CORROSION – MODULE 3
CORROSION
• Any process of deterioration (or destruction) and consequent loss of solid
metallic material through an unwanted chemical or electrochemical attack
by its environment is called corrosion.
WHY DO METALS CORRODE
• Naturally occurring compounds of metals are in thermodynamically stable
state (least energy state), while the extracted metals from these ores have
high-energy state because of the taking up of large amount of energy
during its metallurgy.
• The metals, thus, obtained when exposed to natural environment will have
a tendency to go to their stable lower energy state whenever an
opportunity arises.
TYPES OF CORROSION
DRY OR CHEMICAL CORROSION
• If a metal is exposed to dry environment, the metal gets corroded due to
direct attack of O2 or corrosive gases like SO2, Cl2, or chemical action on
metals through chemical reactions.
• Dry corrosion is classified into
A) Oxidation corrosion
B) Corrosion by other gases
, C) Liquid metal corrosion
# Mechanism of Dry corrosion
→ Oxidation first occurs at the surface of the metal resulting in the formation
of metal ions
→ Oxygen changes to ionic form (O2-) due to transfer of electrons from the
metal which occurs at the oxide film/environment interface.
→ Oxide ions react with the metal ion to form the metal oxide film
# Oxidation corrosion
→ Metals are attacked by dry oxygen (absence of moisture) at extreme
conditions like low or high temperature leading to oxidation type of
corrosion.
→ The alkali and alkaline earth metals are more prone and rapidly oxidized at
low temperatures while metals like Fe, Al, etc. (except noble metals) are
oxidized at high temperatures by the interaction with oxygen.
→ When oxidation starts, a thin layer of oxide film will be formed on the
surface of the metal and the nature of the film decides the further action.
Pilling-Bedworth rule
➢ If the volume of metal oxide formed on the metal surface is greater than
the volume of metal then the oxide layer is protective or non-porous. So,
specific volume ratio is greater than or equal to 1. Hence, no further
corrosion will take place. Eg: Al, Ga, Tl etc.
, ➢ If the volume of metal oxide formed on the metal surface is less than the
volume of metal then the oxide layer is non-protective or porous. So,
specific volume ratio is less than 1. Hence, further corrosion will take place.
Eg: Li, Na, K, Mg etc.
Classification of Oxide layers
➢ Stable Oxide layer: A fine-grain of oxide which forms a compact surface
adhered tightly to the parent metal surface. They are impervious in nature
(cut the penetration of O2) and act as a protective coating. Eg: Oxides of Al,
Sn, Cu etc.
➢ Unstable Oxide layer: This kind of oxide film formed decomposes back to
metal and oxygen. Corrosion is not possible because the oxide layer formed
is very unstable and decomposes back to metal and oxygen. Eg: Oxides of
noble metals such as Pt, Ag, Au etc.
➢ Volatile Oxide layer: This kind of oxide film formed volatilizes as soon as it is
formed. This leads to excessive corrosion. Eg: MoO3
➢ Porous Oxide layer: The oxide layer formed has pores and cracks which
makes O2 accessible to underlying metal surfaces. The corrosion continues
till complete conversion of metal to oxide. Eg: Fe
# Corrosion by other gases
→ The extent of corrosive effect depends mainly on the chemical affinity
between the metal and gas.
→ The thin film formed on the metal surface may be protective or non –
protective
→ Action of Cl2 on Ag:
→ Action of Cl2 on Sn:
→ Action of H2S on Fe:
→ Examples of other gases: SO2, CO2, F2
CORROSION
• Any process of deterioration (or destruction) and consequent loss of solid
metallic material through an unwanted chemical or electrochemical attack
by its environment is called corrosion.
WHY DO METALS CORRODE
• Naturally occurring compounds of metals are in thermodynamically stable
state (least energy state), while the extracted metals from these ores have
high-energy state because of the taking up of large amount of energy
during its metallurgy.
• The metals, thus, obtained when exposed to natural environment will have
a tendency to go to their stable lower energy state whenever an
opportunity arises.
TYPES OF CORROSION
DRY OR CHEMICAL CORROSION
• If a metal is exposed to dry environment, the metal gets corroded due to
direct attack of O2 or corrosive gases like SO2, Cl2, or chemical action on
metals through chemical reactions.
• Dry corrosion is classified into
A) Oxidation corrosion
B) Corrosion by other gases
, C) Liquid metal corrosion
# Mechanism of Dry corrosion
→ Oxidation first occurs at the surface of the metal resulting in the formation
of metal ions
→ Oxygen changes to ionic form (O2-) due to transfer of electrons from the
metal which occurs at the oxide film/environment interface.
→ Oxide ions react with the metal ion to form the metal oxide film
# Oxidation corrosion
→ Metals are attacked by dry oxygen (absence of moisture) at extreme
conditions like low or high temperature leading to oxidation type of
corrosion.
→ The alkali and alkaline earth metals are more prone and rapidly oxidized at
low temperatures while metals like Fe, Al, etc. (except noble metals) are
oxidized at high temperatures by the interaction with oxygen.
→ When oxidation starts, a thin layer of oxide film will be formed on the
surface of the metal and the nature of the film decides the further action.
Pilling-Bedworth rule
➢ If the volume of metal oxide formed on the metal surface is greater than
the volume of metal then the oxide layer is protective or non-porous. So,
specific volume ratio is greater than or equal to 1. Hence, no further
corrosion will take place. Eg: Al, Ga, Tl etc.
, ➢ If the volume of metal oxide formed on the metal surface is less than the
volume of metal then the oxide layer is non-protective or porous. So,
specific volume ratio is less than 1. Hence, further corrosion will take place.
Eg: Li, Na, K, Mg etc.
Classification of Oxide layers
➢ Stable Oxide layer: A fine-grain of oxide which forms a compact surface
adhered tightly to the parent metal surface. They are impervious in nature
(cut the penetration of O2) and act as a protective coating. Eg: Oxides of Al,
Sn, Cu etc.
➢ Unstable Oxide layer: This kind of oxide film formed decomposes back to
metal and oxygen. Corrosion is not possible because the oxide layer formed
is very unstable and decomposes back to metal and oxygen. Eg: Oxides of
noble metals such as Pt, Ag, Au etc.
➢ Volatile Oxide layer: This kind of oxide film formed volatilizes as soon as it is
formed. This leads to excessive corrosion. Eg: MoO3
➢ Porous Oxide layer: The oxide layer formed has pores and cracks which
makes O2 accessible to underlying metal surfaces. The corrosion continues
till complete conversion of metal to oxide. Eg: Fe
# Corrosion by other gases
→ The extent of corrosive effect depends mainly on the chemical affinity
between the metal and gas.
→ The thin film formed on the metal surface may be protective or non –
protective
→ Action of Cl2 on Ag:
→ Action of Cl2 on Sn:
→ Action of H2S on Fe:
→ Examples of other gases: SO2, CO2, F2
