What Is Electroplating and How Does It Work

Electrochemistry is a process by which very thin layers of a selected metal are bonded to the surface of another metal at the molecular level. The process itself involves creating an electrolytic cell: a device that uses electricity to deliver molecules to a particular location. How Electroplating Works Electroplating is the application of electrolytic cells in which a thin layer of metal is deposited onto an electrically conductive surface. A cell consists of two electrodes (conductors), usually made of metal, which are held apart from one another. The electrodes are immersed in an electrolyte (a solution). When an electric current is turned on, positive ions in the electrolyte move to the negatively charged electrode, called the cathode. Positive ions are atoms with one electron too few. When they reach the cathode, they combine with electrons and lose their positive charge. At the same time, negatively charged ions move to the positive electrode, called the anode. Negatively charged ions are atoms with one electron too many. When they reach the positive anode, they transfer their electrons to it and lose their negative charge. The Anode and Cathode In 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 cathode are immersed in a solution that contains a dissolved metal salt—such as an ion of the metal being plated—and other ions that 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. Purpose of Electroplating There are several reasons why you might want to coat a conductive surface with metal. Silver plating and gold plating of jewelry or silverware typically are performed 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 performed simply to increase the thickness of an item. Electroplating Example A 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 Metal Anode Electrolyte Application 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