1. Corrosion process and control (TKK-2289)
15/16 Semester genap
Corrosion process and control (TKK-2289)
Instructor: Rama Oktavian; Vivi Nurhadianty.
Office Hr.: T , Th ; 13-15, F ; 13-15

2. Corrosion types Pitting corrosion
Localized corrosion of a metal surface where small areas corrode preferentially leading to the formation of cavities or pits.
It causes failure by penetration with only a small percent weight-loss of the entire structure.
the most conducive environment for pitting is the marine environment

3. Corrosion types Pitting corrosion Conditions for pitting to occur:
Breaks in the films or other defects
The presence of halogen ions
Stagnant conditions in service

4. Corrosion types Mechanism of Pitting corrosion
the formation of an anode is a prerequisite  a local corrosion cell is developed.
lack of homogeneity at the metal corrosive interface
destruction of a passive film
deposit of debris or solids on the metal surface
formation of an active–passive cell with a large potential difference

5. Corrosion types Pitting process
The formation of anodic sites by disruption of the protective passive film. The metal dissolution takes place at the anode. Pitting is, therefore, initiated.
Due to the continuing metal dissolution, an excess of positive ions M+ is accumulated in the anodic area
The presence of H+ ions and chloride content, prevents repassivation
The increase in the rate of dissolution at the anode increases the rate of migration of the chloride ions
The process continues until the metal is perforated.

6. Corrosion types
Pitting process

7. Corrosion types
Main step of Pitting process

8. Corrosion types Factors affecting pitting corrosion
Chloride ions concentration
Alloying elements
Effect of electrolyte composition
Effect of pH
Effect of temperature

9. Corrosion types
Influence of alloying elements in pitting corrosion of aluminum
Chromium. It is usually added to Al–Mg alloy and Al–Mg–Zn alloys in amounts of 0.1–0.3%.
Copper. It reduces the corrosion resistance of aluminum alloys and increases the susceptibility to pitting
Iron. It exists in the form of FeAl3 and promotes pitting of aluminum alloy
Magnesium. It provides substantial strengthening with good ductility in addition to excellent corrosion resistance

10. Corrosion types Evaluation of pitting damage
Conventional weight-loss techniques cannot be used to evaluate pitting.
What matters in pitting is the depth of the deepest pit
The development of pit depths follows a time function:

11. Corrosion types
Pitting corrosion

12. Corrosion types Pitting corrosion
An example of corrosion damages with shared responsibilities was the sewer explosion that killed 215 people in Guadalajara, Mexico, in April Besides the fatalities, the series of blasts damaged 1,600 buildings and injured 1,500 people.
*Sewer: an artificial conduit, usually underground, for carrying off waste water and refuse, as in a town or city.

13. Corrosion types
Pitting corrosion

14. Corrosion types Pitting corrosion THROUGH PITS SIDEWAY PITS Subsurface
Narrow, deep
Shallow, wide
Undercutting
Elliptical
Horizontal grain attack
Vertical Grain Attack

15. Corrosion types Intergranular corrosion
a form of localized attack on the grain boundaries of a metal or alloy in corrosive media, which results in the loss of strength and ductility.
Intergranular corrosion is less dangerous than stress corrosion.
As corrosion proceeds along the grain boundaries, the grains become weaker particularly at the grain boundaries and they eventually disintegrate

16. Corrosion types Intergranular corrosion
Metals are crystalline materials. They consist of grains
All metals and alloys are joined together by grain boundaries

17. Corrosion types Technical term of steel Sensitization
When steels are slowly cooled through the range 550–850◦C
Stabilized steel
Steels containing stabilizers, such as titanium and niobium, which form
carbides
Chromium depletion
When the concentration of chromium adjacent to the grain boundaries becomes less compared to its concentration away from the grain boundaries
Quenching
cooling rapidly by quenching in water

18. Corrosion types Mechanism of intergranular corrosion
Sensitization in the temperature range 550–900◦C
Diffusion of carbon to a grain boundary and formation of carbide
Weakening of the grain boundary and disintegration of the grains

19. Corrosion types
Mechanism of intergranular corrosion

20. Thank You !