What is Displacement Plating?
Do you think that it is possible to electrically plate one metal onto another metal without using a battery? It is possible! The process is called displacement plating, or electrochemical plating, and it works because the electrochemical potential of each metal is unique. The metals themselves act like a battery because of the potential (charge) difference between them. The first step in displacement plating is to compare the electric potential—the ease of oxidation or reduction—of the metals you wish to use. The Metal Activity Series (also called the Electromotive Series) lists a selection of metals in order of their relative tendency to oxidize (to lose electrons) or reduce (to gain electrons). Metals that are more easily reduced will displace (or replace) those that are more easily oxidized.
The drawing below illustrates the process, and we will use the metals shown, copper and silver, to explain the steps involved. First, by referring to the Metal Activity Series, you may observe that silver (Ag) will displace copper (Cu). Next, as seen in the center of the illustration, the surface of a piece of copper, when introduced to silver ions (Ag+) in solution, begins to dissolve as the copper (Cu0) gives up electrons and becomes oxidized copper (Cu+) in the solution. As the silver ions (Ag+) accept the electrons to become reduced (Ag0), they form a metallic silver layer on the surface of the copper. The right-hand area of the illustration shows the final result: the top layer of copper has been completely replaced by a silver coating.
Displacement plating process: as copper ions replace silver ions in solution, silver metal replaces copper metal on the surface.
Demonstration of displacement plating: silver onto brass. View related burnishing demonstration >>
Historical Use of Displacement Plating
Since the Renaissance, artists and manufacturers have used displacement plating to add silver or gold onto a variety of substrates, often to enhance their appearance. For example, gold was often displacement plated onto the interiors of silver vessels to prevent the silver from tarnishing in the presence of sulfur-laden drinks such as wine. Displacement plating was also often employed in the silvering of brass clock dials. The dial of the tall case clock shown below, made in Philadelphia in the eighteenth century, was silvered by this method. However, because the metal coatings formed by displacement plating are extremely thin (often only a few atoms thick), they do not usually survive the abrasion or corrosion that occurs by use. Consequently, it is difficult to document the early history of this technique.
Tall Case Clock, 1765-75
Movement by William Huston, American
Mahogany, yellow poplar, white cedar, yellow pine; brass, iron, glass
8 feet 3 inches x 21 1/4 x 10 1/2 inches (251.5 x 54 x 26.7 cm)
Purchased with the Germantown Tribute Fund, 1931
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Dial of Tall Case Clock 1930-124-1