Corrosion

definition

• The process by which metals are slowly broken down by chemical reactions with substances in their environments

rusting (equation/reaction type)

• oxygen + iron + water → hydrated iron (III) oxide (rust)

• redox reaction

Where corrosion takes place

General corrosion

• Corrosion only takes place at the surface level of the metal as it is the only part exposed to the environment

Rusting

• Iron goes through corrosion and forms a layer of rust along its surface

• This layer eventually flakes away thus breaking down the surface layer of metal and revealing a new layer of metal

• This process continues until their is no iron left for oxygen and water to react with

Corrosion of aluminium

• Only the outer layer of aluminium goes through corrosion to form aluminium oxide

• This is because as the outer layer corrodes, a protective layer is formed around the rest of the aluminium thus protecting is from corroding.

Test for rusting

1. Place a nail into three separate test tubes

2. Fill the first beaker with water (ensuring to boil it first to remove oxygen) and adding a layer of oil (to stop new oxygen from the air dissolving into the water

3. Fill the second beaker with solid calcium chloride to one beaker (to remove water vapor in the air)

4. Fill the third beaker with water but exclude a layer of oil

5. The third beaker will be the only one in which the nail will rust as it is the only one with suitable conditions for rusting to take place.

Preventing corrosion

Barrier methods

painting

• Effective on big pieces o metals (side of a ship)

Greasing/oiling

• Necessary for objects with moving parts (bike chain)

Electroplating

• Uses electrolysis to cover the iron in a thin layer of another metal to protect the iron underneath

Sacrificial methods

General method

• When a more reactive metal (aluminium/zinc) is added the iron

• The extra metal is more reactive then iron so it will be oxidised as opposed to the iron

• Iron will not rust as it is not exposed to oxygen

Mixture of methods

Galvanising

Sacrificial

• The zinc is more reactive then the oxygen

• During the reaction the zinc will be oxidised as opposed to the iron therefore preventing the iron from rusting

Barrier

• When the zinc is coated around the iron, it acts as a protective layer stopping oxygen from coming into contact with the iron