Corrosion Of Metals

Corrosion is a process of deterioration of metal as a result of its reaction with air or water (present in environment) surrounding it.

RUSTING OF IRON:

The slow conversion of iron into its hydrated oxide, in the presence of moisture and air is called rusting, whereas the hydrated oxide of iron is called rust.

4Fe + 3O₂ → 2Fe₂O₃

Iron Oxygen Ferric oxide

Fe₂O₃ + xH₂O  → Fe2O₃.xH₂O

FerricOxide Water Hydrated ferric oxide

The brownish residue (Fe₂O₃.xH₂O) is commonly called rust and the phenomenon is called rusting. The rust so formed is flaky and easily crumbles from the surface of metal. Thus, fresh iron is exposed to the attack of moist air, to form more rust.

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. The various common methods of preventing the rusting of iron (or corrosion of iron) are given below :

1. By painting. The most common method of preventing the rusting of iron (or corrosion 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. 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.

2. 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.

3. By galvanization. The process of depositing a thin layer of zinc metal on iron objects is called galvanization. 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 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 (roofs) are galvanized to prevent their rusting. The iron pipes used for water supply are also galvanized to prevent rusting.

4. 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. 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 bars and car bumpers made of iron and steel to protect them from rusting and give them a shiny appearance.

5.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. But, stainless steel is too expensive to be used in large amounts.