Shiro Ogawa and Denjiro Watanabe
When nickel plates for vacuum tubes are annealed in hydrogen gas at 900∼1000° in the process of production, their surfaces are often tarnished and show milk-white appearance, which considerably lowers their commercial value. In order to infer the cause of this tarnishing, six kinds of plates were selected and their surfaces were examined by electron diffraction after the heat-treatment. Milk-white surfaces gave generally diffuse rings, which were entirely different from the nickel pattern. Many rings were obtained from two kinds of tarnished surfaces,but another tarnished surface gave only one diffuse ring. Three specimens did not alter their appearances after the treatment. One of them gave a few diffuse rings and elongated spots, while the other two, one of which was made in U.S.A., yielded many comparatively sharp rings. According to our analysis,rings obtained from the two milk-white surfaces were due to(Mg,Ca)SiO3 and NiO,and those from the two lustrous surfaces belonged to nickel and Mg2SiO4 in the one case and to nickel, MnO, MnS and NiO or to nickel, Mg2SiO4, MnO and NiO in the other case. From the fact that a great part of diffraction rings of the milk-white surfaces can be explained as being due to (Mg,Ca)SiO3, it is concluded that the tarnishing is due to silicates of light metals. They were also found on the lustrous surfaces, but because of their minute quantity, appearances of the surfaces were not altered by the heat-treatment. An apparition of the oxide and sulphide of manganese in some cases was due to a comparatively large quantity of this element in the specimen in question. Silicates like (Mg,Ca)SiO3 and Mg2SiO4 must be removed at the time of melting, but if a small quantity of them remains and is dispersed over the whole material, it may gradually aggregate at grain boundaries by the heat-treatment, which finally causes the tarnishing of surfaces.
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