Electro
plating is an
electrochemical process for depositing a thin layer of metal on a
metallic base. Objects can be electroplated in order to avoid
corrosion, to obtain a hard surface or
attractive finish, for
purification purposes of metals (as in the
electrorefining of
copper), to separate metals for analysis, or, as in
electrotyping, to reproduce a form from a mold.
Cadmium,
chromium,
copper,
gold,
nickel,
silver, and
tin are the metals used most often. Typical consumer products included silver-plated
tableware, chrome-plated
automobile accessories, and
jewlery.
In the process of electroplating, the object to be coated is placed in a bath of a salt of the coating metal, and is connected to the negative terminal of an external source of electricity. Another conductor, often composed of the coating metal, is connected to the positive terminal of the electric source. A steady direct current of low voltage, is required for the process. When the current is passed through the solution, atoms of the plating metal deposit out of the solution onto the cathode, the negative electrode. These atoms are replaced in the bath by atoms from the anode (positive electrode), if it is composed of the same metal, as with copper and silver. Otherwise they are replaced by periodic additions of the salt to the bath, as with gold and chromium. In either case an equilibrium is kept until the object is plated. Nonconducting materials may be plated by first being covered with a conducting material such as graphite.
A clean object is important to the electroplating process. The object must be cleaned thoroughly by dipping it into an acid or caustic solution. To eliminate irregularity in the depth of the plate, and to ensure that the grain at the surface of the plate is of good quality and polishable, the current density (amperes per square foot of cathode surface) and temperature must be carefully controlled. Colloids are often added to the bath to improve the surface uniformity.