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 called a cation. 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 further corrosion, this simply process repeats until the metal is completely rusted away. The montage below shows some badly corroded metal objects.
A simple equation to show the rusting or the corrosion of the metal iron is shown below:
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 as well as oxide ions (O2-); that is oxygen atoms that have gained 2 electrons and have been reduced.
Hydrated means that the iron oxide also contains some water molecules which are associated within the giant ionic lattice structure of the iron oxide. The amount of water present is shown by the value of n in the chemical formula for rust; n is a small number usually between 2 and 3. The two half-equations below show the oxidation of the iron atoms to form iron ions and the reduction of the oxygen atoms to form oxide ions. Overall we can say that the corrosion of a metal is a redox reaction where the metal is oxidised and the oxygen is reduced.
You may have set up the experiment shown in the image below in your science lessons. This is a quick and easy method to prove that BOTH water and oxygen are needed for corrosion to occur. Here iron nails are placed in three test-tubes containing a mixture of boiled water, water and a drying agent such as calcium chloride; the iron nails are then inspected daily over a period of a few weeks for any signs of corrosion.
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 off 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.
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 in building such as churches and museums, indeed 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 and not the shiny bronze colour you associate with copper metal. The colour change from
bronze 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 many 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.