Dry (or) Chemical 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 3 main types of dry corrosion.

1. Oxidation corrosion (or) corrosion by oxygen.

2. Corrosion by hydrogen.

3. Liquid‒metal corrosion.

1. Oxidation corrosion (or) corrosion by oxygen

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 (Li, Na, K, etc.) and alkaline‒earth metals (Mg, Ca, Sn, etc.) are rapidly oxidised at low temperature. At high temperature, almost all metals (except, Ag, Au and Pt) are oxidised.

Mechanism of dry corrosion

(i) Oxidation occurs first at the surface of the metal resulting in the formation of metal ions (M²⁺), which occurs at the metal / oxide interface.

 M → M²⁺ + 2e^‒

(ii) Oxygen changes to ionic form (O^2‒) due to the transfer of electron from metal, which occurs at the oxides film / environment interface

 ½O₂ + 2e^‒  → O^2‒

(iii) Oxide ions reacts with the metal ion to form the metal‒oxide film.

M + ½O₂ → M²⁺ + O^2‒ = MO (Metal‒oxide film)

Once the metal surface is converted to a monolayer of metal‒oxide, for further corrosion (oxidation) to occur, the metal ion diffuses outward through the metal‒oxide barrier. Thus the growth of oxide film commences perpendicular to the metal surface (Fig. 4.1).

Nature of oxide film

The nature of oxide film formed on the metal surface plays an important role in oxidation corrosion.

1. Stable oxide layer

A stable oxide layer is a fine‒grained in structure, and gets adsorbed tightly to the metal surface. Such a layer is impervious in nature and stops further oxygen attack through diffusion. Such a film behaves as a protective coating and no further corrosion can develop.

Example: Oxides of Al, Sn, Pb, Cu, etc., are stable oxide layers.

2. Unstable oxide layer

Unstable oxide layer is mainly produced on the surface of noble metals, which decomposes back into the metal and oxygen.

Metal oxide ↔ Metal + Oxygen

Example: Oxides of Pt, Ag, etc., are unstable oxide layers.

3. Volatile oxide layer

The oxide layer volatilizes as soon as it is formed, leaving the metal surface for further corrosion.

Example: Molybdenum oxide (MoO₃) is volatile.

4. Protective (or) Non‒Protective oxide film (Pilling‒ Bedworth rule)

(i) According to Pilling‒Bedworth rule, if the volume of oxide layer formed is less than the volume of metal, the oxide layer is porous and non‒protective.

Example: The volume of oxides of alkali and alkaline earth metals such as Na, Mg, Ca, etc., is less than the volume of metal consumed, the oxide layer formed is porous and non‒protective.

(ii) On the other hand, if the volume of oxide layer formed is greater than the volume of metal, the oxide layer is non‒porous and protective.

Example: The volume of oxides of heavy metals such as Pb, Sn, etc., is greater than the volume of metal, the oxide layer formed is non‒porous and protective.

Pilling‒Bedworth ratios

The ratio of the volume of metal oxide formed to the volume of metal consumed is called. “Pilling‒Bedworth ratio”.

PB ratio = Volume of metal oxide / Volume of metal consumed

If the PB ratio is slightly greater than 1, the metal oxide layer will be protective and non porous.

2. Corrosion by hydrogen

1. Hydrogen embrittlement (at ordinary temperature)

When metals contact metals contact to H₂S at ordinary temperature causes evolution of atomic hydrogen.

 Fe + H₂S → FeS + 2H

This atomic hydrogen diffuses readily into the metal and collects in the voids, where it recombines to form molecular hydrogen.

 H+H → H₂↑

Collection of these hydrogen gases in the voids develop very high pressure, which causes cracks and blisters on metal.

Thus, the process of formation of cracks and blisters on the metal surface, due to high pressure of hydrogen gas, is called hydrogen embrittlement

2. Decarburisation (at higher temperature)

At higher temperature atomic hydrogen is formed by the thermal dissociation of molecular hydrogen.

 H₂ --- ^Heat→ 2H

when steel is exposed to this environment, the atomic hydrogen readily combines with carbon of steel and produces methane gas.

 C + 4H →  CH₄↑

Collection of these gases in the voids develop very high pressure, which causes cracking. Thus, the process of decrease in carbon content in steel is termed as

"decarburisation" of steel.

3. Liquid‒metal corrosion

This is due to the chemical action of flowing liquid metal at high temperature. The corrosion reaction involves

(i) either dissolution of a solid metal by a liquid metal. (or)

(ii) liquid metal may penetrate into the solid metal.