Corrosion is the spontaneous disintegration of metals due to chemical or electrochemical reactions with the environment, causing the formation of compounds like oxides. It occurs as metals strive to return to stable states from higher energy levels, forming oxide films that can be protective or porous. Various methods exist to control corrosion, including proper design, material selection, environmental modifications, and the use of inhibitors.

1. What is Corrosion?
The process of spontaneous disintegration of metals by formation of compounds
like oxides, sulphates, carbonates, etc. on the surface caused by direct chemical or
indirect electrochemical attack by reaction with the environment (moisture and
air).
Note: The term rusting applies to the corrosion of iron and iron base alloys.
Nonferrous alloys corrode but do not rust.

2. Why Corrosion Occurs?
Metals exist in nature in combined forms like oxides, sulphides, etc. which are
thermodynamically stable states. To extract pure metal, energy is supplied and the
extracted metal is at a higher energy level which means it becomes thermodynamically
unstable.
 Metals try to get back to their stable states by combining with other elements, and in
this process, corrosion occurs.
 Metallic ore (mineral or metal in combined form) Pure Metal
Thermodynamically Stable (High Energy)
Unstable

3. During the process of corrosion, the metal surfaces form an oxide
film on their surfaces.
For corrosion to continue further, the nature of the oxide film is
very important. They are of four types:
1. STABLE
2. UNSTABLE
3. VOLATILE
4. POROUS

4. 1. Stable
It is fine grained, tightly sticking and impervious, for example, Al, Sn, Pb and Cu. It
acts as a protective layer and prevents further corrosion.
2. Unstable
The oxide formed decomposes back into metal and oxygen, for example, Ag,
Pt and Au. As a result, there is no corrosion.

5. 3.Volatile
The oxide layer formed volatilizes as soon as it is formed. Fresh metal surface is exposed for further
attack. This causes rapid and continuous corrosion, for example,
molybdenum.
4. Porous
The oxide film is porous, that is, having pores or cracks. Here the O2 penetrates inside and attacks the
underlying metal and hence continuous corrosion occurs, for example, iron and steel.
2Mo + 3𝑂2 → 2Mo𝑂3

6. Pilling–Bedworth Rule
The ratio of the volume of metal oxide to the volume of the underlying metal is called the Pilling–
Bedworth ratio or the specific volume ratio. It helps to predict the extent of corrosion of a metal.
Greater the value of Pilling–Bedworth ratio, lesser is the extent of corrosion.
If the volume of the oxide film is less than the volume of metal laying underneath then the oxide
layer is porous, contains cracks and hence is nonprotective, and if the volume is equal to or
greater than the volume of the underlying metal, the oxide film is continuous, nonporous and
protective, that is, further corrosion does not take place.

7. Mechanism of corrosion:
The two basic theories of corrosion are-
• Dry or chemical corrosion.
• Wet or electrochemical corrosion.

8. Dry/Chemical Corrosion
Also called direct corrosion, it occurs due to direct action of environment on metals in the absence of
moisture. It takes place at a temperature higher than 100℃. It is generally of three types:
a). Oxidation corrosion- This type of corrosion occurs by the direct action of Oxygen only.
b). Corrosion by other gases- In this, dry gases like H2, Cl2, F2, SO2 and CO2 directly attack the metal
surface. Rate of corrosion depends upon the chemical affinity between the metal and gas.
c). Liquid metal corrosion- When a liquid metal flows over a solid metal at high temperature it
weakens the solid metal because of its dissolution in liquid metal and penetration of liquid metal into
solid metal.

9. Wet/Electrochemical Corrosion
It is more common type of corrosion, it occurs under moist or wet conditions. For this corrosion to occur a
metal should be in contact with a conducting liquid or metals of different reactivities are in contact with a
solution. Rusting of iron is the most common example of electrochemical corrosion.
 For this type of corrosion to occur a corrosion cell/electrochemical cell must be formed.

10. To understand electrochemical corrosion we must first understand the necessary
conditions for the formation of electrochemical cell.
I. There must be separate anodic and cathodic areas.
II. There must be electrode potential between the anode and cathode.
III. There must be a metal path connecting the anode and cathode.
IV. Conducting liquid should be present between the two electrodes.
The Reactions involved are as follows:
At Anode:
Metal loses electrons and passes them into the solution through oxidation. corrosion always occurs at
the anodic areas.
M → 𝑀𝑛+
+ n𝑒−
(metal) (metal ion)

11. At Cathode:
Electrons released at the anodic areas are consumed at the cathode. Cathodic
reactions are of two types:-
i. Evolution of hydrogen
This mechanism occurs when anode is larger than cathode and oxygen is absent. When the environment
is acidic, H+ ions of the acid take up electrons and hydrogen gas is evolved.
2𝐻+ + 2𝑒−  𝐻2↑
ii. Absorption of oxygen
This mechanism occurs when anode is smaller than cathode and oxygen is present and the environment is
neutral or alkaline.
½ 𝑂2 + 𝐻2𝑂 + 2𝑒−  2𝑂𝐻−

