • Screening Tests on Materials of Construction for the Thermochemical IS Process

    pp. 113-117

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    Corrosion tests were conducted at two environments in an iodine-sulfur thermochemical water-splitting process. One is 50wt% sulfuric acid solution containing 0.1wt% hydriodic acid at 95°C and 120°C, and the other is gaseous mixture of HI/I2/H2O, the molar ratio of which is 1/1/6, at 200-400°C. In the former environment, Ta, Zr, Quartz glass, and Pb showed corrosion resistance. Although PFA showed corrosion resistance, iodine penetration was observed. In the latter environment, Ti and Hastelloy C-276 showed good corrosion resistance. Inconel 600 and 1Cr-1/2Mo also showed small corrosion rate. However, Inconel 600 suffered grain boundary corrosion.

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