Tin Plating Bath

Acid solutions of tin (stannous) sulfate or flouborate can be electroplated at very high current densities, but the deposit is rather coarsely crystalline unless complex organic brightening additions are used, e.g. cresol sulfonic acid or ß-naphtol and gelatin. The control of this type of solution is rather exacting, and the covering and throwing power are poor.

These disadvantages make the acid solution unsuitable for miscellaneous plating but less detrimental in electroplating moving steel strip to produce electrolytic tin plate. This process has therefore largely replaced hot dipped tinning. Tin plating is carried out at the steel works which produce large coils of bright cold-rolled steel strip. The plant for plating is laid out in a continuous line, through which the strip passes at high speed. Provision is made to join the end of one coil to the beginning of the next without stopping the line.

The strip passes through degreasing etching, rinsing, plating and drying stages in sequence, the length of each being adjusted to give the requisite time at the strip speed used. Although high current densities are used e.g. 200 A/ft2, and only a thin coating is applied (0.00005 in), a considerable length of plating tank is required to allow sufficient time for plating at the high speed of operation, e.g. 100 ft length at 1,000 ft/m. In earlier plants the strip was passed horizontally through a long tank, but as speeds increases, this became impractical and now the strip is posed in vertical loops through very deep tanks. Special precautions need to be taken to avoid undue drag-out of solution.

The deposit is dull and matt, and somewhat porous. To compete with hot-dipped tin plate the strip therefore passes through a very rapid heating and cooling stage, which melt and flow-brighteners the tin. It is finally oiled and cut into the usual standard tin plate sheets. Plant of this type, which must handle steel strip 0.005 to 0.015 in thich and up to 36 in wide at 1,000 ft/m, is enourmously expensive to install and operate, and calls for engineering of a high order. Since tin plate is a commodity of world wide use and very sensitive to price, the whole operation is very finely balanced from an economic point of view.

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