The two conditions necessary for the rusting of iron to occur are:
- Presence of moisture, and
- Presence of oxygen.
The following factors further catalyst the process of rusting.
- Presence of carbon dioxide
- Presence of acids.
- Presence of impurities in their on.
Due to rusting iron object loses its strength.
Prevention of Rusting
There are several ways of preventing rusting of iron. Some of these are discussed below:
1. Barrier protection:
In this method, a barrier film is introduced between iron and atmospheric oxygen and moisture. Barrier protection can be achieved by one of the following ways:
- By painting the surface
- By coating the surface with a thin film of oil or grease.
- By electroplating iron with some non-corrosive metal such as nickel, chromium, copper, etc.
2. Sacrificial protection:
In this method, surface of iron is covered with layer of more active metal like zinc. This active metal loses electrons (undergoes oxidation) in preference to iron and hence, prevents the rusting of iron.
Zinc metal is generally used for protecting iron and the process is called galvanization. Galvanized iron sheets maintain their shine due to the formation of a thin protective layer of basic zinc carbonate, ZnCO3, Zn (OH) 2 due to the reaction between zinc, oxygen, CO2 and moisture in air.
Zinc, magnesium and aluminum powder dissolved in paints can also be applied as protective layers. The well-known aluminum paint contains aluminum powder suspended in varnish.
3. Use of anti-rust solutions:
The alkaline phosphate and alkaline chromate solutions act as anti-rust solutions. When iron articles are dipped into a boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the article from rusting.