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Electrochemistry Encyclopedia
(http://electrochem.cwru.edu/ed/encycl/)
ELECTROPLATING

Mordechay
Schlesinger �

Department of
Physics�

University of
Windsor�

Windsor, ON
N9B 3P4, Canada�

(September, 2002)

Electroplating has, over recent
decades, evolved from an art to an exact science. This development is
seen as responsible for the ever-increasing number and widening types
of applications of this branch of practical science and engineering.
Some of the technological areas in which means and methods of electroplating
constitute an essential component are all aspects of electronics: macro
and micro, optics, opto-electronics, and sensors of most types, to name
only a few. In addition a number of key industries such as the automobile
industry (that uses for example chrome plating to enhance the corrosion resistance of metal parts)
adopt the methods even where other methods, such as evaporation, sputtering,
chemical vapor deposition (CVD) and the like are an option. That is
so for reasons of economy and convenience. By way of illustration it
should be noted that that modern electroplating equips the practitioner
with the ability to predesign the properties of surfaces and in the
case of electroforming
those of the whole part. Furthermore, the ability to deposit very thin
multilayers (less than a millionth of a cm)
via electroplating represents yet a new avenue of producing new materials.

�

Fig.
1. Schematics of an electrolytic cell for plating metal "M"
from a solution of the metal salt "MA".

Electroplatin</span><span class="Normal-0020-0028Web-0029-200e--Char">g
is often also called "electrodeposition", and the two terms
are used interchangeably. As a matter of fact, "electroplating"
can be considered to occur by the process of electrodeposition.
E</span><span class="Normal-0020-0028Web-0029-200e--Char">lectrodeposition
is the process of producing a coating, usually metallic, on a surface
by the action of electric current.
The deposition of a metallic coating onto an object is achieved by pu</span><span class="Normal-0020-0028Web-0029-200e--Char">tting
a negative charge
on the object to be coated and immersing it into a solution which contains
a salt of the metal to be deposited (in other words, the object to be
plated is made the cathode
of an electrolytic cell).
The metallic ions
of the salt
carry a positive charge and are thus attracted to the object. When they
reach the negatively charged object (that is to be electroplated), it
provides electrons
to reduce
the positively charged ions to metallic form. Figure 1 is a schematic
presentation of an electrolytic cell for electroplating a metal "M"
from an aqueous
(water) solution of metal salt "MA".

To
further illustrate the foregoing, let us assume that one has an object
made of one of the common metals, like copper, and that it has been
properly pre-cleaned. We should want to plate it with, say, nickel.
A wire will have to be attached to the object while the other end of
the wire should be attached to the negative pole
of a bat</span><span class="Hyperlink--Char">tery
(or a power supply). To the positive
pole of the battery (or power supply) we connect another wire with its
other end connected to a rod made of nickel. Next we fill the cell
with a solution of the metal salt to be plated. It is possible to use
a molten salt and in some not so common cases, such as the deposition
of tungsten, that is what is done. In most, more common, cases though
the salt is simply dissolved in water. In our present example the nickel
chloride salt dissociates in water
to positively charged nickel cations
and negatively charged chloride anions.
As the object to be plated is negatively charged it attracts the positively
charged nickel cations, and electrons
flow from the object to the cations to neutralize
them (to reduce them) to metallic
form. Meanwhile the negatively charged chloride anions are attracted
to the positively charged nickel
rod (known as the anode of the electrolytic
cell). At the anode electrons are removed from the nickel metal, oxidizing
it to the nickel cations. Thus we see that the nickel dissolves
as ions into the solution. That
is how replacement nickel is supplied to the solution for that which
has been plated out and one retains a solution of nickel chloride in
the cell.

Nickel chloride is used here
to exemplify the process of electroplating for a number of reasons.
First among those is simplicity. It is not recommended, however, that
nickel be used for, say, school science demonstrations because some
individuals are quite allergic to it. We further do not recommend that
chloride salts be used because those are amenable to release chlorine
gas. For school or amateur type demonstration we recommend plating copper
coins with zinc or nickel coins with copper.

History

Before continuing with more detailed
discussion of the subject at hand, a brief history of electroplating
will be presented. The early history of electroplating may be traced
back to around 1800. A university professor, or in modern terms: a chemist,
Luigi Brugnatelli is considered as the first person to apply electrodeposition
process to electroplate gold. Brugnatelli was a friend of Allisandro
Volta (after whom the electric unit "volt"
has been named) who had just a short time before discovered the chemical
principles that would make possible the development of "voltaic"
electrical cells. Volta's first actual demonstration of that was called
"Voltaic Pile". As a consequence
of this development, Brugnatelli's early work using voltaic electricity
enabled him to experiment with various plating solutions. By 1805 he
had refined his process enough to plate a fine layer of gold over large
silver metal objects. He wrote in a letter to the Belgian Journal of
Physics and Chemistry (later reprinted in Britain), which reads:

"I have lately gilt in a complete manner two large si</span><span
class="Normal-0020-0028Web-0029-200e--Char" style=" font-style: italic;
">lver medals, by bringing them into communication by means of a steel
wire, with a negative pole of a voltaic pile, and keeping them one after
the other immersed in ammoniuret of gold newly made and well saturated".

Unfortunately for Brugnatelli,
a disagreement or falling out with the French Academy of Sciences, the
leading scientific body of Europe at the time, prevented Brugnatelli's
work from being published in the scientific journals of his day. His
work remained largely unknown outside of his native Italy except for
a small group of associates. By 1839, however, scientist in Britain
as well as in Russia had independently devised metal deposition processes
similar to those of Brugnatelli's for copper electroplating of printing
press plates. By 1840, this discovery was adapted and refined by Henry
and George Elkington of Birmingham, England for gold and silver plating.
Collaborating with their partner John Wright and using formulae developed
by the latter for potassium cyanide plating baths, the Elkingtons were
able to have the first viable patents for gold and silver electroplating
issued on their name. From Great Britain the electroplating process
for gold and silver quickly spread throughout the rest of Europe and
later to the United States.

With the burgeoning knowledge
and understanding of the subject of electrochemistry and its importance
in understanding the processes of "electrodeposition"
came the ability to deposit other metals. By the 1850's electroplating
methods of bright nickel, brass, tin, and zinc were commercialized and
were applied for engineering and specific commercial purposes. In time,
the industrial age and financial capital had expanded from Great Britain
to the rest of the world. As a result, electrodeposition processes were
expanding in scope and found more and more usage in the production of
a variety of goods and services. While this expansion was taking place,
no significant scientific discoveries were made until the emergence
of the electronic industry in the mid forties of the last century. The
onl