The eating up of metals by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion.
So, reactive metals such as potassium and sodium are kept in kerosene oil to avoid their exposure to the atmosphere.
FACTORS GOVERNING CORROSION OF METALS
The corrosion of metals increases in presence of oxygen.
Metals placed higher in the reactivity series corrode more easily.
The rate of corrosion is enhanced even by the dissolved salts in water which act as an electrolyte. For example, iron corrodes faster in seawater.
PREVENTION OF CORROSION
Barrier protection:
It is done by a coating of one metal with another metal with the help of electricity (electroplating).
So, the metal surface is not allowed to come in contact with the atmospheric agents like air and water and hence it is protected from corrosion.
Sacrificial protection:
Electropositive metal such as zinc or magnesium, are used to cover the metal to be protected.
These metals, being more electropositive get oxidized and save the metal from corrosion.
Cathode protection:
The metal object that is to be protected from rusting is connected to a piece of more electropositive metal like zinc.
The anode is made up of a more reactive element which loses electrons and gets oxidized and thus saves the cathode form rusting.
RUSTING OF IRON
Rust is mainly hydrated iron (oxide, Fe2O3xH2O.
The corrosion of iron is called as rusting. A reddish coating is developed on the surface of articles made up of iron, when it is exposed to the atmosphere.
Rust is a red-brown coloured substance.
It is not only the iron which rusts, even the steel rusts on exposure to damp air (or if exposed to water). But steel rusts less readily than iron.
Rust is an oxide of the metal formed by the reaction with the oxygen present in the atmosphere.
CONDITIONS NECESSARY FOR THE RUSTING
Presence of air (or oxygen)
Presence of water (or moisture)
The presence of pollutants, for example, NO2 and CO2 also enhances rusting.
PREVENTION OF RUSTING
Painting. Coat the surface of the iron with paint. This restricts the contact of air and moisture on the iron object.
By applying grease or oil. This restricts the contact of air and moisture on the iron surface.
Galvanisation:
The process of depositing a thin layer of zinc metal on iron objects is called galvanisation.
This can be done by dipping an iron object in molten zinc metal.
This results to the formation of a thin layer of zinc metal over the iron object.
This layer of zinc metal in turn protects iron objects from exposure to damp air.
Tin-plating and chromium-plating.
Tin and chromium metals are resistant to corrosion. So, the electroplating prevents from rusting of projects.
For example, tiffin-boxes made of steel are nickel-plated in and out to protect them from rusting.
As tin is non-poisonous, it is used for plating tiffin-boxes, hence does not contaminate the food kept in them.
Chromium –plating is done on taps, bicycle handle bars and car bumpers made of iron.
By Alloying
Stainless steel is obtained, when iron is alloyed with chromium and nickel. Stainless steel does not rust at all.
Cooking utensils, Knives, scissors and surgical instruments, etc., are made of stainless steel.
CORROSION OF ALUMINIUM
Aluminium vessels lose their shine and become dull very soon after use.
This occurs due to the corrosion of aluminium metal when exposed to moist air.
The process of thickening layer of aluminium oxide on the surface of aluminium objects by electrolysis known as anodizing.
This process adds on the protection from corrosion.
CORROSION OF COPPER
In damp air copper reacts slowly with the carbon dioxide and water of air to form a green coating of copper carbonate on the surface of the object.
This corrodes the copper. Cleaning of the corroded copper vessels with dilute acid solution dissolves green coloured copper carbonate present on the corroded copper vessels.
