ダイカスト用 Zn-Al 合金の大気腐食特性に及ぼす第三元素添加の効果

抄録

  The effects of third element addition on the atmospheric corrosion resistance of zinc die-cast alloys were investigated by using the cyclic wet and dry test which simulated a diurnal change in specimen temperature and relative humidity in an actual outdoor environment with a constant dew point of 301 K. The amount of chloride ion deposition on the specimen surface was adjusted to 1.0×10^-3 g·m^-2 by the formation of a uniform layer of diluted synthetic sea water. The addition of magnesium (Mg) improved the corrosion resistance of zinc alloys, while the addition of nickel (Ni) degraded the corrosion resistance. X-ray photoelectron spectroscopy measurements indicated that corrosion products formed on Mg-added alloys contained Mg compounds such as oxide, hydroxide, and carbonate. Electrochemical polarization curves of as-polished specimens indicated that the addition of Mg has little effect on anodic and cathodic reaction rates. On the contrary, the addition of Ni or Ti promoted cathodic reaction. The polarization curves measured on specimens after 1 cycle of the cyclic corrosion test indicated that the corrosion products formed on Mg-containing alloys leaded to anodic passivation and inhibited cathodic reaction, while those on Ni-containing alloys accelerated cathodic reaction. Scanning electron microscopic observation on specimens after 1 cycle of the cyclic corrosion test showed that Mg-containing alloys had smooth and uniform surfaces with dense corrosion products, while Ni- or Ti-containing alloys had rough surfaces. It is concluded that the formation of thin and dense corrosion product layer is responsible for the high corrosion resistance of Mg-containing alloys.