CORROSION, ITS TYPES & PREVENTION
Corrosion is the process of gradual deterioration of metal from its surface due to
unwanted chemical or electrochemical reaction of metal with its environment.
The metals are extracted from their metallic compounds (ores). During the extraction, ores are
reduced to their metallic states by applying energy in the form of various processes. In the pure
metallic state, the metals are unstable as they are considered in excited state (higher energy state).
Therefore as soon as the metals are extracted from their ores, the reverse process begins and form
metallic compounds, which are more stable (lower energy state). Hence, when metals are used in
various forms, they are exposed to environment, the exposed metal surface begin to decay (due to
conversion to more stable compound). This is the basic reason for metallic corrosion.
Corrosion (Oxidation of Metal) → Metallic Compound + Energy
M + O → MO
Metallurgy (Reduction of metal) → Metal + O
MO → M + O
Although corroded metal is thermodynamically more stable than pure metal but due to corrosion,
useful properties of a metal like malleability, ductility, hardness, lustre and electrical conductivity are
lost.
TYPES OF CORROSION: Based on the environment, corrosion is classified in to following types:
A. Dry or Chemical Corrosion
B. Wet or Electrochemical Corrosion
A. DRY OR CHEMICAL CORROSION: This type of corrosion is due to the direct chemical attack
of metal surfaces by the atmospheric gases such as oxygen, halogen, hydrogen sulphide,
sulphur dioxide, nitrogen or anhydrous inorganic liquid, etc. The chemical corrosion is
defined as the direct chemical attack on metals by the atmospheric gases present in the
environment. Example: (i) Silver metal undergo chemical corrosion by Atmospheric H 2 S gas
(ii) Iron metal undergo chemical corrosion by HCl gas.
DRY / CHEMICAL CORROSION is further classified into following types:
1. Corrosion by Oxygen or Oxidation corrosion
2. Corrosion by Other gases like Hydrogen
3. Liquid Metal Corrosion
1. CORROSION BY OXYGEN (OXIDATION CORROSION): Oxidation Corrosion is brought about by the
direct attack of oxygen at low or high temperature on metal surfaces in the absence of moisture.
Alkali metals (Li, Na, K etc.,) and alkaline earth metals (Mg, Ca, etc.,) are rapidly oxidized at low
temperature. At high temperature, almost all metals (except Ag, Au and Pt) are oxidized. The
reactions of oxidation corrosion are as follows:
Mechanism: 1) Oxidation takes place at the surface of the metal forming metal ions Mn+ as follows:
2 M → 2Mn+ + 2ne-
2) Oxygen is converted to oxide ion (O2-) due to the transfer of electrons from metal.
n/2 O2 + 2n e- → n O2-
3) The overall reaction is of oxide ion reacts with the metal ions to form metal oxide film.
2 M + n/2 O2 → 2 Mn+ + nO2-
2Mn+ + nO2- → MO (Metal oxide)
Metal + Oxygen → Metal oxide (Corrosion product)
, When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this
film decides the further action. So, the nature of the Oxide formed plays an important part in
oxidation corrosion process. If the film is
(i) Stable layer: A Stable layer is fine grained in structure and can get adhered tightly to the
parent metal surface. Hence, such layer can be of impervious nature (i.e., which cuts-off
penetration of attaching oxygen to the underlying metal). Such a film behaves as
protective coating in nature, thereby shielding the metal surface. The oxide films on Al,
Sn, Pb, Cu, Pt, etc., are stable, tightly adhering and impervious in nature.
(ii) Unstable oxide layer: This is formed on the surface of noble metals such as Ag, Au, Pt.
As the metallic state is more stable than oxide, it decomposes back into the metal and
oxygen. Hence, oxidation corrosion is not possible with noble metals.
(iii) Volatile oxide layer: The oxide layer film volatilizes as soon as it is formed. Hence,
always a fresh metal surface is available for further attack. This causes continuous
corrosion. MoO3 is volatile in nature.
(iv) Porous layer: The layer having pores or cracks. In such a case, the atmospheric oxygen
has access to the underlying surface of metal, through the pores or cracks of the layer,
thereby the corrosion continues unobstructed, till the entire metal is completely
converted into its oxide.
Pilling-Bedworth rule: According to it “an oxide film is protective or non-porous, if the volume of the
oxide is at least as great as the volume of the metal from which it is formed”. On the other hand, “if
the volume of the oxide is less than the volume of metal, the oxide layer is porous (or non-
continuous) and hence, non-protective, because it cannot prevent the access of oxygen to the fresh
metal surface below”.
Specific Volume Ratio = Volume of Oxide formed/ Volume of metal
(P-B Ratio)
Thus, alkali and alkaline earth metals (like Li, K, Na, Mg) form oxides of volume less than the volume
of metals. Consequently, the oxide layer faces stress and strains, thereby developing cracks and
pores in its structure. Porous oxide scale permits free access of oxygen to the underlying metal
surface (through cracks and pores) for fresh action and thus, corrosion continues non-stop. Metals
like Aluminium forms oxide, whose volume is greater than the volume of metal (PB ratio=1.28).
Consequently, an extremely tightly-adhering non-porous layer is formed. Due to the absence of any
pores or cracks in the oxide film, the rate of oxidation rapidly decreases to zero.
2. Corrosion by other gases (by hydrogen):
i. Hydrogen Embrittlement: Loss in ductility of a material in the presence of hydrogen is known as
hydrogen embrittlement.
Mechanism: This type of corrosion occurs when a metal is exposed to hydrogen environment. Iron
liberates atomic hydrogen with hydrogen sulphide in the following way:
Fe + H2 S → Fe S + 2H
Corrosion is the process of gradual deterioration of metal from its surface due to
unwanted chemical or electrochemical reaction of metal with its environment.
