Rusting of Iron

INTRODUCTION
Iron is one of the widely distributed elements in the nature. One of its striking characteristics is that it undergoes rusting on combining with water, air & carbon-dioxide due to which its surface gets covered with a red brown flake coating called "RUST". RUST is affected by moisture, oxygen & carbon - dioxide. RUST is soft and porous and it gradually falls off from the surface of iron material. It is a continuous process and it gradually eats up iron due to which an iron object loses its strength. It is very wasteful process and should be prevented. It is very-very slow process.
Theory
When a piece of iron metal is exposed to humid atmosphere, its gets covered with a red brown substance called "Rust". Rusting of iron can be explained on the basis of electro CHEMICAL THEORY as follows :
1. Water vapours on the surface of the metal dissolved carbon-dioxide and oxygen from the air. Thus the surface of metal is covered with the solution of carbon-dioxide with water, i.e. carbonic acid :-
H2O + CO2 ======> H2CO3 ---------- (i)
This acts as an electrolytic solution of the cell. The carbonic acid and water dissociate to a small extent as follows:
H2CO3 <======> 2 H+ +CO32-
H2O <======> H+ +OH- ---------- (ii)
2. Iron in contact with the dissolved CO2 and O2 undergoes oxidation as follows :-
Fe =========> Fe++ + 2e– -------------(iii)
As a result iron is converted into ferrous (Fe++) ion.
3. The electrons lost by iron are taken up by the H+ ions present on the surface of metal which were produced by the dissociation of H2CO3 and H2O. Thus H+ ions are converted into Hatoms.
H+ + e– =======> H ----------- (iv)
Thus H atoms either react with the dissolved oxygen or oxygen from air to from water.
4H + O2 ======> 2H2O ---------- (v)
The complete reaction may be written as :-
O2+ 4H+ + 4e– ======>2 H2O ------- (vi)
The dissolved oxygen may take up electrons directly to from OH– ions as follows :
O2 + 2H2O + 4e– ======> 4OH– ------(vii)
The over all reaction of the miniature cell will be :-
2Fe(s) + O2(g) + 4H+ (aq) ===> 2Fe++(aq)+2H2O(l)
4. The ferrous ions formed react with the dissolved oxygen or oxygen from air to from ferric oxid as follows :
4Fe++ + O2 + 4H2O ======> 2Fe2O3 + 8H+ (aq)
Ferric oxide then undergoes hydration to form RUST as follows :-
Fe2O3 + xH2O =======> Fe2O3 xH2O
Hydrated ferric oxide (RUST)
Requirements
Apparatus :
i) Boiling tubes
ii) Rubber stopper
iii) Iron nails
iv) Burner
v) Test tube, Holder & Stand
Chemicals :
i) Distilled Water
ii) Dilute HCL
iii) Dilute NaCl Solution
iv) Anhydrous Calcium Chloride
v) Cotton
vi) Mustard Oil
Procedure
i) Take five dry test tubes and mark them A, B, C, D and E. Take 10 pieces of iron nails free from rust.
ii) Moist two iron nails with distilled water and slide it in the test tube marked ‘A’ close the mouth of the tube.
iii) In the test tube ‘B’ take 5 ml of water and boil it so as to remove dissolved air. Slide two nails in this test tube and few drops of mustard oil so as to cover the surface of H2O.
iv) In the test tube ‘C’ take 5 ml of dilute NaCl solution. Slide two nails and close it.
v) In the test tube ‘O’ add two pieces of hydrous calcium chloride and cover it with thin layer of cotton. Again slide 3 nails and close it.
vi) leave these test tubes as such for 4-5 days and observe carefully in which of the tubes rust is formed.
Observations
i) It is observed that rusting takes place in the test tube A and C where as not in B, D, and E.
ii) Moisture, oxygen and carbon-dioxide present in air are responsible for rusting.
iii) It appears that during a compound of iron is formed by the combination of iron oxygen, carbon-dioxide and water.
iv) It is observed that when the process was prolonged in the presence of moist air and acids, rusting spreads quickly.
Methods of Prevention
i) Rusting of iron can be prevented by covering its surface with paints, grease and lacquers.
ii) Galvanisation :- This is one of widely practical methods used for the prevention of rusting. The layer of Zinc on the surface of iron, when comes in contact with moisture, oxygen and carbon-dioxide in air, ferrous a protective invisible thin layer of basic zinc carbonate ZnCO3. Zn(OH)2 due to which the galvanised iron sheets lose their lusture and also tends to protect it from further corrosion.
iii) Rusting of iron can also be prevented by coating its surface with iron (II, III) oxide Fe3O4 or iron (III) phosphate FePo4.
Conclusions
(i) Rust is formed by prolonged action of water oxygen and carbon-dioxide on an iron under other suitable conditions.
(ii) Moisture, Oxygen and carbon-dioxide present in air, all responsible for rusting.

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