6.1.10 Corrosion of Iron

• Most metals react with substances when exposed to the environment for prolonged periods
• This causes degradation of the metal in a process called corrosion
• Corrosion occurs at the surface of the metal only
• Rusting is a chemical reaction between iron, water and oxygen to form the compound iron (III) oxide
• Oxygen and water must be present for rusting to occur
• Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride catalyses the process
• The equation for the rusting of iron is( x represents a varying amount of water):

4Fe (s) + 3O₂ (g) + xH₂O (l) → 2Fe₂O₃.xH₂O (s)

• Rust is a soft solid substance that flakes off the surface of iron easily, exposing fresh iron below which then undergoes rusting
• This means that over time all of the iron rusts and its structure becomes weakened
• This is a major concern as iron is used extensively in industries such as transport and construction
• Aluminium is another metal that undergoes corrosion but in a slightly different way to iron
• Aluminium reacts with oxygen to produce aluminium oxide, Al₂O₃
• The aluminium oxide forms a tough protective layer that covers the aluminium, preventing further corrosion

Investigating Rusting 

• Oxygen and water must be present for rust to occur
• You can investigate the conditions needed for rusting by setting up a series of control test tubes as shown below:

Diagram showing how the conditions for rusting can be investigated

Method:

• Set up the apparatus as shown in the diagram
• The water in the second test tube is boiled to remove any dissolved oxygen
• The oil provides a barrier to prevent oxygen diffusing into the boiled water
• Calcium chloride is a drying agent in the third test tube
• Leave the apparatus for a few days to give it time to react

Results:

• The nail on the left rusts as it is in contact with both air (which contains oxygen) and water
• The nail in the middle does not rust as it is not in contact with air
• The nail on the right does not rust as it is not in contact with water (calcium chloride absorbs any water molecules present due to moisture)
• The results show that both air and water must be present for rusting to occur

Barrier Methods

• Rust can be prevented by coating iron with barriers that physically 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 are common as they are cheap and often the most practical solution

Sacrificial protection & Galvanising

• Sacrificial protection involves using a metal that is more reactive than iron, for example magnesium or zinc 
• The more reactive metal will corrode first, it 'sacrifices' itself to protect the iron 
• Rusting occurs due to iron atoms losing electrons, being oxidised, to form iron (III) ions

Fe   →    Fe³⁺ + 3e^- 

• In sacrificial protection, the more reactive metal will lose electrons more easily as they are more readily oxidised 
• Galvanising is a process where the iron is protected specifically by a layer of zinc 
• ZnCO₃ is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier method
• If the coating is damaged or scratched, the iron is still protected from rusting because zinc preferentially corrodes as it is higher up the reactivity series than iron, acting as a sacrificial metal
• Compared to iron the zinc loses its electrons more readily 

Sacrificial corrosion

• Sacrificial corrosion occurs when a more reactive metal is intentionally allowed to corrode
• An example of this occurs with ships' hulls which sometimes have large blocks of magnesium or magnesium alloys attached
• The blocks slowly corrode and provide protection to the hull in the same way the zinc does by pushing electrons onto the iron which prevents it from being reduced to iron(III) ions