Why Electroplating Is DoneThere are several reasons why you might want to coat a conductive surface with a metal. Silver plating and gold plating of jewelry or silverware typically is done to improve the appearance and value of the items. Chromium plating improves the appearance of objects and also improves its wear. Zinc or tin coatings may be applied to confer corrosion resistance. Sometimes electroplating is done simply to increase the thickness of an item.
How Electroplating WorksElectroplating works like a galvanic cell in reverse. An electrical current reduces cations from a solution so that they can coat a conductive material with a thin layer.
A Closer Look at ElectroplatingIn one form of electroplating, the metal to be plated is located at the anode of the circuit, with the item to be plated located at the cathode. Both the anode and the cathode are immersed in a solution which contains a dissolved metal salt (e.g., an ion of the metal being plated) and other ions which act to permit the flow of electricity through the circuit. Direct current is supplied to the anode, oxidizing its metal atoms and dissolving them in the electrolyte solution. The dissolved metal ions are reduced at the cathode, plating the metal onto the item. The current through the circuit is such that the rate at which the anode is dissolved is equal to the rate at which the cathode is plated.
Electroplating ExampleA simple example of the electroplating process is the electroplating of copper in which the metal to be plated (copper) is used as the anode and the electrolyte solution contains the ion of the metal to be plated (Cu2+ in this example). Copper goes into solution at the anode as it is plated at the cathode. A constant concentration of Cu2+ is maintained in the electrolyte solution surrounding the electrodes:
anode: Cu(s) → Cu2+(aq) + 2 e-
cathode: Cu2+(aq) + 2 e- → Cu(s)
Common Electroplating Processes
|Cu||Cu||20% CuSO4, 3% H2SO4||electrotype|
|Ag||Ag||4% AgCN, 4% KCN, 4% K2CO3||jewelry, tableware|
|Au||Au, C, Ni-Cr||3% AuCN, 19% KCN, 4% Na3PO4 buffer||jewelry|
|Cr||Pb||25% CrO3, 0.25% H2SO4||automobile parts|
|Ni||Ni||30% NiSO4, 2% NiCl2, 1% H3BO3||Cr base plate|
|Zn||Zn||6% Zn(CN)2, 5% NaCN, 4% NaOH, 1% Na2CO3, 0.5% Al2(SO4)3||galvanized steel|
|Sn||Sn||8% H2SO4, 3% Sn, 10% cresol-sulfuric acid||tin-plated cans|