Zinc and cadmium are also plated from double cyanide baths. There is less difficulty in dissolving the anodes and it assists if the solution is alkaline, so that caustic soda is often added, or formed automatically by using the metal oxide and more cyanide. The purpose for which these metals are electroplated is almost always to protect iron and steel from rust. They are not usually polished because appearance is secondary. Although zinc is a fairly reactive metal it soon covers itself with a protective film under neutral conditions, so that it is fairly resistant to the weather and to waters. Moreover, by the sacrificial action previously explained it protects the underlying steel even where the deposit is porous or damaged. However, this necessitates a thicker coating to allow for this sacrificial action.
The potential difference between steel and cadmium at exposed points is small but, especially in slightly alkaline solutions, cadmium protects steel by a slight sacrificial action, but it is not so readily corroded as zinc and the corrosion products are less unslightly. Cadmium is more resistant than zinc to weakly acid solution and is more effective with weakly alkaline solutions such as those used for household washing. But cadmium is very expensive and becoming more so; therefore, although it is preferred form many purpose, particularly for aircraft parts, it is only used where the better service offset its greater cost. Both zinc and cadmium compounds are somewhat toxic, electroplates of these metals should not be used on articles for use with foodstuff.
Both zinc and cadmium can be electroplated in a lustrous condition by means of brightening additions. Alternatively the matt deposits from the simple solution can be chemically brightened after completion of the plating by immersion into simple chemical dips. A dip can also be used to preserve a zinc deposit from the superficial but disfiguring “white rust” a whitish protective film which zinc soon acquired in the atmosphere. A typical passivating dip is an acid solution of sodium dichromate.
Zinc coating can also be electroplated on geometrically simple articles, such as sheet and wire, from an acid zinc sulfate solution. This is practice on a large scale by the manufacturers of such semi fabricated commodities, where the hot dipping process is impractical.
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