Chemistry only

Corrosion is the attack of water and air (oxygen) on metals. The most common form of corrosion is when the metal iron corrodes. The corrosion of the iron is often called rusting. When a metal corrodes it is oxidised, that is it loses electrons and forms a positively charged ion. The corrosion of iron and steel (which is mostly iron) is both expensive to repair and replace and can also be dangerous, as corrosion weakens the metal. Rust is hydrated iron oxide (that is iron oxide which has water associated with it). It is a soft, crumbly compound which in time falls off the metal to expose fresh iron atoms that then undergo corrosion, this simply process repeats until the metal is completely rusted away.

corrosion montage

A simple equation for rusting or the corrosion of iron is shown below:

iron(s) + oxygen(g) + water(l) hydrated iron oxide(s)

Hydrated iron(III) oxide or rust has the chemical formula, Fe2O3.nH2O. It contains Fe3+ ions, that is iron atoms that have been oxidised and lost 3 electrons. the value of n in the chemical formula for rust is a small number usually between 2 and 3, this is simply because rust is not a pure substance but a mixture of substances:

FeFe3+ + 3e
You may have set up the experiment below in your science lesson. This is a quick and easy method to prove that BOTH water and oxygen are needed for corrosion to occur

corrosion practical

Corrosion in other metals

So far all we have really talked about is the corrosion or rusting of iron and steel. Obviously other metals will react with air (oxygen) and water and corrode. As an example consider aluminium and titanium. When these 2 metals corrode a layer of the metal oxide coats the metal surface. However whereas iron oxide or rust is a soft flaky solid that falls off the outside of the iron, aluminium oxide and titanium oxide are hard tough solids that seal of the layer of metal atoms below. We can say they protect the metal from undergoing any further corrosion by basically sealing it from air (oxygen) and water.

statue of liberty

This sealing of a metal surface to protect it from further corrosion also occurs with copper. Copper is a shiny bronze coloured metal which is often used to cover roofs. Many large churches and museums have roofs made of copper that have lasted for hundreds of years. However you may not have spotted these roofs as they are green not a shiny bronze colour! The colour change from bronze colour to green is a result of the corrosion of the copper metal. The copper is oxidised by oxygen and water in the air to form copper oxide which further reacts with carbon dioxide and other pollutants to form a number of compounds, one of which is green copper carbonate. Copper carbonate is one of the compounds responsible for the characteristic green patina seen with copper roofs.

Perhaps the most famous example of this is the Statue of Liberty in New York. When the statue was erected in 1886 it was given a skin made of copper. This would have been a shiny bronze colour but over the years the copper metal has corroded and turned a green colour or patina. This patina consists of a number of compounds including copper carbonate. The patina acts like a second skin and helps protect the copper underneath from undergoing further corrosion.

Key Points

Practice questions

Check your understanding - Questions on corrosion