2/12/24, 9:05 PM
`Corrosion - two case studies:
`aluminium and iron
`Iron rusts, but aluminium does not The chemistry of corrosion Corrosion experiment
` Permeable vs impervious Protecting iron from rusting
`
`Primary Connections: Linking science with literacy
`
`Aluminium and iron in the form of steel are
`the two most widely used metals. Although
`iron has been in use since the beginning of
`the Iron Age (around 1000BC) this has not
`always been the case for aluminium.
`
`Although aluminium is very common in the Earth's crust, the extraction of
`aluminium from its ore, Bauxite, is difficult and it has only really been commercially
`available since the early 1900s.
`
`Both these metals are used in construction
`and industry where exposure to water, air and
`pollution can potentially cause great damage
`to the metals. A quick look around the
`average house soon shows that if iron, or its
`alloy steel, is left out in the wet, it will soon
`corrode and rust. A piece of aluminium in the
`same conditions seems to last quite well,
`showing little signs of corrosion.
`Iron rusts, but aluminium does not
`Aluminium really should not be able to be used as it is in high tension power lines,
`planes, ships, building and cans to name but a few. Its relatively high reactivity
`should make it impossible to use safely. Aluminium can be made to corrode quickly
`in air and even burn violently on heating, but it does not usually react, lasting
`longer than the less reactive iron in normal environments.
`
`What is happening?
`The chemistry of corrosion
`Corrosion, or rusting, occurs when oxygen gas and water react with a metal to form
`a new chemical on the surface of the metal called an oxide. Most metals corrode.
`For some like sodium, it is such a fast and violent reaction that the metal burns in
`contact with water and is soon nothing but oxide or hydroxides!
`
`At the other end of the spectrum is gold and the other precious metals which resist
`corrosion and form only very thin oxide layers or do not react at all with water and
`oxygen.
`
`Like most other metals, aluminium and iron react relatively slowly to form oxides
`and corrode. The reactions can be represented by chemical equations.
`
`The two equations for aluminium and iron respectively are:
`
`FAQ:
`
`Why do cars rust
`more quickly near the
`beach?
`Why do you have to
`clean silver?
`
`Related
`Topics: Metals and non-
`metals
`Properties of metals
`Refining metals
`Iron (Fe)
`Gold (Au)
`Mercury and heavy
`metal pollution
`Production of iron
`Production of
`aluminium
`
`Quiz:
`
`Question 1
`Question 2
`Question 3
`Question 4
`
`Sites:
`
`N/A
`
`Glossary
`
`N/A
`
`Note: This is not the only possible reaction for iron, but it is a fair representation of
`the effect of many different reactions. This process can be speeded up by placing
`the metals in salt water.
`Corrosion experiment
`Try a small experiment with steel wool. Place three pieces of steel wool on a bench
`top. One is kept dry, one is placed in some fresh water (do not cover the steel wool
`
`https://www.primaryconnections.org.au/themes/custom/connections/assets/SBR/data/Chem/sub/corrosion/corrosion.htm#:~:text=When it starts to corr… 1/2
`
`1
`
`APPLE 1027
`
`

`

`Primary Connections: Linking science with literacy
`2/12/24, 9:05 PM
`with water) and the third placed in salt water. Watch for a few
`days. The corrosion will soon become quite evident!
`
`Chemists call corrosion "oxidation". Oxidation literally means "to
`combine with oxygen". Corrosion occurs fastest when the metal
`is in contact with air, water and salt.
`Why does iron rust when aluminium will remain untouched by
`corrosion?
`Permeable vs impervious
`When it starts to corrode, aluminium forms an oxide layer, which
`is impervious to water and oxygen. Iron, on the other hand,
`forms a permeable oxide layer which allows both water and
`oxygen to pass through and keep attacking the underlying iron.
`
`The impervious aluminium oxide layer protects the aluminium
`from further corrosion whereas the rust on the iron flakes off,
`exposing more metal iron to attack.
`Protecting iron from rusting
`The best way to protect iron or steel is to make an impervious layer on the metal
`and protect it the same way as aluminium does naturally. This is why engineers
`paint, grease, oil or tar steel to stop water and oxygen getting to the iron.
`
`Another method is to dip the iron in hot liquid zinc. This is called
`galvanising. What happens is that the zinc binds to the iron and
`then corrodes instead of the iron. An added advantage is that
`the zinc forms an impervious oxide layer like aluminium does
`and this further protects both the iron and the zinc.
`
`Iron and steel can also be protected electrically, by stopping and
`reversing the oxidation process. This occurs when lumps of zinc
`or magnesium are welded onto the hulls of ships or the piers of an oil rig. The more
`reactive zinc or magnesium is corroded before the iron, thus sacrificing themselves
`to corrosion instead of the iron. As a result of this behaviour, zinc or magnesium
`are known as "sacrificial anodes"!
`
`Copyright owned by the State of Victoria (Department of Education and Early
`Childhood Development). Used with Permission.
`
`https://www.primaryconnections.org.au/themes/custom/connections/assets/SBR/data/Chem/sub/corrosion/corrosion.htm#:~:text=When it starts to corr… 2/2
`
`2
`
`