Corrosion of metals

CORROSION OF METALS

Corrosion is a process of deterioration of metal as a result of its reaction with air or water (present in environment) surrounding it.

RUSTING OF IRON:  

The slow conversion of iron into its hydrated oxide, in the presence of moisture and air is called rusting, whereas the hydrated oxide of iron is called rust.

4Fe   + 3O2             2Fe2O3

Iron      Oxygen             Ferric oxide

Fe2O3   + xH2O         Fe2O3.xH2O

Ferric Oxide    Water Hydrated ferric oxide

The brownish residue (Fe2O3.xH2O) is commonly called rust and the phenomenon is called rusting.  The rust so formed is flaky and easily crumbles from the surface of metal.  Thus, fresh iron is exposed to the attack of moist air, to form more rust.

PREVENTION OF CORROSION:

Rusting of iron and corrosion of other metals can be prevented by not allowing air to come in contact with metal. Some methods for doing this are as given below.

• Painting or applying oil: Painting the surface of the metal after cleaning it thoroughly is the most common method of preventing corrosion. The paint keeps air away from the metal surface. Applying grease or oil on the surface has a similar effect. However, if there is a break in the layer of paint, grease or oil, the metal starts corroding at that point.
• Covering with another metal: Depositing a layer of another metal which is more resistant to corrosion is another commonly used method for the prevention of corrosion of metals. The metals mostly used are tin and zinc. Covering a metal with another metal can be done by the following ways:

 Galvanizing: Clean iron sheets are dipped in molten zinc to deposit a layer of zinc on it. This process is called galvanization. The G.I. water pipes available in the market are made of galvanized iron. Galvanized iron does not rust even if there is a scratch on the zinc layer.

Electroplating: The process of electroplating is also used to deposit a metallic layer to prevent corrosion. Tin is electroplated on iron to make food containers commonly called tin cans. However, if the tin layer gets scratched, the iron starts rusting at that place. The tin can is then no longer suitable for storing food. Chromium is electroplated on steel furniture and bicycle handles to prevent rusting. In addition, the chromium layer also gives steel an attractive shine.

Anodizing of aluminum: A layer of aluminium oxide is formed on aluminium upon exposure to air. This prevents further corrosion of aluminium. If this layer is made more uniform, it protects the aluminium underneath more effectively. This is done by a process called anodizing. It consists of passing an electric current through sulphuric acid using aluminium as anode. Oxygen is evolved at the anode which reacts with aluminium to form a uniform protective layer of aluminium oxide.

• Alloying: Some metals become corrosion resistant when alloyed with other metals. For example, stainless steel which is an alloy of iron, chromium and nickel, does not rust easily.