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.

Caustic embrittlement



Caustic embrittlement



It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.



 The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.

Na₂CO₃ + H₂O ---------2 NaOH + CO₂                  

The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate ( Na₂FeO₂). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows

     3Na₂FeO₂ + 4H₂O  --------  6NaOH + Fe₃O₄ + H₂

       6Na₂FeO₂ + 6H₂O + O2  --------  12NaOH + 2Fe₃O₄

So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a required environment. Thus corrosion process develops cracks and making the metal brittle by the deposition of the product.

The corrosion cell can be represented as

 Fe (under stress) / conc. NaOH / dil. NaOH/ Fe (stress free)

      Anode                                                           Cathode

Caustic embrittlement can be prevented by the addition of compounds like sodium sulphite, tannin, lignin, phosphates etc. which blocks the cracks thereby preventing the infiltration of alkali.

Stress Corrosion:



Stress Corrosion

 Stress corrosion of the metal formed by the combined effect of a tensile stress and a specific corrosive environment on the metal., during stress corrosion, the metal or alloy is virtually unaffected over most of its surface, while fine cracks progress through it normal to the direction of tensile stress. The stress on the metal may be internal or external and these stress is due to some mechanical or service conditions. The metal atoms under stress are always at higher energy level so acts as anode and stress free parts of metal acts as cathode under specific corrosive environmental conditions corrosion process starts.

• Brass undergoes corrosion in the presence of ammonia.
• Stainless steel in the presence of Cl^- and caustics.  But best example for stress corrosion is caustic embrittlement.

Pitting Corrosion

Pitting Corrosion

Pitting corrosion is an example for  a localized and accelerated corrosion. When a small particles of dust or water etc are get deposited on a metal (like steel). The portion covered by the dust will not be well-aerated area compared to the exposed surface hence the covered surface becomes anodic with respect to the surface exposed. In presence of an conducting medium (moisture) corrosion starts below the dust part and forming a pit. Once pit is formed the ratio of corrosion increases, because of the formation of smaller anodic and larger cathodic area intense corrosion takes place.

Pitting corrosion is one of the most destructive forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Because of the small sizes of the pits it is highly difficult to identify the pitting corrosion. Pitting corrosion is an autocatalytic process, and once the corrosion products are formed, it further provides the condition for differential aeration below the corrosion product and surrounding metal parts. The pit grows and ultimately may cause failure of the metal.