Rusting of Iron and How to Prevent it?
What is Rusting of Iron?
When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron.
During the rusting of iron, iron metal combines with the oxygen of air in the presence of water to form hydrated iron (III) oxide, Fe2O3.xH2O. (the number of molecules of water x varies, it is not fixed). Rust is red-brown in colour. We have all seen iron nails, screws, pipes and railings covered with red-brown rust here and there. It is not only the iron which rusts, even the steel rusts on being exposed to damp air or water. But steel rusts less readily than iron.
Conditions Necessary for the Rusting of Iron
Rusting of iron needs both air and water. Thus, two conditions are necessary for the rusting of iron to take place:
Presence of air ( or oxygen)
Presence of water ( or moisture)
We know that iron rusts when placed in damp air or when placed in water. Now, damp air also contains water vapour. Thus, damp air alone supplies both the things, air and water, required for the rusting of iron. Again, ordinary water has always some air dissolved in it. So, ordinary water also supplies both the things necessary for rusting.
Prevention of Rusting
The wasting of iron objects due to rusting causes a big loss to the country's economy, so it must be prevented. Several methods are used to protect the iron objects from rusting. Most of the methods involve coating the iron objects with something to keep out air and water ( which cause rusting). The various common methods of preventing the rusting of iron are given below:
- Painting: The most common method of preventing the rusting of iron is to coat its surface with a paint. When a coat of paint is applied to the surface of an iron object, then air and moisture cannot come in contact with the iron object and hence no rusting takes place. The iron articles such as window grills, railings, steel furniture, iron bridges, railway coaches, ships and bodies of cars, buses and trucks etc. are all painted to protect them from rusting.
- Applying Grease or Oil: When some grease or oil is applied to the surface of an iron object, then air and moisture cannot come in contact with it and hence rusting is prevented. For example, the tools and machine parts made of iron and steel are smeared with grease or oil to prevent their rusting.
- Galvanisation: The process of depositing a thin layer of zinc metal on iron objects is called galvanisation. It is done by dipping an iron object in molten zinc metal. A thin layer of zinc metal is then formed all over the iron object. This thin layer of zinc metal on the surface of the iron objects protects them from rusting because zinc metal does not corrode on exposure to damp air. The iron sheets used for making buckets, drums, dust bins and sheds are galvanised to prevent their rusting. The iron pipes used for water supply are also galvanised to prevent rusting.
How zinc can prevent rusting? Zinc is a quite reactive metal. The action of air on zinc metal forms a very thin coating of zinc oxide all over it. This zinc oxide coating is hard and impervious to air and hence prevents the further corrosion of zinc metal. Galvanised iron object remains protected against rusting even if a break occurs in the zinc layer. This is because zinc is more easily oxidised than iron. So, when the zinc layer on the surface of galvanised iron object is broken, the zinc continues to corrode but iron object does not corrode or rust.
- Chromium-Plating and Tin-Plating: Tin and chromium metals are resistant to corrosion. So, when a thin layer of tin metal (or chromium) is deposited on iron and steel objects by electroplating, then they are protected from rusting. Tin is used for plating tiffin-boxes because it is non-poisonous and hence does not contaminate the food kept in them. Chromium-plating is done on bicycle handle bars and car bumpers made of iron and steel to protect them from rusting and give them a shiny appearance.
- Rusting of iron Can be Prevented by Alloying it to Make Stainless Steel: When iron is alloyed with chromium and nickel, then stainless steel is obtained. Stainless Steel does not rust at all. Cooking utensils, knives, scissors and surgical instruments, etc, are made of stainless steel and do not rust at all.