Corrosion of Metals (Edexcel GCSE Chemistry)

• Corrosion is the destruction of materials by chemical substances in their environment which act on them over a period of time
• Most metals can corrode in the presence of oxygen to form the corresponding metal oxide
• Corrosion is caused by redox reactions:
  • The metal loses electrons and is oxidised while the oxygen gains electrons and is reduced

• Rusting is the name given specifically to the corrosion of iron in the presence of water and oxygen from the air:

iron + water + oxygen ⟶ hydrated iron(III)oxide

Barrier Methods

• Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen
• However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust
• Unlike some other metals, once iron begins to rust it will continue to corrode internally as rust is porous and allows both air and water to come into contact with fresh metal underneath any barrier surfaces that have been broken or scratched
• Common barrier methods include: paint, oil, grease and plastic

Barrier Methods for Preventing Corrosion of Metals

• Electroplating is a process where the surface of one metal is coated with a layer of a different metal
• The metal being used to coat is a less reactive metal than the one it is covering
• The anode is made from the pure metal used to coat
• The cathode is the object to be electroplated
• The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode

A piece of iron being electroplated with tin. The electrolyte is tin(II) chloride, a water-soluble salt of tin

Uses of electroplating

• Electroplating is done to make metals more resistant to corrosion or damage, e.g. chromium and nickel plating
• When people talk about a 'tin can', the amount of tin is very small (only about 1%). The can is made from steel and has a very thin coat of tin on the interior surface that resists corrosion from the liquids inside
• It is also done to improve the appearance of metals, e.g. silver plating cutlery and jewellery