Rusting

Rusting of iron is the most common type of corrosion. Iron when exposed to moist air (air containing large quantity of water vapour) gets covered with a layer of brown powdery material. Formation of brown powdery material on the surface of iron in the presence of moist air is called rusting of iron.Rust mainly consists of hydrated ferric oxide (Fe2O3.xH2O). The reaction which describes rusting of iron is,

4Fe(s) + 3O2(g) + xH2O  →  Fe2O3.xH2O(s)
iron oxygen water from air hydrated ferric oxide (rust)

Rust is soft, porous and powdery substance. It falls off from the surface of iron of its own. This exposes the lower layers of iron to the atmosphere. As a result, the rusting continues, and gradually the iron loses its strength. Iron does not rust is dry air. Iron needs oxygen and water for rusting.

The following factors accelerate the rusting of iron.

  1. The presence of gases such as, carbon dioxide (CO2), oxides of sulphur (SO2, SO3) and oxides of nitrogen (NO, NO2) in air increases the rate of rusting of iron.
  2. The presence of electrolytes, such as sodium chloride, in water increases the rate of rusting. It is because of this reason that iron rusts faster in sea water than in distilled water.
  3. Presence of impurities of less electro positive metals in iron increases the rate of rusting.
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