Types of Corrosion

Differential metal corrosion (galvanic corrosion)

When the two dissimilar metals are in contact with each other, the metal with lower reduction potential (anode) undergoes oxidation and the metal with higher electrode potential (cathode) undergoes reduction. The potential difference between the metals causes driving force for corrosion. The greater the electrode potential difference, the higher is the corrosion rate. Thus anodic area always undergoes corrosion and cathodic area remains unaffected.

Differential aeration corrosion

Differential aeration corrosion occurs when a metal surface is exposed to differential air concentration or oxygen concentration. The part of the metal exposed to air acts as cathode and part of the metal less aerated acts as anode. Consequently poorly aerated region undergoes corrosion.

Pitting corrosion

Pitting corrosion is a localized and accelerated corrosion resulting in the formation of pit or cavity, around which the metal is relatively unattacked. Pitting corrosion results when small particles of dust or water etc. are get deposited on a metal. The portion covered by the dust will not be well oxygenated compared to the exposed surface. Hence the covered surface becomes anodic with respect to the surface exposed. In presence of a moisture corrosion starts below the dust part. This causes pits or cavities in the metal. Hence it is called pitting corrosion. Once a pit is formed the rate of corrosion increases. This is because of the formation of small anodic area. The cathode induces the small anodic area to corrode faster by accepting electrons from the anodic are resulting in the enlargement of the pit.

Eg: (1) Less aerated part of the machinery.

  (2) Less aerated parts of barbed wire.

  (3) Cracks in metals which are not well aerated.

Stress corrosion:

When metals or alloys are subjected to metallurgical operations such as welding, bending & quenching a part of the metal or alloy undergoes stress. The metal atoms under stress position have high energy and act as anodic with respect to unstressed position. Under specific environment stressed portion undergoes corrosion initiating the crack. The corrosion medium is highly selective and specific in their action.

For example,

(1)  Season cracking of brass - Brass undergoes stress corrosion in the presence of ammonia.

(2)  Caustic embrittlement of Steel - Stainless steel in the presence of Chloride ions and caustics.

Caustic embrittlement:

It is a type of stress corrosion that takes place in boilers which works between the pressures of 10-20 atmospheres. Fine hairline cracks may be presented at the stressed portion of the boiler. Boiler water usually contains some sodium carbonate (Na₂CO₃). This enters the cracks and at high pressure & temperature, Na₂CO₃ undergoes hydrolysis producing NaOH.

The NaOH remains in the cracks and CO₂ escapes. The concentration of alkali increases gradually and when it reaches 10% a galvanic cell is formed between the iron under stress and iron in the main body. The iron surrounded by dilute NaOH is the cathode and the iron under stress acts as anode and undergoes corrosion resulting in boiler failure or boiler explosion.