Download free books at BookBooN.comInorganic and Applied Chemistry
Example 6- L:
Aluminium oxide as protection against corrosionIron in the form of steel is widely used as building material for houses, bridges, cars and more. Therefore
corrosion of iron is a phenomenon that has great practical application.Steel is a mixed product where the main part is iron atoms plus other metals and carbon. This means that
the surface of steel is irregular and such inhomogenities means that the surface of steel is easily oxidised.
In such anodic regions each iron atom gives 2 electrons in the following reaction:Fe(s) Fe2+ + 2 e-The given electrons by this process are transported to the cathodic regions of the steel following the earlier
stated principles of transport of electrons that we have looked at.In such cathodic regions on the steel Fe2+ ions react with the oxygen in the air and forms rust which is
chemically hydrated iron (III) oxide of variable composition. The reaction may simplified be written as:4 Fe2+(aq) + O 2 (g) + (4 + 2 n) · H 2 O(l) 2 Fe 2 O 3 · n H 2 O(s) + 8 H+(aq)Due to the transport of electrons from the anodic regions to the cathodic regions on the steel surface the
rust will often form a distance away from the site where the iron atoms are oxidised. This is sketched in
the figure below:Figure 6- 11: Corrosion of aqueous environment
The electrochemical corrosion of a steel surface in the presence of water.But what can be done in order to increase the life time for a steel surface? One of the possibilities is to
paint the steel surface in order to hinder oxygen and water molecules from getting into contact with the
iron atoms. One of the other possibilities is to treat the steel with a more corrosion preventive metal. E.g.
is it common to treat the steel with zinc in a so-called galvanisation process closely related to electrolysis,Electrochemistry