Electroplating

Electroplating uses electricity to coat a metal object with a thin layer of another metal. This improves appearance and resistance to corrosion.

Key idea

Electroplating is a type of electrolysis. Ions in solution move, and electrons flow in wires, to deposit a metal on the object you want to coat.

Why electroplate?

• Appearance: shiny jewellery (silver-plated)
• Corrosion resistance: tin-plated steel cans
• Hard, wear-resistant surfaces: chrome-plated parts

Setting up the cell

• Cathode (negative): the object to be coated (e.g. a spoon)
• Anode (positive): usually the plating metal (e.g. a copper strip)
• Electrolyte: a solution containing ions of the plating metal (e.g. copper(II) sulfate)
• Power supply: anode is positive, cathode is negative

Electrons flow in the external circuit from anode to cathode. Metal ions move through the solution to the cathode.

What happens at the electrodes?

Cathode (reduction): metal ions gain electrons and become solid metal on the object.

$\text{Cu}^{2+}(aq) + 2e^- \to \text{Cu}(s)$

Anode (oxidation): atoms of the anode metal lose electrons to make ions that go into solution.

$\text{Cu}(s) \to \text{Cu}^{2+}(aq) + 2e^-$

Using a metal anode keeps the ion concentration steady, so plating is smooth.

Observations

• Cathode gets a thin, even metal layer
• Metal anode slowly dissolves
• With copper(II) sulfate (blue), the colour stays nearly constant if a copper anode is used

Choosing materials

• Match electrolyte to the plating metal: AgNO$_3$ for silver, CuSO$_4$ for copper.
• Use the same metal as the anode for steady plating. With an inert anode, the solution is used up and plating slows.

Common misconceptions

• Cathode is negative in electrolysis (not positive).
• The object to be coated must conduct electricity and be clean; dirt prevents smooth plating.
• Thicker plating needs more time and controlled current; turning up current too high gives rough, dull deposits.