by Chris Woodford. Last updated: June 18, 2013.
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.
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.
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).
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 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 them a protective outer later. 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).
Find out more
On this website
On other sites
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:
- Electroplating: how it works: From finishing.com, includes a few examples of safe and simple electroplating experiments you can try yourself.
- Instructables: Simple, cheap electroplating: Another DIY electroplating experiment using simple, household items.
- Electroplating a quarter: Clearly and simply explained in this brief little video from chemistry teacher Mr Kent.
- Electrolysis: Electroplating: This short, clear (2-minute) BBC Schools video reviews the science of electroplating copper with zinc. (Sorry if you cannot watch this; BBC videos are sometimes not be available to viewers in all countries.)
- Electroplating: Basic Principles, Processes and Practice by Nasser Kanani. Elsevier, 2004. A detailed introduction for chemistry students, as well as manufacturers.
- Electroplating: Engineering Handbook by Lawrence J. Durney (ed). Springer, 1984. Another detailed reference, mainly intended for people working in the metal finishing industry.
- Electroplating and Electrorefining of Metals by Alexander Watt, Arnold Philip. Watchmaker, 2005. A reprint of old material first written about a century ago, so really of interest only for the historically curious.