Thursday, April 2, 2009

Electroplating of Silver, Gold, Copper, Zinc and Cadmium

Coating Metal for Electroplating Process

You can plating metal surface using Silver, Gold, Copper, Zinc or Cadmium. Those materials are chosen because of their characteristic of stand to corrode. It is convenient to deal with the electroplating of all the above metals together, because the same type of solution is used in each case, viz, the double cyanide solution. Cyanides are the salts of the very weak hydrocyanic acid (prussic acid). The cyanides of sodium and potassium are soluble in water, but the cyanides of the heavy metals are insoluble; nevertheless these insoluble cyanides readily dissolve in a solution of sodium of potassium cyanide to form a double salt.

NaCN + AgCN → NaAg(CN)2 ↔ Na+ + Ag(CN)2-

This double salt dissociates in solution to yield sodium ions and a negatively charged double cyanide ion which contains the heavy metal. Although most of the heavy metal is bound up in the anion and not available for plating, this double cyanide ion further dissociates to be in equilibrium with a very small amount of positively charged heavy metal ion, which are available for plating.

As they are used up by deposition, more are immediately provided by further dissociation of the double cyanide anion. The advantages of this type of electroplating solution over solutions of simple salts of the metal concerned are twofold; firstly, the deposit is of much finer grain size, and hence is smoother and more reactive metal, when this is first placed into the plating solution.
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