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Corrosion

If a metal is reactive, its surface may be attacked slowly by air and water (moisture) in the atmosphere. The metal reacts with the oxygen of air and water vapour of air forming compounds on its surface. The formation of these compounds tarnishes the metal, that is, it makes the surface of the metal appear dull. The compounds formed on the surface of the metal are usually porous and gradually fall off from the surface of the metal, and then the metal underneath is attacked by air and water. This process takes place repeatedly and in the end the action of air and water gradually eats up the whole metal.
 

The eating up of metals by the action of air and moisture on their surface is called corrosion. 

Most of the metals corrode when they are kept exposed to damp air (or moist air). For example, iron metal corrodes when kept in damp air for a considerable time. When an iron object is kept in damp air for a considerable time, then a red-brown substance called ‘rust’ is formed on its surface. Rust is soft and porous, and it gradually falls off from the surface of the iron object, and then the iron below starts corroding. Thus, corrosion of iron is a continuous process which ultimately eats up the whole iron object. The corrosion of metals is a highly undesirable process. A large amount of metal is lost every year because of corrosion. In general, the more reactive a metal is, the more readily it corrodes.

 

The corrosion of iron is called rusting. While other metals are said to ‘corrode’, iron metal is said to ‘rust’. In fact, most of the examples of corrosion which we come across in our daily life are due to the rusting of iron.

 


The rusting of iron is actually the most troublesome and damaging form of corrosion. We will now discuss the rusting of iron and its prevention in detail.
 

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, Fe2O3xH2O. This hydrated iron (III) oxide is called rust. So, rust is mainly hydrated iron (III) oxide, Fe2O3xH2O (the number of molecules of water x varies, it is not fixed). We have all seen iron nails, screws, pipes and railings covered with red-brown rust here and there. It is not only iron which rusts, even steel rusts on being exposed to damp air (or on being kept in water). But steel rusts less readily than iron. We will now describe the conditions which are necessary for the rusting of 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.

1. Presence of air (or oxygen)
2. Presence of water (or moisture)

 

We know that iron rusts when placed in damp air (moist air) or when placed in water. Now, damp air (or moist air) also contains water vapour. Thus, damp air alone supplies both air and water, required for the rusting of iron. Again, ordinary water has always some air dissolved in it. So, ordinary water alone also supplies both air and water, needed 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 object with ‘something’ to keep out air and water which cause rusting. The various common methods of preventing the rusting of iron are given below:


1. Rusting of Iron can be Prevented by 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, air and moisture cannot come in contact with the iron object and hence no rusting will take place. The window grills, railings, steel furniture, iron bridges, railway coaches, and bodies of cars, buses and trucks, etc., are all painted to protect them from rusting.


 

 

Rusting of Iron can be Prevented by 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.
 

Rusting of Iron can be Prevented by Galvanisation
The process of depositing a thin layer (or coating) of zinc metal on iron objects is called galvanisation. Galvanisation 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 iron object protects it from rusting because zinc metal does not corrode on exposure to damp air. The iron sheets used for making buckets, drums, dustbins and roofs of sheds are galvanised to prevent their rusting.

 

Rusting of Iron can be Prevented by Tin-Plating and Chromium-Plating
Tin and chromium metals are resistant to corrosion. So, when a thin layer of tin metal or chromium metal is deposited on iron and steel objects by electroplating, the iron and steel objects can be protected from rusting. For example, tiffin-boxes made of steel are nickel-plated from inside and outside to protect them 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 bar and car bumper made of iron and steel to protect them from rusting and gives 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. But stainless steel is too expensive to be used in large amounts. Please note that in the ‘stainless steel formation’ method of rust prevention, iron is not coated with anything.





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