Corrosion of Metals

        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.

                                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.

Chemical bonding and their types

Chemical bonding and their types-

Chemical bond -
A bond is any force which holds two atoms together. The formation of bond between two atoms is due to some redistribution or regrouping of electrons to form a more stable configuration. Several atoms are assembled and held together to form thousands of molecules which participate in the building and function of physical and biological systems.

Types of chemical bond-
The regrouping of electrons in the combining atoms may take place in either of three ways-

(1) Electrovalent or polar or ionic bond======> by a transfer of one or more electrons from one atom to another.

(2) Covalent bond =======> by a sharing of one or more pairs of electrons between the combining atoms.

(3) Co-ordinate Bond =======> by a combination of the two processes of transfer and sharing of electrons.



(1) Electrovalent or polar or ionic bond-

Ionic bond formation takes place between atoms of strongly electropositive and strongly electronegative elements. An element preceding and inert gas in the periodic table is strongly electronegative and the element immediately following the inert gas is strongly electropositive. For example- chlorine is electronegative while sodium is electropositive. According to W. kossel (1916),a transfer of electrons takes place from the outermost shell of the electropositive atom to the outermost shell of the electronegative atom, resulting in the formation of stable positive and negative ions respectively which are held together by electrostatic forces of attraction to form a molecule or more precisely and ion pair .


A   +   B ------> A+  +  B-

The atoms involved are electrically neutral before combining. The element A which has lost its electrons is known as electropositive where as element B which has gained the electrons is termed electronegative element. The compound formed by electron transfer is termed as electrovalent by Langmuir (1919) because the resulting compound is electrolyte. It is also called Polar since the molecules develops a positive and a negative pole.
               The electrovalent compounds always exist in ionic form, are hard and nonvolatile, have high melting and boiling points because of stronger nature of the bond and are soluble in polar solvents and because of the presence of ions conduct electricity in solution or in the fused state. The electrovalent compounds having identical electronic configuration exhibit the phenomenon of isomorphism.

 Na   +   Cl  ---->   Na+    +     Cl-

 Ca   +   O  -----> Ca2+   +   O2-

(2) Covalent bond (Nonpolar Bond)-

Covalent bond formation first suggested by G.N. Lewis (1916) consists in sharing or holding a pair of electrons in partnership between two combining atoms, so that the pair counts towards the electronic grouping of both atoms. By this mechanism also, the stability akin to the inert gas is attained by each atom. For each pair of electrons to be shared between two atoms in each of the constituent atom contributes one electron.

   A•  +   B• ----->  A : B

This type of linkage which is the result of equal contribution and equal sharing of electrons is known as covalent bond. The compound formed by electron sharing is termed as covalent or or nonpolar by Langmuir (1919).
              The covalent compounds always exist in molecular form, are non electrolytes or non ionizable, soluble in organic solvents and have low melting and boiling points because of weaker nature of the bond. They are usually liquids or gases and are generally soft, easily fusible and volatile. They are non conducting in the fused state or in solution. The covalent bond is rigid and directional and as such there is a  possibility of position isomerism and stereoisomerism amongst these compound.
              covalent linkage is common in organic compounds, although inorganic compounds also have it. In covalent compounds one pair of shared electrons corresponds to a single bond, two pairs to double bond, three pairs of electrons or 6 shared electrons to triple Bond.
        Some common examples from three categories are-

 H •  +  H•  ------> H-H

   :            :
 :O:   +   :O:   --------> O=O

                                    _
 N:.  +   .:N   ------>  N=N
                                 

(3) Co-ordinate Bond (Semipolar or Dative Bond)-

Co-ordinate bond is also formed by mutual sharing of electrons but in this case the two electrons that are shared come from the same atom. The shared pair of electrons is called lone pair. The atom which provides the pair of electrons is called the donor and the atom accepting this pair is called the acceptor. After the formation of the bond, the lone pair of electrons is held in common. This sort of bonding is called co-ordinate (Sidgwick) or dative (Menzies). In this mechanism although the sharing is equitable the contribution is one sided and therefore a slight polarity develops in the molecule. For this reason this bond is also called semipolar (Sugden). This type of linkage is represented by an arrow pointing away from the donor atom (or pointing towards the acceptor atom). Usually the donor is an atom which has already acquired stable electronic configuration and the acceptor is generally two short of the stable configuration.

    :         :                  :   :
  :A:  +  B:  ------>   :A:B:
    :         :                  :   :
In this case atom A is dinor and atom B is acceptor because atom A is given a lone pair of electrons to atom B.

For example- In O3(Ozone) form a co- ordinate bond.

   :         :        :              :    :      :
 :O:  + :O: + :O: -----> :O::O:->O:
                                                  :

Trichloromethane (chloroform)

Trichloromethane  (chloroform)      CHCl3

 Trichloromethane is a trichloro derivative of methane. It is commonly called as chloroform.

Methods of preparation-

 It is prepared by the chlorination of methane in the presence of sunlight.
         Sunlight
CH4 -------------------> CH3Cl + HCl
           +Cl2


             Sunlight
CH3Cl ---------------> CH2Cl2+ HCl
                +Cl2


             Sunlight
CH2Cl2 ---------------> CHCl3+ HCl
                  +Cl2           

Properties-

(1) colourless heavy liquid

(2) sweet taste 

(3) it is oxidised in the sunlight and form a harmful gas carbonyl chloride which is commonly called as phosgene.

                    Sunlight
2CHCl3+O2 ---------> 2COCl2+HCl                                           

Phosgene is a very poisonous gas. So it is not used in the anaesthetic purpose.

Process of storage for chloroform-

(1) For the prevent from sunlight chloroform is stored in coloured bottle.
(2) for the prevent from air chloroform is fully filled in the bottle.
(3) before the storage of chloroform in the bottle it is mixed with ethanol 0.6 to 1%. Ethanol reacts with phosgene and form non poisonous diethyl carbonate.

2C2H5OH + COCl2 ------->(C2H5)2CO3  +  2HCl


Uses of chloroform-

(1) it is used as a anaesthetic but its heavy dose is very lethal.
(2) it is used as a solvent for oil,fat ,rubber,raisin etc.
(3) it is used for preparation of chloropicrin ,chloretone etc.
(4) it is used for conservation of animals in the laboratory.
(5) it is used for making medicines.
(6) it is used as a laboratory reagent in the laboratory.
(7) in the present time chloroform is used for making freon refrigerant R-22.