Electroplating
Last updated: August 30, 2009.
There's no such thing as alchemy—magically changing common chemical elements into
rare and valuable ones—but electroplating is possibly the next best
thing. The idea is to use electricity to coat a relatively mundane
metal, such as copper, with a thin layer of another, more precious
metal, such as gold or silver. Electroplating has lots of other uses,
besides making cheap metals look expensive. We can use it to make
things rust-resistant, for example, or to produce a variety of useful
alloys like brass and bronze. How does this amazing process work?
Let's take a closer look!
Photo: Electroplating in action—an exhibit at Think Tank (the science museum in Birmingham, England). These two forks are the electrodes and the blue solution (copper sulfate) is being used to copper-plate one of them.
What is electroplating?
Electroplating involves passing an electric current through a solution called an
electrolyte. This is done by dipping two terminals called
electrodes into the electrolyte and connecting them into a
circuit with a battery or other power supply. The electrodes and
electrolyte are made from carefully chosen elements or compounds.
When the electricity flows through the circuit they make, the
electrolyte splits up and some of the metal atoms it contains are
deposited in a thin layer on top of one of the electrodes—it becomes electroplated. All kinds of metals can
be plated in this way, including gold, silver,
tin, zinc, copper,
cadmium, chromium, nickel,
platinum, and lead.
Photo: Gold-plated: When astronaut Ed White made the first American spacewalk in 1965, he was wearing a gold-plated visor on his helmet to protect his eyes from solar radiation.
Photo by courtesy of Great Images in NASA (GRIN).
Electroplating is very similar to electrolysis
(using electricity to split up a chemical solution), which is the reverse of the process by which
batteries produce electric currents. All these things are examples of
electrochemistry: chemical reactions caused by or producing
electricity that give scientifically or industrially useful
end-products.
How does electroplating work?
First, you have to choose the right electrodes and electrolyte by figuring out the
chemical reaction or reactions you want to happen when the electric
current is switched on. The metal atoms that plate your object come from out of the
electrolyte, so if you want to copper plate something you need an electrolyte
made from a solution of a copper salt, while for gold plating you need a
gold-based electrolyte—and so on.
Next, you have to ensure the electrode you want to plate is completely clean.
Otherwise, when metal atoms from the electrolyte are deposited onto
it, they won't form a good bond and they may simply rub off again.
Generally, cleaning is done by dipping the electrode into a strong
acid or alkaline solution or by (briefly) connecting the
electroplating circuit in reverse. If the electrode is really clean,
atoms from the plating metal bond to it effectively by joining very
strongly onto the outside edges of its crystalline structure.
Artwork: Electroplating copper onto brass.
Now we're ready for the main part of electroplating. We need two electrodes made from
different conducting materials, an electrolyte, and an electricity
supply. Generally, one of the electrodes is made from the metal we're
trying to plate and the electrolyte is a solution of a salt of the
same metal. So, for example, if we're copper plating some brass, we
need a copper electrode, a brass electrode, and a solution of a
copper-based compound such as copper sulfate solution. Metals such as
gold and silver don't easily dissolve so have to be made into
solutions using strong and dangerously unpleasant cyanide-based chemicals.
The electrode that will be plated is generally made from a cheaper
metal or a nonmetal coated with a conducting material such as
graphite. Either way, it has to conduct electricity or no electric
current will flow and no plating will occur.
We dip the two electrodes into the solution and connect them up into a circuit so
the copper becomes the positive electrode (or anode) and the zinc
becomes the negative electrode (or cathode). When we switch on the
power, the copper sulfate solution splits into ions (atoms with too
few or too many electrons). Copper ions (which are positively
charged) are attracted to the negatively charged brass electrode
and slowly deposit it on it—producing a thin later
of copper plate. Meanwhile, sulfate ions (which are negatively
charged) arrive at the positively charged copper anode, releasing electrons
that move through the battery toward the negative, brass electrode.
It takes time for electroplated atoms to build up on the surface of the negative electrode.
How long exactly depends on the strength of the electric current you
use and the concentration of the electrolyte. Increasing either of
these increases the speed at which ions and electrons move through
the circuit and the speed of the plating process. As long
as ions and electrons keep moving, current keeps flowing and the plating process continues.
Why use electroplating?
Electroplating is generally done for two quite different reasons. Metals such as gold
and silver are plated for decoration: it's cheaper to have gold- or
silver-plated jewelry than solid items made from these heavy,
expensive, precious substances. Metals such as tin and zinc (which
aren't especially attractive to look at) are plated to give a
protective outer later to a metal. For example, food containers are
often tin plated to make them resistant to corrosion, while many
everyday items made from iron are plated with
zinc (in a process called galvanization) for the same reason.
Some forms of electroplating are both protective and decorative. Car fenders and "trim," for example, were once
widely made from tough steel plated
with chromium to make them both attractively shiny and
rust-resistant (inexpensive and naturally rustproof plastics are now more likely
to be used on cars instead). Alloys such as brass and bronze can be plated too, by
arranging for the electrolyte to contain salts of all the metals that
need to be present in the alloy. Electroplating is also used for
making duplicates of printing plates in a process called
electrotyping and for electroforming (an alternative to
casting objects from molten metals).
Photo: This car wheel is made from aluminum metal plated with
nickel
in a more environmentally friendly process developed by Metal Arts Company, Inc.
The Microsmooth™ process uses about 30 percent less electricity, nearly 60 percent less natural gas, and half the water that conventional plating processes need. Photo by Metal Arts Company, Inc.
courtesy of US Department of Energy (DOE).
Further reading
Electroplating is something you can easily experiment with at school or (with the help
of an adult) at home. Here are some sites you can safely explore:
