In another form of barrel plating operation, the parts lie at the bottom of an open cup-shaped tub rotatable about an axis at about 45o to the horizontal. There is a negative contact sited centrally in the base, and an anode is hung above the parts. The tub is filled with plating solution and rotated. For removal, the contents are dumped out through a sieve. Barrel plating does not produce such satisfactory deposits as tank plating, for the action on any one parts is at best intermittent and some parts may receive an inordinately thin deposit.
The precise procedure for the cleaning and pre-treatment of different metals before electroplating can only be mentioned briefly here. Iron and steeel articles can be directly plated with all the metals previously mentioned. However, copper must be deposited from the cyanide rather than from the acid type of solution since otherwise a loose powdery deposit would be produced chemically. This does not apply to nickel-silver or brass, but for silver plating these metals a special strike initial treatment is necessary to circumvent a similar chemical deposition of silver.
Zinc and zinc alloys are electroplated in very large quantities in the form of zinc base alloy pressure die castings, and are subsequently exposed to severe corrosive conditions, e.g. on the outside of motor cars. The great chemical activity of zinc not only makes the etching and initial plating difficult, but also demands an unusually sound deposit. Any metallic coating is strongly cathodic to zinc, so that severe attack occurs to this base metal where it is exposed at pores, etc. The corrosion products formed there are more bulky than the metal from which they were formed and force up the adjacent plating into blisters. Zinc is invariably given a thin initial plating of copper from a double cyanide solution; the other desired electroplates such as nickel and copper follow.
