9. ELECTROCHEMISTRY AND CORROSION
The surface of almost any metal begins to decay more or less rapidly when it comes into
contact with the gaseous or liquid medium surrounding it. This is actually owing to the
chemical interaction between the metal and the gases present in the air, or the water
and the substances dissolved in it. So any process of chemical decay of metals due
to the action of the surrounding medium is called corrosion. The corrosion or
rusting of iron is the most important case of corrosion and owing to its enormous
practical and economic importance, the phenomenon has received much attention.
The simplest case of corrosion is that which occurs when metals come into contact with
gases. The surface of the metal becomes coated with the corresponding compounds
namely oxides, sulphides, basic carbonates which often form a compact layer protecting
the metal from further attack by the gases. On the other hand, corrosion caused by
contact with a liquid (e.g water) and the substances dissolved in it is different from that
caused by air. The chemical compounds formed in the previous case may dissolve and
thus allows corrosion to penetrate further into the metal. Such type of corrosion is called
chemical or dry corrosion. Moreover, since water containing dissolved salts is a
conductor of electric current, it promotes electrochemical processes which enhance
corrosion. Such type of corrosion is known as electrochemical or wet corrosion
because here corrosion occurs through ionic reactions in the presence of moisture.
However, experimentally it has been observed that air and water are necessary for
corrosion, but neither alone will do. Thus bright iron nails in a dessicator do not rust and
neither do nails kept in air free water.
Hence
Corrosion is an unintentional gradual destruction of a metal by chemical or
electrochemical attack.
Common types of corrosion may be expressed by one or more of the following reactions.
(a) Combination of metal with nonmetal in absence of water: M + X → MX
eg., at high temperature in dry air, 2Fe + O2 → 2 FeO
Page 1 of 9
, (b) Combination of metal with oxygen in presence of water:
A proposed mechanism consists of the following steps:
(1) Fe(s) → Fe2+ + 2e-
(2) H+ + e- → H
(3) 4H + O2(g) → 2 H2O
(4) 4 Fe2+ + O2(g) + 4 H2O → 2 Fe2O3 + 8 H+
In step (1) Fe acts as an anode, becoming oxidized to Fe2+. Electrons move through the
iron bar to a moist surface, where H+ ions in water accept them, forming neutral H atoms
(Step 2). Step (3) – H atom reacts with O2 in the air to produce H2O.
Step (4) - Fe2+ ions are oxidized to Fe3+ in the presence of water to form rust, a hydrated
form of ferric oxide (Fe2O3. xH2O).
Oxidation is the loss of electrons by the substance being oxidized.
Anode is the electrode at which oxidation occurs.
Reduction is the gain of electrons by the substance being reduced.
Cathode is the electrode at which reduction occurs
(c) Displacement of hydrogen from acids or acid solutions:
M + H2Y → MY + H2
Here Y may be sulphate, chloride, sulphite etc. Examples are, attack on iron by sulphuric
acid or hydrochloric acid, and tarnishing of silver by H2S in presence of moisture: 2Ag +
H2S → Ag2S + H2
(d) Displacement of hydrogen from water which usually contains small amounts of
dissolved inorganic substances:
M + 2 H2O → M(OH)2 + H2
The rusting of iron in water is a form of corrosion involving this reaction.
(e) Displacement of one metal ions by another metal from salt solution:
M’ + MSO4 → M’SO4 + M
eg., Fe + CuSO4 → FeSO4 + Cu
Page 2 of 9
The surface of almost any metal begins to decay more or less rapidly when it comes into
contact with the gaseous or liquid medium surrounding it. This is actually owing to the
chemical interaction between the metal and the gases present in the air, or the water
and the substances dissolved in it. So any process of chemical decay of metals due
to the action of the surrounding medium is called corrosion. The corrosion or
rusting of iron is the most important case of corrosion and owing to its enormous
practical and economic importance, the phenomenon has received much attention.
The simplest case of corrosion is that which occurs when metals come into contact with
gases. The surface of the metal becomes coated with the corresponding compounds
namely oxides, sulphides, basic carbonates which often form a compact layer protecting
the metal from further attack by the gases. On the other hand, corrosion caused by
contact with a liquid (e.g water) and the substances dissolved in it is different from that
caused by air. The chemical compounds formed in the previous case may dissolve and
thus allows corrosion to penetrate further into the metal. Such type of corrosion is called
chemical or dry corrosion. Moreover, since water containing dissolved salts is a
conductor of electric current, it promotes electrochemical processes which enhance
corrosion. Such type of corrosion is known as electrochemical or wet corrosion
because here corrosion occurs through ionic reactions in the presence of moisture.
However, experimentally it has been observed that air and water are necessary for
corrosion, but neither alone will do. Thus bright iron nails in a dessicator do not rust and
neither do nails kept in air free water.
Hence
Corrosion is an unintentional gradual destruction of a metal by chemical or
electrochemical attack.
Common types of corrosion may be expressed by one or more of the following reactions.
(a) Combination of metal with nonmetal in absence of water: M + X → MX
eg., at high temperature in dry air, 2Fe + O2 → 2 FeO
Page 1 of 9
, (b) Combination of metal with oxygen in presence of water:
A proposed mechanism consists of the following steps:
(1) Fe(s) → Fe2+ + 2e-
(2) H+ + e- → H
(3) 4H + O2(g) → 2 H2O
(4) 4 Fe2+ + O2(g) + 4 H2O → 2 Fe2O3 + 8 H+
In step (1) Fe acts as an anode, becoming oxidized to Fe2+. Electrons move through the
iron bar to a moist surface, where H+ ions in water accept them, forming neutral H atoms
(Step 2). Step (3) – H atom reacts with O2 in the air to produce H2O.
Step (4) - Fe2+ ions are oxidized to Fe3+ in the presence of water to form rust, a hydrated
form of ferric oxide (Fe2O3. xH2O).
Oxidation is the loss of electrons by the substance being oxidized.
Anode is the electrode at which oxidation occurs.
Reduction is the gain of electrons by the substance being reduced.
Cathode is the electrode at which reduction occurs
(c) Displacement of hydrogen from acids or acid solutions:
M + H2Y → MY + H2
Here Y may be sulphate, chloride, sulphite etc. Examples are, attack on iron by sulphuric
acid or hydrochloric acid, and tarnishing of silver by H2S in presence of moisture: 2Ag +
H2S → Ag2S + H2
(d) Displacement of hydrogen from water which usually contains small amounts of
dissolved inorganic substances:
M + 2 H2O → M(OH)2 + H2
The rusting of iron in water is a form of corrosion involving this reaction.
(e) Displacement of one metal ions by another metal from salt solution:
M’ + MSO4 → M’SO4 + M
eg., Fe + CuSO4 → FeSO4 + Cu
Page 2 of 9
