how do iron galvanised
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Galvanising is a method of rust prevention. The iron or steel object is coated in a thin layer of zinc. This stops oxygen and water reaching the metal underneath - but the zinc also acts as a sacrificial metal. Zinc is more reactive than iron, so it oxidises in preference to the iron object.
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1. Liquid Cooling (Pouring Temp to Freezing Temp.)
2. Solidification Shrinkage (Liquid to Solid)
3. Solid Cooling (Freezing Temp. to Room Temp.)
The liquid shrinkage is rarely a problem, because the molten metal in the gating system continues to flow in mold cavity to compensate the shrinkage of liquid in the cavity.
As the metal changes its state from Liquid to crystalline solid, the new atomic arrangement is formed. The significant amount of shrinkage occurs in this stage and it varies from alloy to alloy. Not all materials contract upon solidification, some actually expand such as Grey Cast Iron (7340 kg/m3) where low density Graphite flakes (2226 kg/m3) are formed as part of solid structure. Thus Solidification shrinkage of Grey CI is negative (Expansion = -(Contraction), easy enough). This is also taken care (Partly or fully) of by careful design of mold and gating system that ensures directional solidification.
2. Solidification Shrinkage (Liquid to Solid)
3. Solid Cooling (Freezing Temp. to Room Temp.)
The liquid shrinkage is rarely a problem, because the molten metal in the gating system continues to flow in mold cavity to compensate the shrinkage of liquid in the cavity.
As the metal changes its state from Liquid to crystalline solid, the new atomic arrangement is formed. The significant amount of shrinkage occurs in this stage and it varies from alloy to alloy. Not all materials contract upon solidification, some actually expand such as Grey Cast Iron (7340 kg/m3) where low density Graphite flakes (2226 kg/m3) are formed as part of solid structure. Thus Solidification shrinkage of Grey CI is negative (Expansion = -(Contraction), easy enough). This is also taken care (Partly or fully) of by careful design of mold and gating system that ensures directional solidification.
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