ELECTROPLATING - Silver plating and gold plating of jewelry or silverware typically are 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.

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Electroplating

What Is Electroplating?

by Anne Marie Helmenstine, Ph.D.

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.

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 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.

Why Electroplating Is Done

There 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 are 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.

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% Na3PO4buffer	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

Anne Marie Helmenstine, Ph.D.

·   Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

·   Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

·   ThoughtCo and About Education chemistry expert since 2001.

·   Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

Anne Marie Helmenstine, Ph.D.​

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 https://www.thoughtco.com/what-is-electroplating-606453