Corrosion & Its Control
Dr Champa Verma
Associate Professor
JNGEC, Sundernagar, Mandi (HP)
, Introduction
• Corrosion has been defined as a slow but constant decay of
metal by attack of surrounding. It starts at surface and
slowly proceed towards inner layers of metal.
• Most of the metals except few metals such as gold,
platinum (called noble metal) are prone to corrosion.
• Typical examples of corrosion
1. Rusting of iron due to formation of hydrated ferric oxide.
2. Formation of green film of basic copper carbonate [CuCO3
+ Cu(OH)2] on surface of copper when exposed to moist air
containing CO2.
, Mechanism or Causes of Corrosion
Except few metals like gold, platinum (noble metal) other metals are found in
nature as their compounds (such as oxides, hydroxides, carbonates, chlorides,
sulphides, phosphates, silicates etc.) which are called their ore.
Metals are thus obtained by extraction from their ores by reduction process.
In nature, when metals exists as their compounds (or ore) they are stable and
they are in the low energy states.
However, during extraction of metals from their ores, free metals are become
less stable and are in the higher energy state than in the ionic state.
So, metals have a tendency to back to the ionic state and hence metal atoms
are prone to get attacked by environment .
This is the main reason for corrosion of metals.
Mineral Extraction Corrosion Corrosion
or Metal
(M) product
ore (Mn+) by reduction -ne
(Mn+)
+ ne
, Types of Corrosion
1. Chemical (or dry) corrosion: It involves direct
chemical attack of atmospheric gases like CO2, O2,
H2S, SO2, halogen and inorganic acid vapors on metal.
2. Electrochemical (or wet)corrosion:
• It occurs due to setting up of a large number of tiny
galvanic cells in metals in presence of an impurity as
well as in presence of moisture. Generally impurity
(more active metal) act as anode and other metal act
as cathode so anode is the area where corrosion
occurs.
• Example, rusting of iron in moist atmosphere.
Dr Champa Verma
Associate Professor
JNGEC, Sundernagar, Mandi (HP)
, Introduction
• Corrosion has been defined as a slow but constant decay of
metal by attack of surrounding. It starts at surface and
slowly proceed towards inner layers of metal.
• Most of the metals except few metals such as gold,
platinum (called noble metal) are prone to corrosion.
• Typical examples of corrosion
1. Rusting of iron due to formation of hydrated ferric oxide.
2. Formation of green film of basic copper carbonate [CuCO3
+ Cu(OH)2] on surface of copper when exposed to moist air
containing CO2.
, Mechanism or Causes of Corrosion
Except few metals like gold, platinum (noble metal) other metals are found in
nature as their compounds (such as oxides, hydroxides, carbonates, chlorides,
sulphides, phosphates, silicates etc.) which are called their ore.
Metals are thus obtained by extraction from their ores by reduction process.
In nature, when metals exists as their compounds (or ore) they are stable and
they are in the low energy states.
However, during extraction of metals from their ores, free metals are become
less stable and are in the higher energy state than in the ionic state.
So, metals have a tendency to back to the ionic state and hence metal atoms
are prone to get attacked by environment .
This is the main reason for corrosion of metals.
Mineral Extraction Corrosion Corrosion
or Metal
(M) product
ore (Mn+) by reduction -ne
(Mn+)
+ ne
, Types of Corrosion
1. Chemical (or dry) corrosion: It involves direct
chemical attack of atmospheric gases like CO2, O2,
H2S, SO2, halogen and inorganic acid vapors on metal.
2. Electrochemical (or wet)corrosion:
• It occurs due to setting up of a large number of tiny
galvanic cells in metals in presence of an impurity as
well as in presence of moisture. Generally impurity
(more active metal) act as anode and other metal act
as cathode so anode is the area where corrosion
occurs.
• Example, rusting of iron in moist atmosphere.
