This PPT is describing the form of corrosion and especially for pitting and intergranular corrosion and its prevention techniques.

1. Jeevanantham P
18PCH004
II-MSc Chemistry
e-mail : jeevananthamche@gmail.com

2. Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced
in the material.
Pitting is considered to be more dangerous than uniform corrosion damage because it is
more difficult to detect, predict.
 Pits can be either hemispherical or cup-shaped.
Pitting corrosion

3. Pitting is initiated by:
1. Localized chemical or mechanical damage to the protective oxide film.
2. Localized damage to, or poor application of a protective coating.
3. The presence of non-uniformities in the metal structure of the component,
e.g. nonmetallic inclusions.

4. Mechanism

5. Types of pitting corrosion :

6. Historical Effect Caused by Pitting Corrosion :
A single pit in a critical point can cause a great deal of damage. One example is the
explosion in Guadalajara, Mexico on April 22, 1992, when gasoline fumes
accumulated in sewers destroyed kilometers of streets. The vapors originated from a
leak of gasoline through a single hole formed by corrosion

7. Pitting corrosion can also help initiate stress corrosion cracking, as happened when a
single eyebar on the Silver Bridge, West Virginia failed and ended up killing 46
people on the bridge in December, 1967.

8. Prevention
Proper materials selection will have higher resistance to pitting
Controlling oxygen level.
Cathodic & Anodic protection.
Good surface finish required.
Controlling the pH, chloride concentration and temperature.

9. INTERGRANULAR CORROSION

10. Localized attack adjacent to grain boundaries with relatively little
corrosion of grains.
It’s form of surface corrosion along the grain boundaries of metal.
Intergranular corrosion can occur in many alloys system such as stainless
steel, nickel and aluminium base alloys.

11. Cause of Intergranular Corrosion :
• Impurities at the grain boundaries.
• Enrichment of one of the alloying elements.
• Segregate one of metal in alloy in the grain boundaries and cause
intergranular corrosion.

12. Mechanism

14. Types of Intergranular Corrosion
1. Knife line
attack
2. Weld decay

15. • Formation of Chromium carbide precipitants at the grain boundaries
occurs when stainless steel heated to temperature of 400-900degree C.
• This leads to intergranular corrosion.
Knife Line Attack
Prevention of KLA

16. Weld decay
• Weld decay is a form of intergranular corrosion, usually of stainless
steels or certain nickel-base alloys, that occurs as the result of
sensitization in the heat-affected zone during the welding operation.
Prevention of Weld decay

17. Summary
•Pitting - Localized corrosion by cavities or holes
are formed in the materials.
• intergranular- Localized corrosion adjacent to the grains
boundaries in the materials.
• Prevention - Material selection, Anode/cathode
protection, control the environment, etc…

18. References :
• https://www.corrosionclinic.com/types_of_corrosion/weld_decay_kn
ife_line_attack.htm
• https://www.slideshare.net/e_gulfam/intergranular-corrosion
• https://www.slideshare.net/SandipThorat6/metal-corrosion-and-its-
prevention
• https://www.nace.org/resources/general-resources/corrosion-
basics/group-1/pitting-corrosion
• Wikipedia