Corrosion and its control, Lecture notes of Chemistry

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Chapter II

CORROSION and its CONTROL

CORROSION AND ITS

CONTROL

Corrosion is defined as the gradual

destruction or deterioration of

metals or alloys by the chemical or

electrochemical reaction with its

environment.

DEFINITION

Based on the environment, corrosion is classified in to

1. Dry or Chemical corrosion
2. Wet or Electrochemical corrosion. Dry or Chemical corrosion: Dry corrosion is due to the attack of metal surfaces by the atmospheric gases such as oxygen, hydrogen sulphide, sulphur dioxide, nitrogen etc. There are three main types of dry corrosion;
3. Oxidation corrosion (or) corrosion by oxygen
4. Corrosion by hydrogen.
5. Liquid – metal corrosion. CLASSIFICATION OF CORROSION

OXIDATION CORROSION

 Oxidation corrosion is brought about by the direct

attack of oxygen at low or high temperatures on

metal surface in the absence of moisture.

 Alkali metals like (Li, Na, K, etc) and alkalineearth

metals (Mg, Ca, Sr, etc) are rapidly oxidised at low

tempt.

 At high temperature, almost all metals (expect Ag,

Au and Pt) are oxidized.

Oxygen changes to ionic form (O 2 ) due to the

transfer of electron from metal, which

occurs at the oxide film / environment

interface.

½ O 2 + 2e → O^2

Oxide ions reacts with the metal ion to form

the metal oxide film.

M + ½ O 2 → M2+^ +O 2 ≡ MO (Metaloxide film)

NATURE OF OXIDE FILM

The nature of oxide film formed on the metal surface plays in important role in oxidation corrosion. (i) STABLE OXIDE LAYER A stable oxide layer behaves as a protective coating and no further corrosion can develop. Example: oxides of Al, Sn, Pb, Cu, etc., are stable oxide layers.

10

Porous Metal Oxide

Having pores or cracks.Oxygen will diffuse through the pores and cause further corrosion. Iron is an example of this type of corrosion. Entire metal is converted to its oxide. Eg. Alkali and Alkaline earth metals. metal metal Further attack Through pores and Crack continues

• O 2 Porous metal oxide Exposed surface

11 metal metal metal Exposed surface Volatile metal oxide Fresh surface Exposed for further attack

• O 2

Volatile Metal Oxide

These oxide layers evaporate as soon as they are formed and facilitates further Corrosion. Rapid corrosion occurs. After sometime, the metal itself will disappear. Eg. Molybedum oxide

13 (PILLINGBEDWORTH RULE) According to Pilling–Bedworth rule,

1. If the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is porous and non protective. Example: Oxides of alkali and alkaline earth metals.
2. If the volume of the oxide layer formed is greater than the volume of the metal, tightly adhering layer which is non porous is formed. Example: Al,Cu,Sn.

14

CORROSION BY OTHER GASES

 (^) Gases like SO 2 , CO 2 , Cl 2 , H 2 S etc – induce corrosion action on metals  (^) Corrosion effect depends on the chemical affinity between the metal and the gas.  (^) The degree of attack by gases depends on the formation of protective or nonprotective films on the metal surface. (1) If the film is nonporous or protective, the intensity of attack decreases. Eg. AgCl film – attack of Cl 2 on Ag (2) If the film is porous or nonprotective, the surface of the whole metal is gradually destroyed. Eg. Dry Cl 2 gas attack on Sn – formation of volatile SnCl 4

• fresh surface is exposed for further attack

(b) DECARBURISATION

At higher temperature atomic hydrogen is

formed by the thermal dissociation of molecular

hydrogen.

H 2 → 2H

When steel is exposed to this environment, the

atomic hydrogen readily combines with carbon

of steel and produces methane gas.

C + 4H → CH 4 ↑

Collection of these gases in the voids develop

very high pressure, which causes cracking. Thus

the process of decreases in carbon content in

steel is termed as “Decarburisation” of steel.

This is due to the chemical action of

flowing liquid metal at high temperature.

The corrosion reaction involves either,

(i) Dissolution of a solid metal by a liquid

metal (or)

(ii) Liquid metal may penetrate in to the

solid metal.

LIQUID METAL CORROSION

WET (or) ELECTROCHEMICAL CORROSION

Wet corrosion occurs under the foll. conditions,

 When two dissimilar metals are in contact with each

other in the presence of an aqueous solution or

moisture.

 When two dissimilar parts of the same metal are in

contact with an aqueous solution of an electrolyte.

20 MECHANISM OF WET CORROSION

According to electrochemical theory of corrosion,

the wet corrosion involves two steps.

a) Oxidation or metal dissolution

b) Reduction

Metal dissolution occurs always at anode leading to

the formation of metal ions and electrons

M → Mn+^ + ne

At cathode, electron consumption resulting in either

evolution of hydrogen or absorption of oxygen

depending on the nature of corrosion environment.