On the Grain Boundary Oxidation of Aluminium Brass in High-Temperature Water and Steam

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  • アルミニウム黄銅の高温水および蒸気による粒界酸化について
  • アルミニウム オウドウ ノ コウオンスイ オヨビ ジョウキ ニ ヨル リュウカイ サンカ ニ ツイテ

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Feed heater tubes of aluminium brass used for high temperature and high pressure boilers are liable to intergranular corrosion in hot water or steam. On this problem of intergranular corrosion we have conducted fundamental researches with tubes used in the actual plant and with pieces heated in autoclaves, using an electron microscope and the electron or X-ray diffraction methods. The following conclusion has been reached. (1) The intergranular corrosion products detected by the electron diffraction method were ZnAl2O4, CuAl2O4, γ-Al2O3, ZnO, Cu2O, CuO, etc., and the larger part of these products were Zn or Al oxides. It was also found that the atom density of the alloying element in the vicinity of the grain boundary was reduced to a level closely approximating that of pure copper. (2) In the analyses of the metal phase of the oxidized region by the X-ray diffraction method, it was found that the lattice constant did not change, and decrease in the amount of Zn was noticed. (3) From the above results, it is inferred that the dissolved oxygen in hot water and steam penetrates into the inside of the metal through grain boundaries and combines with elements having large affinity to oxygen such as Zn or Al, that is, the intergranular corrosion of aluminium brass in hot water or steam arises from the grain boundary oxidation.

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