How does electroplating coat an object?

1. A shiny tap, a gold-colored ring, a silvery screw: many metal things wear a very thin coat of another metal. How do you put a coat of metal on a steel key?
2. First the key is scrubbed in a cleaning bath and rinsed. Even a greasy fingerprint would stop the new metal from sticking, and the coat would peel off.
3. The clean key hangs in a bath of blue liquid with copper dissolved in it. A bar of solid copper hangs beside it. Wires connect both to a power supply.
4. Switch on, and electricity flows. Tiny charged bits of copper, called ions, leave the copper bar and are pulled through the liquid toward the key.
5. Look very closely at the key. Each copper ion that touches it is given electricity from the wire and turns into solid copper. Row by row, the coat builds up.
6. Lift the key out and rinse it. It is covered all over with shiny copper, in a layer thinner than a hair. The copper bar has shrunk a little: that is where the coat came from.

This plating tank is running. Tap each part to find out its job.

• Power supply. The power supply pushes electricity around the loop, always in the same direction. Turn it up and the coat grows faster.
• Wires. Wires carry the electricity. Tiny particles called electrons flow through one wire to the key and come back to the power supply through the other.
• Copper bar. The copper bar is where the coat comes from. Little by little its copper goes into the liquid, so the bar slowly gets thinner.
• Plating bath. The bath is water with copper dissolved in it, which is what makes it blue. Bits of metal can travel through the liquid from the bar to the key.
• Copper ions. An ion is a tiny bit of metal with an electric charge. Copper ions have a positive charge, so they are pulled toward the key, which is negative.
• The key. The object being plated hangs on a metal hook, so electricity can reach it. Whatever is wired to the negative side gets the coat.
• New metal layer. Each ion that reaches the key turns into solid copper and stays there. Millions and millions of them build a thin, even layer over the whole key.

Two greasy keys are waiting. Clean just one of them, then dip both in the bath and switch on. Lift them out and compare.

Experiment: One key is cleaned and the other is still greasy. What happens when both are plated?

1. Press Clean. Only the first key goes into the cleaner.
2. Dip both keys in the plating bath and switch the power on.
3. When the coat is thick enough, switch off and lift the keys out. Look closely at each one.

What we found: The clean key came out with a smooth, even coat of copper. On the greasy key the copper could not touch the steel wherever there was grease, so the coat was patchy and flaked off. That is why every part is cleaned and rinsed before it is plated.

Turn on JavaScript to try it yourself.

A customer wants a smooth copper coat between 8 and 12 micrometers thick. Clean the key, pick a current, and switch off at the right moment.

Lots of metal things are not what they seem. A shiny bathroom tap is usually brass with a very thin coat of chrome on top. Many silvery screws are steel with a coat of zinc to stop them rusting. Putting a thin coat of one metal on top of another is called plating, and the most common way to do it uses electricity.

The object is cleaned until it is spotless, then hung in a bath of liquid that has metal dissolved in it. A bar of the same metal hangs in the bath too. Wires connect the object and the bar to a power supply, making a loop that electricity can flow around.

When the power is switched on, tiny charged bits of metal called ions are pulled through the liquid toward the object. When an ion touches the object it turns back into solid metal and sticks. More and more arrive and build up a layer. At the same time the metal bar slowly dissolves, which keeps the bath topped up.

The longer the power stays on, and the stronger the current, the thicker the coat. Platers have to be patient, though. If they turn the current up too high, the metal piles on too quickly and comes out rough and dull instead of smooth and shiny. Plating baths contain strong chemicals, so the work is done in factories and laboratories by trained people wearing gloves and goggles.

Key words

Ion

A tiny bit of matter that carries an electric charge.

Anode

The bar of metal that the coat comes from. It is joined to the positive side.

Cathode

The object being coated. It is joined to the negative side.

Electrolyte

A liquid that conducts electricity because it contains ions.

Current density

How much electricity flows onto each bit of the surface.

Micrometer

One thousandth of a millimeter. A human hair is about 70 micrometers thick.

Check your thinking

1. Why must an object be cleaned before it is plated?

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   So the new metal can touch the surface and stick. Grease gets in the way. Where the metal cannot touch the surface, the coat peels off.

2. Where does the metal for the coat come from?

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   From the bar of metal in the bath, which slowly dissolves. The bar gives up its metal to the bath, and the bath gives it to the object.

3. How can you make the coat thicker?

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   Leave the power on for longer. The longer the electricity flows, the more metal arrives and the thicker the coat grows.

4. What goes wrong if the current is turned up much too high?

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   The coat comes out rough and dull. The metal piles on too quickly to settle into a smooth layer.

5. In the bath, what carries the electric charge from the anode to the cathode?

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   Ions moving through the solution. Electrons flow only in the wires and the metal parts. In the electrolyte, charge is carried by ions.

6. The same charge is passed through a copper bath and a silver bath. Why is far more silver deposited?

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   Each silver ion needs one electron, each copper ion needs two, and silver atoms are heavier. The number of atoms deposited is the number of electrons divided by the ion's charge. Silver's charge is plus one, and each atom also has more mass.