The metals are extracted from their metallic compounds (ores). During the extraction, ores are
reduced to their metallic states by applying energy in the form of various processes. In the pure
metallic state, the metals are unstable as they are considered in excited state (higher energy state).
Therefore as soon as the metals are extracted from their ores, the reverse process begins and form
metallic compounds, which are more stable (lower energy state). Hence, when metals are used in
various forms, they are exposed to environment, the exposed metal surface begin to decay (due to
conversion to more stable compound). This is the basic reason for metallic corrosion.
Corrosion (Oxidation of Metal) → Metallic Compound + Energy
M + O → MO
Metallurgy (Reduction of metal) → Metal + O
MO → M + O
Although corroded metal is thermodynamically more stable than pure metal but due to corrosion,
useful properties of a metal like malleability, ductility, hardness, lustre and electrical conductivity are
lost.
TYPES OF CORROSION: Based on the environment, corrosion is classified in to following types:
A. Dry or Chemical Corrosion
B. Wet or Electrochemical Corrosion
A. DRY OR CHEMICAL CORROSION: This type of corrosion is due to the direct chemical attack
of metal surfaces by the atmospheric gases such as oxygen, halogen, hydrogen sulphide,
sulphur dioxide, nitrogen or anhydrous inorganic liquid, etc. The chemical corrosion is
defined as the direct chemical attack on metals by the atmospheric gases present in the
environment. Example: (i) Silver metal undergo chemical corrosion by Atmospheric H 2 S gas
(ii) Iron metal undergo chemical corrosion by HCl gas.
DRY / CHEMICAL CORROSION is further classified into following types:
1. Corrosion by Oxygen or Oxidation corrosion
2. Corrosion by Other gases like Hydrogen
3. Liquid Metal Corrosion
1. CORROSION BY OXYGEN (OXIDATION CORROSION): Oxidation Corrosion is brought about by the
direct attack of oxygen at low or high temperature on metal surfaces in the absence of moisture.
Alkali metals (Li, Na, K etc.,) and alkaline earth metals (Mg, Ca, etc.,) are rapidly oxidized at low
temperature. At high temperature, almost all metals (except Ag, Au and Pt) are oxidized. The
reactions of oxidation corrosion are as follows:
Mechanism: 1) Oxidation takes place at the surface of the metal forming metal ions Mn+ as follows:
2 M → 2Mn+ + 2ne-
2) Oxygen is converted to oxide ion (O2-) due to the transfer of electrons from metal.
n/2 O2 + 2n e- → n O2-
3) The overall reaction is of oxide ion reacts with the metal ions to form metal oxide film.
2 M + n/2 O2 → 2 Mn+ + nO2-
2Mn+ + nO2- → MO (Metal oxide)
Metal + Oxygen → Metal oxide (Corrosion product)
, When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this
film decides the further action. So, the nature of the Oxide formed plays an important part in
oxidation corrosion process. If the film is
(i) Stable layer: A Stable layer is fine grained in structure and can get adhered tightly to the
parent metal surface. Hence, such layer can be of impervious nature (i.e., which cuts-off
penetration of attaching oxygen to the underlying metal). Such a film behaves as
protective coating in nature, thereby shielding the metal surface. The oxide films on Al,
Sn, Pb, Cu, Pt, etc., are stable, tightly adhering and impervious in nature.
(ii) Unstable oxide layer: This is formed on the surface of noble metals such as Ag, Au, Pt.
As the metallic state is more stable than oxide, it decomposes back into the metal and
oxygen. Hence, oxidation corrosion is not possible with noble metals.
(iii) Volatile oxide layer: The oxide layer film volatilizes as soon as it is formed. Hence,
always a fresh metal surface is available for further attack. This causes continuous
corrosion. MoO3 is volatile in nature.
(iv) Porous layer: The layer having pores or cracks. In such a case, the atmospheric oxygen
has access to the underlying surface of metal, through the pores or cracks of the layer,
thereby the corrosion continues unobstructed, till the entire metal is completely
converted into its oxide.
Pilling-Bedworth rule: According to it “an oxide film is protective or non-porous, if the volume of the
oxide is at least as great as the volume of the metal from which it is formed”. On the other hand, “if
the volume of the oxide is less than the volume of metal, the oxide layer is porous (or non-
continuous) and hence, non-protective, because it cannot prevent the access of oxygen to the fresh
metal surface below”.
Specific Volume Ratio = Volume of Oxide formed/ Volume of metal
(P-B Ratio)
Thus, alkali and alkaline earth metals (like Li, K, Na, Mg) form oxides of volume less than the volume
of metals. Consequently, the oxide layer faces stress and strains, thereby developing cracks and
pores in its structure. Porous oxide scale permits free access of oxygen to the underlying metal
surface (through cracks and pores) for fresh action and thus, corrosion continues non-stop. Metals
like Aluminium forms oxide, whose volume is greater than the volume of metal (PB ratio=1.28).
Consequently, an extremely tightly-adhering non-porous layer is formed. Due to the absence of any
pores or cracks in the oxide film, the rate of oxidation rapidly decreases to zero.
2. Corrosion by other gases (by hydrogen):
i. Hydrogen Embrittlement: Loss in ductility of a material in the presence of hydrogen is known as
hydrogen embrittlement.
Mechanism: This type of corrosion occurs when a metal is exposed to hydrogen environment. Iron
liberates atomic hydrogen with hydrogen sulphide in the following way:
Fe + H2 S → Fe S + 2H
