Pitting corrosion is a localized form of corrosion that leads to the formation of small cavities or holes in the material. It occurs when small areas become active (anodic) while the surrounding areas remain passive (cathodic). This creates galvanic cells that drive the corrosion process. Pitting corrosion initiates at defects on the material surface and then propagates in an autocatalytic manner. It is most common in alloys protected by a passive film when exposed to environments containing chlorides, oxygen, and stagnant conditions. Proper material selection, surface finishing, controlling environmental factors like pH and chloride levels, and using protective coatings or cathodic protection can help prevent pitting corrosion.

1. Presentation on
Pitting Corrosion
Prepared by-
RAHUL DEO SONAR
Jr. Metallurgist
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Metallurgy Dept.- Dammam Lab-KSA

2. INDEX
Definition of Pitting Corrosion
Mechanism
Common Environments of Pitting Corrosion
Prevention
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3. Definition
It is a form of entirely localized corrosion.
Leads to formation of cavities or holes in
the material.
Considered to be the most dangerous and
insidious type of corrosion.
Rate of corrosion is higher than uniform
corrosion because of large cathode/anode
ratio.
Difficult to detect and measure.
Its is autocatalytic in nature.
Occurs usually in the direction of gravity.
Fig.1: Pitting corrosion on a Stainless
Steel surface
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4. Mechanism of Pitting Corrosion
Principle: Metal oxidation results in localized acidity that is maintained by the
continuously undergoing cathodic & anodic half-reactions, this acidity breaks the
passive layer, which creates a potential gradient and electromigration of anions will
start in the pit .
Three stages of pitting corrosion:
 Initiation of pit.
 Propagation of pit (at this point autocatalytic in nature).
 Perforation or Termination of pit.
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5. Let us take the figure below, we are having a passivating metal in contact with sea
water(oxygen rich).
The reaction taking place will be:
M  M2+ +2e- …...........................anodic
O2 + 2H2O + 4e-  4OH- ………...cathodic
 Here the oxygen rich fluid in contact with metal becomes cathodic and the bottom
part becomes anodic.
 Metal at the pit will be ionized to release e- and
form M2+.
 This e- will travel to the top of the pit & reacts
with O2 & H2O to form hydroxide ions(OH-).
 Now the M2+ ions will react with OH- ions to
form rust.
 More anodic dissolution takes place in the pit so we
will have more M2+ ion concentration & promotes
pits.
 Deeper the pit lesser the oxygen content and higher
will be the pitting corrosion rate.
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6. Common environments of pitting corrosion
 Occurs in alloys that are protected by passivating film like stainless steel, Al alloys.
 Materials having poor surface finish may undergo this corrosion.
 Presence of dirt on surface.
 Stagnant water conditions favour pitting(low velocity).
Cholride environment.
Oxidizing metal ions with chlorides are aggressive pitters.
Prevention
 Proper material selection(e.g. SS316 with Mo) will have higher resistance to pitting.
 Controlling oxygen level.
 Cathodic & Anodic protection.
 Good surface finish required.
 Agitation of stagnant fluids.
Controlling the pH, chloride concentration and temperature.
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