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 Uniform corrosion
The uniform thinning of a metal without any localized attack
Corrosion does not penetrate very deep inside.
Familiar example is the rusting of steel in air
Factors that can cause corrosion

3. Corrosion types Uniform corrosion
Corrosion can proceed in a dry environment without any moisture if traces of sulfur compounds or H2S or other pollutants are present in the air.

4. Corrosion types
Uniform corrosion

5. Corrosion types Uniform corrosion
Effect of humidity
Corrosion can be caused in the atmosphere when about 70% of the humidity is present.
In the presence of such humidity, an invisible thin film of moisture is formed on the surface of a metal.

6. Corrosion types Uniform corrosion
Water layer
If visible water layers are formed on the metal surface, corrosion initiates. Splashing of seawater, rain and drops of dew provide the wet environment.
Dew Formation
If the dew becomes acidic, due to the presence of SO2, it increases the rate of corrosion.

7. Corrosion types Uniform corrosion
Mechanism of uniform corrosion
In atmospheric corrosion, a very thin layer of electrolyte is present. Corrosion products are formed close to the metal surface.

8. Corrosion types Uniform corrosion At the anodic area
Mechanism of uniform corrosion
At the anodic area
At the cathodic area
The OH ions react with the Fe++ ions produced at the anode

9. Corrosion types Uniform corrosion
Mechanism of uniform corrosion
With more access to oxygen in the air, Fe(OH)2 oxidizes to Fe(OH)3 and later it loses its water
Ferrous hydroxide is converted to hydrated ferric oxide or rust by oxygen

10. Corrosion types Uniform corrosion Tarnishing of silver ware
Example of uniform corrosion
Tarnishing of silver ware
Tarnishing of electrical contacts
Rusting of steels in open air
Corrosion of offshore drilling platforms
Failure of distillation columns
Corrosion of underground pipes
Corrosion of heat exchanger tubes

11. Corrosion types Galvanic corrosion
Definition
Occurs when two metals with different electrochemical potentials or with different tendencies to corrode are in metal-to-metal contact in a corrosive electrolyte.
When two metals with different potentials are joined, such as copper ( V) and iron (−0.440 V), a galvanic cell is formed.
The driving force for corrosion is a potential difference between different materials

12. Corrosion types
Galvanic corrosion
Mechanism

13. Corrosion types Galvanic corrosion
Mechanism
The positive ions (Fe++) flow from the anode (iron) to cathode (copper) through the electrolyte, which is water.
The hydrogen ions (H+) are discharged at the copper cathode, and ultimately hydrogen is released.
The Fe++ ions travel towards the cathode and OH− towards the anode. They combine to form insoluble iron hydroxide, Fe(OH)2.

14. Corrosion types Galvanic corrosion
Factors affecting galvanic corrosion
Position of metals in the galvanic series.
The magnitude of galvanic corrosion primarily depends on how much potential difference exists between two metals.
the metals selected should be close to each other in the galvanic series to minimize galvanic corrosion.
Example: aluminum should not be joined to steel, as aluminum being more active would tend to corrode

15. Corrosion types Galvanic corrosion The nature of environment.
Factors affecting galvanic corrosion
The nature of environment.
Consideration must be given to the environment that surrounds the metal.
Water containing copper ions, like seawater, is likely to form galvanic cells on a steel surface of the tank

16. Corrosion types Galvanic corrosion
Factors affecting galvanic corrosion
Area, Distance and Geometric Effects.
The anode to cathode area ratio is extremely important as the magnitude of galvanic corrosion is seriously affected by it

17. Corrosion types Galvanic corrosion
Factors affecting galvanic corrosion
Area, Distance and Geometric Effects.
The solution conductivity varies inversely with the length of the conduction path.
If two different metals are far away from each other, there would be no risk of galvanic corrosion, because of very little current flow.
As current does not flow around the corners, the geometry of the
circuit affects the degree of galvanic corrosion

18. Corrosion types Galvanic corrosion
Example of galvanic corrosion
Galvanic corrosion of steel pipe with brass fittings.
Galvanic corrosion of the body of the ship in contact with brass or bronze propellers.
Galvanic corrosion between the tubes and the tube sheet in heat exchangers.
Galvanic corrosion of steel coated with copper due to the defects in copper coating

19. Corrosion types Crevice corrosion
Caused by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence of voids, gaps and cavities between adjoining surfaces.
Presence of narrow spaces between metal-to-metal or non-metal to metal components
Presence of cracks, cavities and other defects on metals
Deposition of biofouling organisms and similar deposits
Deposition of dirt, mud or other deposits on a metal surface

20. Corrosion types Crevice corrosion
The conventional steels, like SS 304 and SS 316, can be subject to crevice corrosion in chloride containing environments
Factors affecting crevice corrosion

21. Corrosion types
Crevice corrosion
Effect of bulk concentration

22. Corrosion types Crevice corrosion Mechanism
Most of the mechanism is based on certain type of concentration cells. a difference in metal ions exists between the crevice and outside, hence, a corrosion cell is formed.
A high concentration of oxygen on the surface outside the crevice and a low oxygen concentration inside a crevice creates a differential aeration cell, which initiates crevice corrosion

23. Corrosion types
Crevice corrosion
Mechanism

24. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Developed by Oldfield and Sutton suggested four stages of crevice corrosion

25. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Stage 1- the initial reaction when stainless steel is placed in a oxygenated neutral chloride solution
The solution in the crevice becomes slowly depleted in oxygen

26. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Stage 1
Oxygen is depleted and the solution inside becomes deoxygenated
The metal ion concentration is increased

27. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Stage 2- the cathodic reduction of oxygen proceeds outside the crevice and slow dissolution of the metal takes place inside the crevice

28. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Stage 3- accelerated corrosion takes place due to the breakdown of the passive film

29. Corrosion types Crevice corrosion
Mathematical model for crevice corrosion
Stage 4- the crevice corrosion continues to propagate.
Example of crevice corrosion
- Development of leaks at the rolled joint of the pipe and pipe bottom in a heat exchanger.
- Crevice corrosion of a tubing in a hydraulic oil cooler.

30. Thank You !