Rusting is a natural phenomenon where metals corrode. Rusting is very common in iron. Only impure irons or alloys that contain iron can rust and other metals go corrosion in a similar way. 
When iron comes in contact with the moisture-containing water it reacts. Iron having positive(+ve) charge combines with oxygen having negative(-ve) charge. This chemical reaction produces Iron Oxide(Fe2O3) or Ferric Oxide most commonly called rust.
In Ferric Oxide(Fe2O3) reaction Iron(Fe) loses its electron to Oxygen(O2) atom. This process is called oxidation. Rust forms when Iron undergoes oxidation. Not all oxidation forms rust.
When impure Iron comes in contact with water, oxygen, other strong oxidants or acids it rusts.Rusting can of many types and color. Most common are red rust, brown rust, green rust, yellow rust, and black rust.
Rusting is similar to combustion. Unlike fire, though the reaction is much slower does not create a flame. But it slowly eats up the iron.
Not only water but also polluted air containing a huge amount of Carbon dioxide(CO2) causes rusting. CO2 also contains Oxygen. CO2 and H2O (water) react to form weak carbonic acid. As the acid attacks the iron, water breaks into Hydrogen(H2) and Oxygen(O2). Thus, the oxidation reaction takes place which further causes rusting.
Factors affecting Rusting:
1. Moisture-Iron exposed to rain or other sources of water rusts.
2. Acids- When Iron comes in contact to Acid Rain, it rust.
3. Salt- Iron tends to rust faster by the presence of salt in the sea.
4. Impurities- Impure Iron tends to rust faster than the pure irons.
Rusting causes metals to decay, making it flabby, weak and crumbly. It completely eats away the material and at a time nothing is left over. It is a slow chemical process.
We can prevent Iron from rusting. We just have to stop Iron from coming
in any contact of Oxygen and water.
Step to prevent Iron from Rusting:
1. Galvanization- Coating of Zinc over Iron.
2. Painting- Coating of paint over Iron.
3. Greasing- Applying grease over iron.
