Electroplating, the process of coating a metal object with a thin layer of another metal by means of electrolysis. The electroplated coating is usually no more than .002 inch (.05 mm) thick. Electro-forming is a similar process except that the thin layer is applied to a nonmetal that is later destroyed.
Electroplating is used to give metal objects a better appearance or to protect them from corrosion, wear, or rust. Tableware, trays, decorative pieces, and jewelry are plated with gold or silver to make them more attractive. Copper is coated with chromium to protect it from corrosion. For the same reason iron and steel are plated with nickel, chromium, tin, zinc, or cadmium. Tin cans, for example, are tin-plated steel, and the chrome trim on automobiles is chromium-plated steel. Platinum, palladium, and rhodium are used to coat other metals with a hard, corrosion-resistant surface.
First, a container is filled with a solution of a salt of the metal that is to form the coating. For example, if copper is to form the coating, the solution will consist of copper sulfate (a salt of copper) mixed with water. This solution is called the electrolytic bath. The object to be plated is immersed in the bath. A metal bar, composed either of the metal that is to form the coating or of a metal that is not affected by the electrolytic bath, is also immersed in the bath. The entire apparatus is called an electrolytic cell.
The object to be coated is connected to the negative terminal of an electric battery or other source of direct current, and becomes the cathode (the electrode through which negative charge enters an electrical device). The metal bar is connected to the positive terminal of the electric power source and becomes the anode (the electrode through which negative charge leaves).
When electric power is applied, electrolysis of the electrolytic bath occurs. The bath gives up its metal content to the surface of the cathode. This coating forms an alloy with the metal of the cathode, and adheres to the cathode after the cathode has been removed from the bath. As the electroplating process continues, the metal salts in the bath are used up. If the anode is a bar of the coating metal, the bar dissolves in the bath at the same rate that the bath gives up its metal to the cathode. If the anode is made of another metal, salts of the coating metal must be added to the bath as metal becomes deposited on the cathode.
The longer the process continues, the greater the thickness of the coating on the cathode. If the cathode and the metal with which it is to be coated will not combine into an alloy, the cathode is first plated with a metal that will form an alloy with it. The plated cathode is then plated with the desired metal. For example, steel to be silver-plated is first plated with copper, because steel and silver will not form an alloy.