2. INTRODUCTION
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• Corrosion it is defined as the process of decomposition or
destruction of a metallic material in presence of environmental
factors through chemical reaction.
• A large number of equipments are used for manufacturing of drug
when the solution which is corrosive in a nature comes in contact
with the equipment it can destroy the performance and fabrication of
equipment therefore great care should be adopted during the
selection of a material for construction of equipment.
3. • The term corrosion is defined as an act or process
of gradual wearing away of a metal due to
chemical or electro-chemical reaction by its
surroundings such that the metal is converted into
an oxide, salt or some other compound.
• A substance known as the rust results from such a
process. In other words, the corrosion indicates
the deterioration and loss of material due to
chemical attack.
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INTRODUCTION
4. TYPES OF CORROSION
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Corrosion may be of dry and wet type.
Dry Corrosion is a type in which
absence of aqueous environment but
usually in the presence of gases and
vapours mainly at a high temperature.
Wet Corrosion is due to attack of
aqueous media
8. THERIEROS OF CORROSION
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1) Chemical action theory or direct corrosion
2) Electrolytic theory or electro-chemical corrosion
3) Galvanic action theory
4) High-temperature oxidation.
Each of the above theory of corrosion will now be
briefly described.
9. Chemical action theory or direct
corrosion
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The direct corrosion is the simplest corrosion produced by
means of a chemical attack and it includes oxidation in which
the oxygen of the atmosphere combines with all or a part of the
surface of material.
Common examples are as follows:
(i) Acid pickling or chemical bath used to clean the metal
surfaces
(ii) Corrosion of copper flashing or sheet metal in the
atmosphere
(iii) Reactions of dry chlorine, hydrogen, etc.
(iv) Rusting of iron and steel
(v) Tarnishing of silverware, etc.
10. Electrolytic theory or electro-
chemical corrosion
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This is the commonly accepted theory of corrosion.
According to this theory, the corrosion takes place due to
chemical reaction in combination with electrolysis.
It takes place at or near room temperature when the metal
comes into contact with moisture or with aqueous solutions of
salts, acids or bases.
For electro-chemical corrosion to occur, the following
conditions should be satisfied simultaneously:
There should be an electrolyte.
The current should be passing through the circuit.
There should be a difference of potential between a metal
and its surrounding or between different parts of the same
metal.
The circuit must be closed.
11. GALVANIC THEORY
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The galvanic corrosion occurs when two dissimilar metals are in
electrical contact with each other and are exposed to an electrolyte.
For instance, a less noble metal like zinc will dissolve and form the
anode whereas the more noble metal such as copper will act as the
cathode.
Thus the anode metal is made to corrode or dissolve continuously by
the galvanic action. It is therefore necessary to observe that the direct
contact between dissimilar metals is avoided in the fabrication work to
prevent the corrosion of the anodic metal.
12. High-temperature oxidation
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The rusting of ferrous alloys at high temperatures forms scales and
oxides. It indicates the high-temperature dry corrosion. The other form
of the high-temperature corrosion occurs when the liquid metals flow
through other metals.
The corrosion is due to the tendency of the solid to dissolve in the
liquid metal upto the solubility limit at the given temperature.
The liquid-metal attack may take any of the following three
forms:
(i) Simple solution of the solid metal, or
(ii) Formation of chemical compound, or
(iii) Selective extraction of one of the component metals in a solid
alloy.
13. Corrosion of Ferrous Metals:
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The main constituent of the ferrous metals is iron. The
term rusting is sometimes used to refer the corrosion of
ferrous metals. The ferrous metals corrode most easily.
14. Following are the two reasons for the corrosion to
develop in the non-ferrous metals i.e., metals which
do not contain iron as their main constituent:
(i) Contact between unprotected dissimilar metals
where moisture is present; and
(ii) Contact with lime, cement and some varieties of
timber which may release acids, alkalies or salts when
damp.
The two important non-ferrous metals liable to corrode
are lead and zinc. The copper is unaffected by cement
or lime.
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Corrosion of Non Ferrous Metals:
