GCSE OCR Gateway Chemistry C6.1p Corrosion of iron and other metals

GCSE OCR Gateway Chemistry C6.1p Corrosion

C6.1p To be able to describe the process of corrosion and the conditions that cause corrosion in iron and other metals.

Corrosion in iron and other metals

Causes and conditions of the corrosion of iron

Iron easily corrodes to form brown rust a flaky substance that peels off the surface of the exposed iron.

It is possible to show that rust is produced from the action of both water and oxygen on the surface of the iron.

Chemically the reaction is an oxidation type reaction in which hydrated iron(III) oxide is formed as in the typical equation below:

4Fe + xH₂O + 3O₂ ⟶ 2Fe₂O₃.xH₂O

This reaction can be confirmed using the following experiment in which simple test tube reactions are set up with either air or water removed.  In both cases, there is a reduction or a complete absence of the formation of rust. 

The first test tube is the control experiment in which there is an iron nail immersed in water and in contact with air.  The iron nail rusts in this tube after a couple of days.

The second tube contains an identical iron nail in water which has been boiled for a significant length of time in order to remove all dissolved oxygen.  This set up is then sealed with a layer of oil to prevent air re-dissolving in the water.  No corrosion is observed after several days.

The third tube contains an identical iron in absolutely dry air.  Calcium chloride (CaCl₂) in the tube removes all water from the air in the tube.  After several days the iron nail has not corroded.

The fourth tube contains the same nail this time immersed in salt water and in contact with air.

If salt water is used instead of water in the control experiment then we observe an acceleration of rust formation.  We see this in the results of the experiment in the photo below:

These results to this simple experiment suggest that water and air are required for iron to corrode.

Corrosion in some other metals

Other metals do corrode but not as severely or as rapidly as iron

Copper

Copper we know and see turns green on exposure to air and rainwater.  Copper looks very attractive when first constructed as we see below:

The green colour is due to the formation of verdigris or basic copper carbonate on exposure to air water and carbon dioxide as we can see here on the Minneapolis City Hall building below:

Aluminium

If aluminium corrodes then its tough adhesive surface layer of one molecule thick aluminium oxide (Al₂O₃) has been broken through.  Usually this occurs in the presence of strong alkali like sodium hydroxide (NaOH) or strong acid such as sulphuric acid (H₂SO₄).