How electroplating works

Plenty of shiny metal things are coated in a thin layer of a different metal. One common way to put it on is electroplating. It also turns up in school chemistry, where students may plate a coin in class.

The basic idea

Electroplating covers a metal object with a thin layer of another metal. It is one of the industrial uses of electrolysis, where an electric current passes through a solution.

The object to be coated is the cathode, the negative electrode. It sits in a solution of a salt of the coating metal. The anode, the positive electrode, is usually a block of that same metal.

When the current flows, positively charged metal ions move towards the cathode. There they turn back into metal and build up a layer on the object. In the classroom copper experiment, a copper anode slowly dissolves as this happens.

Getting a good coating

In the school copper experiment, the copper on the cathode often comes out powdery and uneven. The Royal Society of Chemistry explains that a much lower current gives a shiny coating that is very hard to rub off. That slower, even coating is the basis of electroplating.

The coating also tends to be thicker on the parts of an object nearest the anode.

Why things are plated

Gold or silver plating makes a ring of cheaper metal look like gold or silver. The inside of a steel food can is plated with tin, which keeps oxygen and water away from the steel.

Industry plates objects to resist wear and corrosion, to make them slide more easily, to reflect light, to carry electricity or to look better. Plating also builds up worn parts, and it lays down the copper tracks on printed circuit boards.

Where the industry began

John Wright of Birmingham found that potassium cyanide worked as a solution for gold and silver plating. His associates, George and Henry Elkington, took out the first patents for electroplating in 1840. They founded the electroplating industry in Birmingham, and from there it spread around the world.

Plating chemicals and safety

The Royal Society of Chemistry's small classroom practical lists safety spectacles among its equipment.

In plating works, the Health and Safety Executive (HSE) warns that hexavalent chromium compounds and nickel sulphate can cause occupational asthma. Once someone's lungs are sensitised, even low levels can set off an attack.

The NHS lists nickel among the metals that can cause allergic contact dermatitis, a skin reaction.

Where plating is part of a business, the Control of Substances Hazardous to Health (COSHH) Regulations require employers to plan, manage and monitor the harmful substances they use.

Some household products can harm people or the environment if they go out with the rubbish. GOV.UK points people to their council to find out whether it takes hazardous waste such as chemicals.

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