1. CHAPTER 4 CORROSION AND ITS PREVENTION
Introduction of Corrosion
Facts about Corrosion
Causes of Corrosion
Types of corrosion
E ffects of corrosion
Factors affecting corrosion
Prevention or control of corrosion

2. Introduction of Corrosion
The branch of Engineering that deals with the study of corrosion and its prevention safely is known as corrosion Engineering.
Corrosion:
The loss of material(metals or alloys) or its useful properties by chemical interaction with its environment is known as corrosion.
Examples;
Rusting of iron.
Blackening of silver articles in atmosphere.
Fogging of nickel.

4. Facts about corrosion Corrosion is a natural process.
Corrosion is an electrochemical process.
Corrosion leads to a tremendous loss.
Corrosion is the reverse of metal extraction.

5. Causes of corrosion
Most metals exist in nature in combined forms that is in the form of ore. So extraction of metal from its ores is an unnatural process in which component of a stable system is being separated.
Ore extraction Metal corrosion corrosion product
Stable gain es mteta stable loss of es stable
During the process of extraction a number of steps like concentration, Roasting ,and Smelting are involved and finally the ore is reduced into metal.
therefore metals in their finally refined state are highly unstable and have a tendency to revert back in their original state that is to stable state by the process of corrosion(oxidation).

6. Example; Rusting of iron is the most popular example of corrosion.
Usually the Surface of metal is covered with a thin layer of oxide. When this layer is broken, bare surface is exposed to the environment and this area act as an anode while the remaining area act as cathode.
since the medium is exposed to the atmosphere, it contain dissolved oxygen. Thus at cathode oxygen reduction take place while at anode iron corrodes.
4 Fe Fe e Oxid. At anode
O2 + 4H2O + 8 e OH Red. At cathode
___________________________
4Fe + 6H2O + 3O Fe(OH) Fe2O3. nH2O

7. Types of corrosion 1. Uniform corrosion
This is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal.

8. 2. Galvanic Corrosion
Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

9. 3. Crevice or contact corrosion
Crevice or contact corrosion is the corrosion produced
at the region of contact of metals with metals or
metals with nonmetals. It may occur at washers,
under barnacles, under applied protective films.

10. 4. Corrosion in Concrete
Concrete is a widely-used structural material that is
frequently reinforced with carbon steel reinforcing
rods, post-tensioning cable or pre-stressing wires. The
steel is necessary to maintain the strength of the
structure, but it is subject to corrosion.

11. 5. Erosion corrosion
Erosion corrosion is the result of a combination of an
aggressive chemical environment and high fluid-surface
velocities.
6. Dealloying
Dealloying is a rare form of corrosion
found in copper alloys, gray cast iron,
and some other alloys. Dealloying
occurs when the alloy loses the active
component of the metal and retains the
more corrosion resistant component
in a porous "sponge" on the metal surface.

13. Effects of corrosion Reduced Strength Downtime of equipment
Lost surface properties
Reduced value of goods

14. Factors affecting corrosion
1. Presence of impurities in metals
Speed of corrosion increases with the presence of
impurities in the metals because these impurities help
in setting up the voltaic cells.
2. Presence of electrolyte
Electrolytes present in water also increases the rate of
corrosion e.g. corrosion of iron in sea water takes
place in large extent than in distilled water because
sea water contains salts i.e.electrolytes.

15. 3. Position of metals in electrochemical series
Highly reactive metals undergo corrosion faster than
least reactive metals. Reactivity of metals can be
found from the electrochemical series.
e.g.Au Cu Pb Fe Zn Al Mg Na K
Reactivity increases
4. Presence of carbon dioxide in water
Presence of carbon dioxide in natural water also
increases the rusting of iron because it acts as an
electrolyte and increases the flow of electron from
one place to another.

16. 5. Presence of protective coating
When the iron surface is coated with the metal,
which is more reactive than the iron, then the rate of
corrosion is retarded e.g. when iron is coated with
zinc, iron is protected from rusting. 

17. PREVENTION OF CORROSION
Corrosion is slow but continuously eating away the metal and consequence of rusting.
Rusting can be avoided by not letting moist air come in contact with the metal surface .Following methods could gain fully be adopted to minimize rusting and corrosion

18. Methods of preventing corrosion and rusting
Tarring
Painting
Enameling
Galvanizing
Sheradising
Tin plating
electroplating

19. Tarring:
metal is dipped in hot coal tar so that a film of it is sticks to the surface which protect the surface from rusting and corrosion.
Pipes or ends of poles to be embedded in the earth are usually given this protective treatment.

20. 2. Painting:
Paints usually the lead paints are applied on the surface to be protected. Exposed metal surfaces as in case of roof and bridge structure are given this treatment which has to be repeated after regular interval of time.

21. 3.Enameling:
Enamels consist of high grade bases like zinc oxide or lead oxide ground in oil or varnish. they dry slowly leaving a hard tough and elastic film which is smooth and durable.Enamle painted surfaces are washable and are not affected by acids,alkali,gases or steam. Even tough they are more costly than ordinary paints yet because of their durability. They are equally good for use both on external and internal work.

22. 4.Galvanizing:
Depositing a fine film of zinc on the iron/steel surfaces is termed as galvanizing.
The surfaces to be galvanizing is first cleared of all foreign matter by giving it an acid wash to be followed by a wash of clean water. the surface is then dried and dipped in molten zinc.The fine film of zinc that get deposited protect the surface from contact with atmosphere and consequence oxidation.
Removal of the zinc film caused by scratches.

23. 5.Sheradising:
Surface to be treated is cleaned of all foreign deposits by washing it with acid solution and then with clean water.
It is then dried and covered with zinc dust and enclosed in steel boxes to be heated in a furnace under controlled temperatures.
Molten zinc spreads over the whole surface and on cooling forms a thin protective layer.
Sherading gives better protection than galvanizing.

24. 6.Tin plating:
After cleaning the surface with acid wash followed by wash with clean water and drying, it is dipped in a bath of molten tin.
A protective covering of tin layer if left on the surface.

25. 6. Electroplating:
By the process of electrolysis a thin film of nickel,cromium,copper or zinc is deposited on the surface to be protected.
The surface to be protected is made the cathode and the metal to be deposited is made the anode.