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 Stress Corrosion Cracking
the failure of a metal resulting from the co-joint action of stress and chemical attack.
a combination of static tensile stress, environment and a metallurgical condition which leads to component failure.
characterized by fine cracks which lead to failure of components are potentially the structure concerned.
Under tensile stress, and in a suitable environment, some metals and alloys crack . . .

3. Corrosion types Stress Corrosion Cracking Requirement for SCC:
A susceptible metal.
A specific environment
A tensile or residual stress.

4. Corrosion types Stress Corrosion Cracking
Common sources of tensile stresses are:
1: External stresses caused by
Centrifugal forces
Temperature variation
2: Residual stresses due to
Forming
Heat treatment
Welding
Machining
Grinding

5. Corrosion types Stress Corrosion Cracking
Application of fracture mechanics
The fracture toughness of most materials is determined by a parameter called ‘stress intensity factor,’ Ki
Failure occurs when the stress intensity factor reaches a critical value, K1c

6. Corrosion types Stress Corrosion Cracking
Application of fracture mechanics
Failure occurs when the stress intensity factor reaches a critical value, K1c

7. Corrosion types Stress Corrosion Cracking Types of SCC:
Chloride stress corrosion cracking
Caustic stress corrosion cracking
Sulfide stress corrosion cracking
Seasonal cracking

8. Corrosion types
Stress Corrosion Cracking
Types of SCC:

9. Corrosion types
Stress Corrosion Cracking
Types of stress in SCC:

10. Corrosion types
Stress Corrosion Cracking

11. Corrosion types Stress Corrosion Cracking SCC mechanism:
The following are generally the sites for crack initiation.
Surface discontinuities
Corrosion pits
Grain boundaries

12. Corrosion types Stress Corrosion Cracking
The process of SCC consists of three stages:
Crack Initiation
Crack Propagation
Brittle Fracture

13. Corrosion types Stress Corrosion Cracking
The process of SCC consists of three stages:
Crack Initiation
SCC is initiated by stress concentrations at defects on the material surface. The defect may be an existing material defect. The defect may also be a result of pitting corrosion, crevice corrosion, intergranular corrosion or local galvanic corrosion.

14. Corrosion types Stress Corrosion Cracking
The process of SCC consists of three stages:
Crack Initiation

15. Corrosion types Stress Corrosion Cracking
The process of SCC consists of three stages:
Crack Propagation
The passivation film at the tip of the crack is broken due to plastic deformation. Pure and normally very active metal is exposed and will be attacked by corrosion. Grow rate will be a combination of corrosion and cracking. The crack starts to grow when the stress concentration at the end of the crack (KI) exceeds the threshold stress intensity factor for stress corrosion cracking (KISCC).

16. Corrosion types Stress Corrosion Cracking
The process of SCC consists of three stages:
Brittle Fracture
When the stress concentration at the end of the crack exceeds the critical stress intensity factor (KI > KIC), there will be a rapid, unstable brittle fracture.

17. Corrosion types
Examples of typical SCC

18. Corrosion types
Examples of typical SCC

19. Corrosion types
Examples of typical SCC

20. Corrosion types Characteristic of SCC
Stress, either residual or applied, is required.
Generally all alloys are susceptible to SCC
SCC of a specific alloy is caused by only a few chemical species in the environment.
Conditions of cracking are specific to alloy and environment.
The mode of cracking may be intergranular or transgranular.
SCC cracks are microscopically brittle in appearance.

21. Corrosion types
Environmental and materials factor in SCC

22. Corrosion types Mechanism of SCC Mechano–electrochemical model
Film rupture model
Embrittlement model
Adsorption model

23. Corrosion types Mechanism of SCC Mechano–electrochemical model
a grain boundary precipitate anodic to the grain boundary would provide an active path for localized corrosion to proceed.
the removal of the protective film at the crack tip by plastic deformation would facilitate the onset of localized corrosion.

24. Corrosion types Mechanism of SCC Film rupture model
based on a repetitive cycle comprising (a) localized film disruption, (b) localized attack at the point of disruption and (c) film repair.
Embrittlement model
based on the postulate that an electrochemical process embrittles the materials in the vicinity of a corroding surface.
Adsorption model
Adsorption of damaging species causes weakening of cohesive bonds between surface metal atoms by specific damaging species.

25. Corrosion types Effect of environmental factors on SCC Specific ions
Not all environments promote SCC and hence, the environments causing SCC are specific.

26. Corrosion types Effect of environmental factors on SCC
Effect of oxygen
oxygen is essential for SCC to take place. Oxygen only acts as a reducible species
Role of hydrogen
role of hydrogen in SCC is very crucial

27. Corrosion types
SCC in stainless steel

28. Corrosion types
SCC in steel
Effect of Alloy Composition

29. Corrosion types SCC in steel Effect of temperature
The susceptibility of austenitic steels to SCC increases with temperature.

30. Corrosion types SCC in steel Stress level
SCC would never occur in the absence of either corrosive environment or stress. SCC can occur both by residual and tensile stress.
Effect of chloride concentration

31. Thank You !