Sunday, June 7, 2009

Tin Plating

Tin is a very white metal with high reflective in the polished or freshly solidified condition. It has high resistance to corrosion especially against fruit acids, milk products and other foodstuffs, and is not attacked by sulfur compounds. Its salts are not poisonous. The metal is soft, easily melted and readily soft soldered. It is thus ideally suited as a coating metal for steel and copper vessels used with foodstuffs, and has been so used for centuries. The demand has now almost outrun the supply so that has become a very expensive metal. Although tin is cathode to steel during corrosion in air, and thus accelerates rusting at pores and breaks, the position is reversed in the absence of air, as in a sealed food can. The tin then sacrificially protect the steel at pores. Tin is an ideal coating for electrical wires and other electrical connections because of its suitability for soldering and its immunity to tarnish by sulfur bearing rubber insulation. Tin has traditionally been applied to other metals in the molten state as described later, but it can readily be electroplated either from acid or alkaline solution.

Alkaline plating is carried out in a hot solution of sodium stannate (Na2SnO2) formed by dissolving tin oxide in caustic soda. This is another example of an electroplating solution with the main reverse of the metal in the negative anion but in equilibrium with a small concentration of tin cations, from which the actual deposition takes place. The speed of tin plating, the covering and throwing power are execellent, although the deposit is matt. The rate of deposition is 0.0013 in/h at 20 A/ft2, but an even faster rate of deposition can be obtained from more concentrated solutions of potassium stannate, this is more expensive. Both solution can be fed from tin anodes, but great care has to be taken to maintain, which ruins the bath. Satisfactorily anode performance is evident from a characteristic yellow green appearance.

Other electroplating:
Zinc and Cadmium Electroplating
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