3. Special steels
page 14
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Photo of iron ore
Picture 3.6 Iron ore being loaded into a lorry.
The nature of corrosion
The combination of iron with oxygen in moist conditions is an energetically favourable process. That is why we don’t find iron on its own in the ground. We find it as iron oxide in an iron ore. In order to free the iron from its oxide we have to supply energy in a Blast Furnace (see The chemistry of steelmaking). The iron we extract tends to re-form its oxide in an electrochemical process we call corrosion.
Corrosion graphic
Picture 3.7. the cycle of corrosion.
The chemistry
The chemical changes that happen during corrosion can be shown as follows:
Chroium layer
Picture 3.8 The chromium content protects stainless steel by forming an oxide layer.
How stainless steel resists corrosion
If iron and chromium are exposed to oxygen, it is the chromium that reacts to form an oxide. Due to its high chromium content, stainless steel forms a very thin layer of chromium(III) oxide as soon as the surface is exposed to the oxygen in the atmosphere. The layer of oxide is so thin that the metal can still shine through it, but it is thick enough to prevent the oxygen and water attacking the metal underneath and so no corrosion takes place.

The protection is permanent because even if the protective layer is scratched, the chromium in the steel will react with oxygen in the air to immediately re-form the protective layer.

Other forms of corrosion protection
Stainless steel is a very effective way of preventing rusting. The trouble is that it is expensive because chromium is expensive. Other cheaper methods of preventing corrosion are used when the degree and durability of the protection are less critical.

Protection How it is applied Uses
Galvanising Coating with a layer of zinc. Protects even when scratched Car bodies
Chromium plating Coating with a layer of chromium. Protection lost when scratched Pipes and ornamental work
Painting Covering with a layer of paint. Protection lost when scratched Large structures such as ships and buildings
Organic coating Covering with a layer of plastic Electrical and office equipment
Tin Plating Coating with a layer of tin. Protection lost when scratched Food and drink cans
Question 3-3.
a) What conditions are needed for iron to become hydrated iron(III) oxide?

b) What form of corrosion protection would be suitable for the steel structures of a sports stadium?


Summary                   Close
  • iron will corrode when exposed to the atmosphere
  • stainless steel resists corrosion
  • the chromium in stainless steel forms a very thin protective oxide layer