ダイカスト用亜鉛合金の大気腐食特性に及ぼす合金元素の影響

抄録

  The effect of alloying elements on the atmospheric corrosion behavior of zinc die-casting alloys was investigated using the cyclic wet and dry test which simulated a diurnal change in specimen temperature and relative humidity in an actual outdoor environment. The deposition amount of chloride ions on the specimen surface was adjusted to 1.0×10^-3 and 1.0 g•m^-2 by forming of a uniform layer of diluted synthetic seawater. The addition of aluminum and mischmetal (MM) improved the corrosion resistance of zinc die-casting alloys, while that of Cu degraded the corrosion resistance. Polarization measurements indicated that Cu addition promotes the reduction reaction of dissolved oxygen. Corrosion products formed on Cu-containing specimens also promoted the cathodic reaction. On the contrary, the corrosion products on Al-containing and MM-containing alloys had the ability to suppress the anodic reaction. The value of average corrosion depth and the composition of corrosion products in the accelerated corrosion test were compared to those obtained by atmospheric exposure tests. It was concluded that the cyclic wet and dry test, in which diluted synthetic seawater is used as a test solution, is appropriate for an accelerated atmospheric corrosion test for zinc alloys.