Electroplating is one kind of surface finishing. There are many other kinds. Everyone has seen and handled electroplated objects, even if they didn't know it. Examples include kitchen and bathroom faucets, inexpensive jewelry and the trim on some automobiles. There are thousands of examples. In fact, it is certain that nearly every piece of metal you have ever seen has been through some kind of surface finishing process. There are three basic reasons for surface finishing: to improve appearance, to slow or prevent corrosion (rust) and to increase strength and resistance to wear (in the case of "engineering" finishes). An object may be processed for any or all of these reasons.
The term electroplating means the coating of an object
with a thin layer of metal by use of electricity. The metals most often
used are gold, silver, chromium, copper, nickel, tin and zinc, but many
others are used also. The object to be plated is usually a different metal,
but can be the same metal or a non-metal, such as a plastic grille for
an automobile.
Electroplating usually takes place in a "tank" of solution containing
the metal to be deposited on an object. This metal is in a dissolved form
called ions. An ion is an atom that has lost or gained one or more electrons
and is thus electrically charged. You cannot see ions, but the solution
may show a certain color; a nickel solution, for example, is typically
emerald green. The deposited metal, however, will be gray or silver in
appearance.
When certain metallic chemicals dissolve in water, the metal atoms of
these chemicals are freed to move about, but lose one or more electrons
(negative charges) and, as a result, are positively charged. The object
to be plated is negatively charged and attracts the positive metal ions,
which then coat the object to be plated and regain the lost electrons
to become metal again.
A familiar example of this process is the experiment often performed in
which a key is plated with copper. The key (called the cathode) is connected
to the negative terminal of a battery and is placed in a solution of vinegar,
a weak acid. The positive terminal of the battery is connected to a piece
of copper (called the anode--and often just a copper wire), which is placed
in the solution. The acid slowly dissolves the wire, making copper ions
that are then attracted to the key, regaining their lost electrons and
becoming copper metal again, but now in the form of a thin coating on
the key. The battery forces all this to happen and prevents the deposited
copper from re-dissolving.
Now look at the illustration. Positively charged copper
ions are free in the solution, but are being attracted by the negatively
charged key. As the ions contact the key, they regain their lost electrons
and become copper metal and stick to the key wherever they touched it.
This is the basic process of electroplating, and all forms of it work
the same way.
- Source:
http://www.aesf.org
Other Common Finishing Processes
There is another plating process, discovered in 1946, called electroless plating. It earned that name because it operates without using electricity; the action is purely chemical and runs by itself, once started.
Electrical and chemical processes are not the only ways
to coat an object. Another important process is called vapor deposition.
In this process, the metal to be deposited is converted into a vapor that
is allowed to condense on the object to be 
coated.
Many beautiful finishes can be obtained by this process, which is carried
out in a vacuum chamber.
Another increasingly important surface finishing process is powder coating. It depends on the fact that opposite charges attract, just as in electroplating. The object to be coated is electrically charged and is sprayed with a non-metallic powder that sticks to it. The object is then passed through an oven where the powder particles melt and run together to make a smooth finish. Articles commonly coated in this way include lawn mower frames, sports equipment, playground equipment, lawn furniture and the insides of refrigerators, washing machines and dryers.
Aluminum is everywhere and all of it has been anodized. This process involves producing a very thin, invisible layer of the oxide on the aluminum, which protects it from the kinds of corrosion that affect aluminum, such as salt air from the ocean. The oxide is a chemical combination of aluminum and oxygen. It can be dyed to produce any desired color.
Finally, the most common surface finishing process of all--painting--is one with which everyone is familiar. The broad category of paints includes lacquers, enamels, varnishes and stains. Many steel items are painted rather than plated, such as automobiles. - Source: http://www.aesf.org
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