UNIT - II

Corrosion and Its Control

Corrosion
The surface of almost all the metals begin to decay more or less rapidly when exposed to
atmospheric gases, water or other reactive liquid medium.

The process of decay metal by environmental attack is known as corrosion.

Metals undergo corrosion and convert to their oxides, hydroxides, carbonates, sulphides etc.
E.g. Iron undergoes corrosion to form reddish brown colour rust [Fe2O3. 3H2O].

Copper undergoes corrosion to form a green film of basic carbonate [CuCO3 + Cu (OH) 2]

1.2.1 Causes of corrosion

1. The metals exist in nature in the form of their minerals or ores, in the stable combined forms
as oxides, chlorides, silicates, carbonates, sulphides etc.
2. During the extraction of metals, these ores are reduced to metallic state by supplying
considerable amounts of energy.
3. Hence the isolated pure metals are regarded as excited states than their corresponding ores.
So metals have natural tendency to go back to their combined state (minerals/ores).

When metal is exposed to atmospheric gases, moisture, liquids etc., and the metal surface reacts
and forms more thermodynamically stabled compounds.

Effects of corrosion

1. Wastage of metal in the form of its compounds.

2. The valuable metallic properties like conductivity, malleability, ductility etc. are lost due
to corrosion.
3. Life span and efficiency of metallic parts of machinery and fabrications is reduced.

1.2.2 Theories of corrosion

Dry corrosion or Chemical corrosion

This type of Corrosion occurs mainly through the direct chemical action of atmospheric gasses
like O2, halogens, H2S, SO2, N2 or anhydrous inorganic liquid with the metal surface.

There are three types of chemical Corrosion:

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(1.) Oxidation corrosion

(2.) Corrosion due to other gases

(3.) Liquid metal corrosion

(1) Oxidation Corrosion: This is carried out by the direct action of oxygen low or high
Temperatures on metals in absence of moisture. Alkali metals and Alkaline earth metals are
rapidly oxidized at low temperatures. At high temperature all metals are oxidized (except Ag,
Au, and Pt).

M ----- M2+ + 2e- (Oxidation)

O2 + 2e- ----2O2- (Reduction)

M + O2 -----M2+ + 2O2- (Metal oxide)

Fig.1.17 Oxidation corrosion

Mechanism: Initially the surface of metal undergoes oxidation and the resulting metal oxide
scale forms a barrier which restricts further oxidation. The extent of corrosion depends upon the
nature of metal oxide.

(a) If the metal oxide is stable, it behaves has a protective layer which prevents further

Corrosion.

E.g., the oxide films of Al, Sn, Pb, Cu, Cr, W etc. are stable and therefore further corrosion is
prohibited.

(b) If the metal oxide unstable, the oxide layer formed decomposes back into metal and oxygen.
Oxidation corrosion is not possible.

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E.g., Ag, Au and Pt do not undergo oxidation corrosion.

(c) If the metal oxide layer is volatile, then the oxide layer volatilizes after formation and leaves
the underlying metal surface exposed for further attack. This causes continuous corrosion which
is excessive in molybdenum oxide (MoO3).

(d) If the metal oxide layer is porous, the oxide layer formed has pores or cracks. In this case the
atmospheric oxygen penetrates through the pores or cracks and corrode the underlying metal
surface. This cause continuous corrosion till conversion of metal into its oxide is completed.

Ex: Alkali and alkaline earth metals (Li, Na, K, Mg etc.)

(2) Corrosion due to other gases: This type of corrosion is due to gases like SO2, CO2,
Cl2, H2S, F2 etc. In this corrosion, the extent of corrosive effect depends mainly on the chemical
affinity between the metal and the gas involved. The degree of attack depends on the formation
of protective or non protective films on the metal surface which is explained on the basis of
Pilling Bedworth rule.

(i) If the volume of the corrosion film formed is more than the underlying metal, it is strongly
adherent; non-porous does not allow the penetration of corrosive gases.

Ag + Cl2 ----2AgCl (protective film)

(ii) If the volume of the corrosion film formed is less than the underlying metal, it forms

Pores/cracks and allow the penetration of corrosive gases leading to corrosion of the underlying
metal.

Ex. In petroleum industry, H2S gas at high temperature reacts with steel forming a FeS scale. Fe
(steel) + H2S FeS (porous)

(3) Liquid metal corrosion: This corrosion is due to chemical action of flowing liquid
metal at high temperatures on solid metal or alloy. The corrosion reaction involves either
dissolution of a solid metal by a liquid metal or internal penetration of the liquid metal into the
solid metal.

Eg. Coolant (sodium metal) leads to corrosion of cadmium in nuclear reactors.

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Wet corrosion or electrochemical corrosion
 This type of Corrosion occurs where a conducting liquid is in contact with the metal. This
corrosion occurs due to the existence of separate anodic and cathodic parts, between
which current flows through the conducting solution.

 At anodic area, oxidation reaction occurs there by destroying the anodic metal either by
dissolution or formation of compounds. Hence corrosion always occurs at anodic parts.

Mechanism: Electrochemical corrosion involves flow of electrons between anode and cathode.

The anodic reaction involves dissolution of metal liberating free electrons.

M----- Mn+ + ne-

The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of
oxygen which depends on the nature of corrosive environment.

Evolution of hydrogen: This type of corrosion occurs in acidic medium.

E.g. Considering the metal Fe, anodic reaction is dissolution of iron as ferrous ions with

Liberation of electrons.

Fig.1.18 Hydrogen evolution corrosion

Anode: Fe---- Fe2+ + 2e- (Oxidation)

The electrons released flow through the metal from anode to cathode, whereas H+ ions of acidic
solution are eliminated as hydrogen gas.

Cathode: 2H+ + 2e- ----H2 (Reduction)

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The overall reaction is: Fe + 2H+ -----Fe2+ + H2

This type of corrosion causes displacement of hydrogen ions from the solution by metal ions. All
metals above hydrogen in electrochemical series have a tendency to get dissolved in acidic
solution with simultaneous evolution of H2 gas. The anodes are large areas, whereas cathodes
are small areas.

Absorption of oxygen: For example, rusting of iron in neutral aqueous solution of

electrolytes in presence of atmospheric oxygen. Usually the surface of iron is coated with a thin
film of iron oxide. If the film develops cracks, anodic areas are created on the surface. While the
metal parts act as cathodes. It shows that anodes are small areas, while the rest metallic part
forms large

Cathodes. The released electrons flow from anode to cathode through iron metal.
-
At anode: Fe ----Fe2+ + 2e (Oxidation)
- -
At cathode: ½ O2 + H2O + 2e ---2OH (Reduction)

Overall reaction: Fe2+ + 2OH- ----- Fe (OH)2

If oxygen is in excess, ferrous hydroxide is easily oxidized to ferric hydroxide.

4Fe (OH) 2 + O2 + 2H2O → 4Fe (OH) 3

The product called yellow rust corresponds to Fe2O3. 3H2O.

Fig.1.19 Oxygen absorption corrosion

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