Practical: Electroplating a Metal Object

Electroplating coats the surface of an object with a thin layer of a different metal. The coating may improve appearance—for example, a shiny metal can be deposited on jewellery or cutlery—or improve resistance to corrosion. A continuous, firmly attached coating acts as a barrier between the underlying object and substances in its surroundings.

Electroplating does not turn the whole object into the coating metal. Only its surface is changed, so the object can retain the useful properties and lower cost of the metal underneath.

The electrolysis arrangement

Electroplating uses a direct current supply and an electrolyte containing ions of the metal that will form the coating. In a common school method for copper-plating an iron nail:

• the clean iron nail is the cathode and is connected to the negative terminal;
• a copper strip is the anode and is connected to the positive terminal;
• aqueous copper sulfate is the electrolyte because it supplies mobile $\mathrm{Cu^{2+}}$ ions;
• both electrodes are partly immersed but must not touch.

The same principle can be used with other metals: the object is the cathode, while the electrolyte contains ions of the desired coating metal. A soluble anode made from that metal can replenish the ions removed from the solution.

How the metal coating forms

When the direct current is switched on, positive copper ions migrate through the electrolyte towards the negatively charged object. At its surface, each copper ion gains two electrons and becomes a copper atom:

$\mathrm{Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)}$

This is reduction because electrons are gained. The copper atoms remain on the object, gradually producing a copper coating. The cathode therefore gains copper and increases in mass.

At the copper anode, copper atoms lose electrons and enter the solution as copper ions:

$\mathrm{Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-}$

This is oxidation because electrons are lost. The anode gradually becomes thinner. Copper ions are removed from the solution at the cathode and replaced at the anode, so the copper anode helps maintain the supply of $\mathrm{Cu^{2+}}$ ions.

Electrons move through the wires and power supply, whereas ions move through the electrolyte. These two kinds of charge movement complete the circuit.

Carrying out copper electroplating

Use a beaker, aqueous copper sulfate, a copper strip, the metal object, connecting leads and a low-voltage direct current supply. Wear eye protection and follow the teacher's instructions for handling and disposing of the solution.

1. Polish or otherwise clean the object so that grease, dirt and oxide do not prevent the copper from adhering. Rinse it if instructed.
2. Connect the object to the negative terminal of the switched-off supply.
3. Connect the copper strip to the positive terminal.
4. Suspend both electrodes in the copper sulfate solution. Immerse the area to be plated, keep the clips out of the solution and ensure that the electrodes do not touch.
5. Switch on the direct current and allow electrolysis to continue until a visible coating has formed, recording changes at both electrodes.
6. Switch off the supply before removing the object. Rinse and dry it as instructed, then inspect the coating.

The expected observation is a reddish-brown copper layer on the object. A successful coating should cover the intended surface rather than flake away. Cleaning matters because deposited atoms attach to the surface: grease or oxide can produce an uneven or poorly attached layer.

If mass is measured, the object must be clean and dry both before and after plating; otherwise retained solution or water could give a misleading apparent mass gain.

Linking the practical to corrosion protection

A coating can reduce corrosion by preventing oxygen, water or other reacting substances from reaching the underlying metal. Its effectiveness depends on the coating being continuous and sufficiently resistant to the conditions of use. If it is damaged, the exposed underlying metal may no longer be protected. Electroplating can therefore improve appearance, resistance to corrosion, or both.