Chemical Finishing Techniques : Displacement Plating (Electrochemical Plating)

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 produces only a thin coating of metal on an object because the process is self-terminating: once the surface of the metal to be plated has been completely covered by a layer of the plating metal the exchange of the two metals stops. In the example shown here, there is no longer a potential difference between the surface of the copper piece (which is now silver) and the silver ions in solution. In the activities section you will find a laboratory procedure for displacement plating silver onto brass, and to the right is a video demonstration of this procedure.

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 Works 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 1930-124-1 [ More Details ]

Dial of Tall Case Clock 1930-124-1

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