Corrosion of Metals
If a metal is reactive, its surface may be attacked slowly by the 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 metals, that is ,it makes the the surface of metal appear dull. The compounds formed on the surface of metal are usually porus and gradually fall off from the surface of metal, and then the metal underneath is attacked by air and water. This process goes on and on. In this way, the action of air and water gradually eats up the whole metal. At some places ( especially in industrial areas) there are some acidic gases in the air which mix with rain water to form chemicals such as acids. These acids also attack the surface of metals and eat them up slowly.
The eating up of metals by the action of air, moisture or a chemical ( such as an acid) on their surface is called corrosion. Most of the metals corrode when they are kept exposed to moist air.
For example- Iron metal corrodes when kept in moist air for a considerable time. When an iron object is kept in moist 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 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 iron is called rusting while other metals are said to corrode, iron metal is said to rust.
If a metal is reactive, its surface may be attacked slowly by the 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 metals, that is ,it makes the the surface of metal appear dull. The compounds formed on the surface of metal are usually porus and gradually fall off from the surface of metal, and then the metal underneath is attacked by air and water. This process goes on and on. In this way, the action of air and water gradually eats up the whole metal. At some places ( especially in industrial areas) there are some acidic gases in the air which mix with rain water to form chemicals such as acids. These acids also attack the surface of metals and eat them up slowly.
The eating up of metals by the action of air, moisture or a chemical ( such as an acid) on their surface is called corrosion. Most of the metals corrode when they are kept exposed to moist air.
For example- Iron metal corrodes when kept in moist air for a considerable time. When an iron object is kept in moist 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 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 iron is called rusting while other metals are said to corrode, iron metal is said to rust.
Rusting of Iron-
When an iron object is left in moist 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(|||) oxide, Fe2O3.xH2O. This hydrated iron(|||) oxide is called rust. So rust is mainly hydrated iron(|||) 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 rust on being exposed to moist air (or on being kept in water). But still rusts less readily than iron .
Conditions necessary for the rusting of iron-
Rusting of iron (or corrosion 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 moist air or when placed in water. Now, moist air also contains water vapour. Thus, moist 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 alone also supplies both the things, 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 (or corrosion). 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 (or corrosion 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 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.
(2) 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 their and moisture cannot come in contact with it and hence rusting is prevented.
(3) Rusting of iron can be prevented by galvanization-
The process of depositing a thin laye of zinc metal on iron object is called galvanization. Galvanisation is done by dipping and 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 objects protects them from rusting because zinc metal does not corrode on exposure to moist air.
(4) 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, then the iron and steel objects are protected from rusting.
(5) 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.
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