The process used in electroplating
is called electrodeposition. The item to
be coated is placed into a container containing
a solution of one or more metal salts. The
item is connected to an electrical circuit,
forming the cathode (negative) of the circuit
while an electrode typically of the same
metal to be plated forms the anode (positive).
When an electrical current is passed through
the circuit, metal ions in the solution
are attracted to the item. The result is
a layer of metal on the item. However, considerable
skill and craft-technique is required to
assure an evenly-coated finished product.
This process is analogous to a galvanic
cell acting in reverse.
A more detailed description
of the electrodeposition process follows:
The anode and cathode in the electroplating
cell are connected to an external supply
of direct current, a battery, or more commonly
a rectifier. The anode is connected to the
positive terminal of the supply, and the
cathode (article to be "plated")is
connected to the negative terminal. When
the external power supply is switched on,
the metal at the anode is oxidized from
the 0 valence state to form cations with
a positive charge. These cations associate
with the anions in the solution. The cations
are reduced at the cathode to deposit in
the metallic, 0 valence state. Example:
In an acid solution Cu is oxidized to Cu++
by losing two electrons. The Cu++ associates
with the anion SO4 2- in the solution to
form copper sulfate. At the cathode, the
Cu++ is reduced to metallic Cu by gaining
two electrons.The plating is most commonly
a single metallic element, not an alloy.
However, some alloys can be electrodeposited,
notably brass and solder.
Electroplating is used
in many industries for functional and/or
decorative purposes. Some well known examples
are chrome-plating of steel parts on automobiles.
Steel bumpers become more corrosion-resistant
when they have been electroplated with
first nickel and then chromium.
- Steel camshafts resist
wear better when they have been electroplated
- Plain steel or aluminum
parts in light fixtures become beautiful
when they are electroplated with nickel
and then decorative chromium or brass.
- Steel bolts last much
longer because they are sold with a coating
of zinc that has been applied by electroplating.
Zinc electroplating and passivation provides
a double protection system for steel components.
Virtually all types of steel can be protected
Newly developed electrolytes
and process methods are able to provide
greatly increased corrosion prevention.
In addition to the well
known yellow full passivation, there are
blue, olive and black variants available
to meet modern requirements. Modern electrolytes
can produce brilliant chrome like finishes.
Specially developed processes produce improved
metal distribution over complex shapes.
Alloy zinc deposits offer extra performance.
Passivation processes (also
known as conversion coatings) are usually
applied to zinc deposits to improve component
life. These coatings used to be based on
hexavalent chromium chemistry providing
unique surface corrosion resistance which
will withstand even the most extreme conditions
over prolonged periods but have recently
been superseded by trivalent chromium chemistry
on both health and environmental grounds.
Electroplating can be used
to silver plate copper or brass electrical
connectors, since silver tarnishes much
more slowly and has a higher conductivity
than those metals. The benefit of the silver
is lower surface electrical resistance resulting
in a more efficient electrical connection.
Silver plating is also popular for RF connectors
because radio frequency current flows primarily
on the surface of its conductor; the connector
will thus have the strength of brass and
the conductivity of silver.
Low force/low voltage separable
connectors used in telecommunications switchgear,
computers, and other electronic devices
are typically plated with gold or palladium
over a barrier layer of nickel. The tail
ends of these connectors, which are usually
joined to the device by soldering, are plated
with a tin/lead alloy, or pure tin.
Modern electroplating was
invented by Italian chemist Luigi V. Brugnatelli
in 1805. Brugnatelli used his colleague
Alessandro Volta's invention of five years
earlier, the voltaic pile, to facilitate
the first electrodeposition. Unfortunately,
Brugnatelli's inventions were repressed
by the French Academy of Sciences and did
not become used in general industry for
the following thirty years.
By 1839, scientists in Britain
and Russia had independently devised metal
deposition processes similar to Brugnatelli's
for the copper electroplating of printing
press plates. Soon after, John Wright of
Birmingham, England discovered that potassium
cyanide was a suitable electrolyte for gold
and silver electroplating. Wright's associates,
George Elkington and Henry Elkington were
awarded the first patents for electroplating
in 1840. These two then founded the electroplating
industry in Birmingham England from where
it spread around the world.
One of American physicist
Richard Feynman's first projects was to
develop technology for electroplating metal
onto plastic. Feynman successfully developed
this technology, allowing his employer to
keep commercial promises he had made but
could not have fulfilled otherwise.
On June 28, 1988, four workers
at an electroplating plant in Auburn, Indiana
were asphyxiated by hydrogen cyanide gas
produced when muriatic acid was mixed with
zinc cyanide in a cleaning operation. A
fifth victim died two days later.