Tuesday, November 30, 2010

Differential Aeration Corrosion

Differential Aeration Corrosion
Definition: Corrosion of metals arising as a result of the formation of an oxygen concentration cell due to the uneven supply of air on the metal surface is known as differential aeration corrosion.

Differential aeration corrosion occurs when a metal surface is exposed to differential air concentrations or oxygen concentrations. The part of the metal exposed to higher oxygen concentration acts as cathodic region and part of the metal exposed lower oxygen concentration acts as anodic region. Consequently, poorly oxygenated region undergoes corrosion.

At the anode (less O2 concentration),      M ------------Mn+ + ne
At the cathode (more O2 concentration),   H2O + ½ O2 + 2e------  2OH-

Waterline corrosion:

Waterline corrosion
 Waterline corrosion s a case of differential aeration corrosion, more prevalent in cases such as ocean going ships, water storage steel tanks, etc., in which a portion of the metal is always under water.
The waterline corrosion takes place due to the formation of differential oxygen concentration cells. The part of the metal below the water line is exposed only to dissolved oxygen while the part above the water is exposed to higher oxygen concentration of the atmosphere. Thus, part of the metal below the water acts as anode and undergoes corrosion and part above the waterline is free from corrosion. A distinct brown line is formed just below the water line due to the deposition of rust.

13 comments:

  1. Nice job..thanks!!

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  2. can you explain why the part with less O2 acts as anode perhaps by using eqns. like Nernest eqn?

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  3. well explained the aeration theory

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  4. John Yang---I don't agree much. Further explanation is needed. Nernst equation cannot be use here, but a lot of people do. Tafel equation should be used. When you use Tafel equation, you would find that the host's explanation is problematic.

    --Nernst equation is for thermodynamic process, not for kinetics
    --Tafel equation is for kinetics


    --Nernst equation is for thermodynamic process, not for kinetics

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  5. Waterline corrosion is limited to some points or it can occur in industrial plants & processes as well , because many industries are dealing with liquids and they use instruments like Tank Level Transmitter and Tank Level Sensor so is there any possibility to avoid this in industrial processes . although these instruments are corrosion resistant but if I focus on overall plant what would you say?

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  6. can u explane the corrosion that we found in many types of valves of valves that we use in industries

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  7. Nice explanation. Finally I understood

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