12. Difference Between Chemical & Electrochemical
Corrosion

13. TYPES OF CORROSION
Electrochemical corrosion is of two types:
1. Differential Aeration Corrosion (DAC) or Concentration Cell Corrosion
Examples:
 Pitting Corrosion
 Waterline Corrosion
 Stress Corrosion
 Corrosion Under a drop of water, leaf, stone, etc.
 Caustic Embrittlement
2. Differential Metallic Corrosion (DMC) or Bimetallic Corrosion
Examples:
 Galvanic Corrosion

14. Differential Aeration Corrosion(DAC)/Concentration cell corrosion
Concentration cell corrosion takes place when different areas of a metal surface are in contact with an
electrolyte of different concentration (caustic embrittlement) or varying aeration (pitting corrosion,
waterline corrosion, corrosion under a block of wood or a drop of water, etc).

15. Galvanic corrosion
It is also called bimetallic corrosion or differential metallic corrosion. It occurs when two
dissimilar metals (for example, Zn and Cu) are electrically connected and exposed to an
electrolyte and then the metal higher in the electrochemical series undergoes corrosion.
Example:
In a Zn-Cu couple
Zn  𝑍𝑛2+ + 2𝑒− (At Anode)
2𝐻++ 2𝑒−  𝐻2 ↑

16. Factors Influencing Corrosion
•Nature of the metal
a). Position in the galvanic series : Greater the difference in the position of the two metals greater is the
corrosion of the anodic metal.
b). Purity of the metal : Pure metals are less prone to corrosion.
•Nature of the corroding environment.
a). Temperature: With the rise in temperature, rate of corrosion also increases as temperature rises. At
higher temperatures, even the passive metal can become active and get corroded.
b). Humidity: Corrosion increases with the increase in humidity of the atmosphere.

17. Protection against Corrosion (Corrosion Control)
The various methods by which corrosion can be controlled and prevented are as follows:
1. Protection by proper designing
2. Material selection
3. Modifying the environment
4. Modification of properties of the metal
5. Use of inhibitors
6. Cathodic protection or electrochemical protection
7. Use of protective coating.

18. Protection by proper designing:
1. Avoid direct contact of two dissimilar metals in a corroding medium. If it cannot be avoided then use an insulating
fitting between them.
2. When two dissimilar metals in direct contact are to be used then-
 They should be close in the galvanic series.
 The anodic material should have a large area.
 Joints should be welded with same metal/alloy.
 Moisture retention is as low as possible. Sharp corners should be avoided and storage containers should be
designed in such a way that they can be completely drained and cleaned.

19. Material Selection
 Using pure metals: Impurities in metals cause heterogeneity that leads to corrosion.
 Using metal alloys: The metals can be made corrosion-resistant by alloying them with suitable alloying
elements.

20. Modifying The Environment
1. De-Aeration: Removal of gases like oxygen and carbon dioxide from the environment reduces
corrosion.
2. Deactivation: It involves addition of chemicals that remove oxygen.
3. Dehumidification- Alumina or silica gel may be added to remove moisture from the environment.

21. Modification Of The Properties Of The Metal
The properties of the metals can be modified to decrease the effects of corrosion. It can be done by:
1. Alloying- Alloys are more resistant to corrosion than pure metals.
2. Refining- As pure metals are more corrosion resistant, refining can increase the corrosion resistance of
the metal.
3. Annealing- It is a heat treatment given to the metals to reduce residual stress.
ALLOYING
ANNEALING

22. Use of Inhibitors
A substance that when added to the corrosive medium effectively decreases the corrosion of the metal is
known as a corrosion inhibitor. These are of two types:
(i) Anodic inhibitors- It gets absorbed on the anodic surface forming a passive film on it.
(ii) Cathodic inhibitors- It slows down reactions taking place at the cathode.

23. Cathodic Protection or Electrochemical Protection
The metal to be protected is made to behave like a cathode. This can be done in two ways:
(i) Sacrificial Anode Cathodic Protection Method (SACP): The metal structure to be protected is
connected to a more reactive metal. The more reactive metal behaves as anode and the iron structure
becomes cathodic. The anode which is sacrificed to protect the base metal is called Sacrificial Anode.
(ii) Impressed current cathodic protection (ICCP): Direct current from an external source is applied in
opposite direction to nullify the corrosion current and convert corroding metal from anode to
cathode.
SACP 
ICCP

24. Use Of Protective Coatings
The metal structure to be protected is covered with metallic or nonmetallic coatings. These coatings
separate the base metal from the corrosive environment and protect the base metal from corrosion.

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