Friday, April 11, 2008

Electrotyping Baths, Double Salt Baths and Barrel Plating Baths: Special Purpose Baths

A number of applications for nickel plate require baths of special composition. Bath fomulas and preferred operation conditions are shown in the table Special Purpose Nickel Bath.

Electrotyping Baths
The ammonium salt increases conductivity and provides buffer action at high pH. Boric acid is not included because it leads to undesirable stress and to cracking of the plate at the low temperature high pH conditions necessary for covering or spreading over the surface of a treated was mold. Air agitation is commonly employed to permit operation at the upper end of the current density range. Modern practice for electrotypes and stereotypes is to apply nickel from standard warm fouborate or especially sulfamate baths on sheet of vinyl plastic, leads or aluminum backed instead of on waxes.

The Double Salt Baths
Double salt bath solution of the oldest nickel plating baths, finds little application in the country. It can be employed only at room temperature (20 – 30 oC) and low current densities. It was used for deposition of tin coating on, for example, brass, costume jewelry and novelties. Brighteners like gum tragacanth, gum arabic or cadmium salt was often used.

Barrel Plating Baths
Barrel Nickel Plating is carried out with large numbers of small parts which are too difficult to rack. This solution being somewhat more concentrated than double salt solution, has the wider range necessary for the barrel plating of small parts in bulk. The brighteners used same as on the double salt baths. Modern proprietary warm bright nickel baths based on the Watts solution, usually with higher nickel chloride content and using some of the same brighteners used in rack plating but in different proportions, are now available for barrel plating to yield a superior product at higher rates of plating. The modern baths are required to give bright deposits over a very broad current density range down very lowest. Usually a very low foaming surfactant, similar to the ones used in the air-agitated baths, is added to decrease drag out.

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