overview

1. SCC
Prepared By- Pranav Sharma

2. SCC
• Stress-corrosion cracking (SCC) is a cracking phenomenon that occurs in
susceptible alloys and is caused by the conjoint action of a surface tensile stress
and the presence of a specific corrosive environment.

3. Types

4. Factors

5. Environmental Factor
• Temperature
• pH
• Electrochemical potential
• Solute species
• Solute concentration
• Oxygen concentration

7. Special Agents

8. Microrganisms
• Temperature is important factor affecting microbial growth.Microbial life is
possible within the range of 21 to 230°F (–5 to 110°C). Microorganisms are also
classified as to the temperature range in which they thrive, as in the following
table:

9. Contd..
• The methods by which microorganisms increase the rate of corrosion of
metals and/or their susceptibility to localized corrosion in an aqueous
environment are:
1. Production of metabolites. Bacteria may produce organic acids, inorganic
acids, sulfides, and ammonia, all of which may be corrosive to metallic
materials.
2. Destruction of protective layers. Organic coatings may be attacked by
various microorganisms, leading to the corrosion of the underlying metal.
3. Hydrogen embrittlement. By acting as a source of hydrogen and/or
through the production of hydrogen sulfide, microorganisms may influence
hydrogen embrittlement of metals.
4. Formation of concentration cells at the metal surface and, in particular,
oxygen concentration cells. A concentration cell may be formed when a
biofilm or bacterial growth develops heterogeneously on the metal surface.
Some bacteria may tend to trap heavy metals such as copper and cadmium
within the extracellular polymeric substance, causing the formation of ionic
concentration cells. These lead to localized corrosion.

10. Contd..
5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite corrosion
inhibitors used to protect aluminum and aluminum alloys from nitrate and ammonia.
6. Stimulation of electrochemical reactors. An example of this type is the evolution of
cathodic hydrogen from microbially produced hydrogen sulfide.

11. Mechanical Factors
• Threshold stresses and stress-intensity factors, the presence of a stress-
independent crack-growth regime, and the dependence of cracking to strain rate
are important features in determining the susceptibility of alloys to SCC.
• The stress-intensity factor, K, is a parameter that describes the relationship
between the applied stress and crack length for specific specimen geometries
• Figure shows the stress-intensity factor, K, as a function of the crack propagation
rate, da/dt. The threshold is defined in this figure by the minimum detectable
crack growth rate. The threshold stress intensity is generally associated with the
development of a plastic zone at the crack tip

13. • The slow-strain-rate technique provides an excellent way to determine the
susceptibility of an alloy to SCC (Ref 13, 14). However, the strain-rate behavior
strongly depends on the alloy/environment combination.
• For example, for most materials, the critical strain rate, at which the maximum
susceptibility is obtained, is 10–6/s. This critical strain rate points to a cracking
mechanism whereby the rate of anodic dissolution is equal to the rate of
protective film formation. If a higher strain rate is applied, the mechanical fracture
will be more rapid than the rate of anodic dissolution

14. • The fracture toughness of most materials is determined by a parameter called
‘stress intensity factor,’ Ki. It is a measure of the concentration of stresses at the tip
of the crack. It is given by

15. Fracture Role

16. Zones of Transition

17. Metallurgical Factor
• Strong influence of alloy composition and microstructure on the susceptibility to
SCC is given by austenitic stainless steels, where chromium and molybdenum
promote the formation of passive films on the surface. Trace elements such as
carbon at concentrations greater than 0.03 wt%, may cause sensitization by
forming chromium carbides at the grain boundaries and depleting zones around
the carbides of chromium, thereby rendering the steel susceptible to intergranular
SCC (IGSCC). Austenitic stainless steels will fail transgranularly in high-temperature
chloride solutions.

18. Mechanism

19. Mechanism
• The mechanism of further development and growth depends on the combination
of material and environment. The cracks may be transgranular, intergranular or a
mixture of these. In particular, two mechanisms seem to dominate transgranular
crack development and growth:
1.Accelerated anodic dissolution at the crack tip where some material is subject to
continual plastic deformation.
2. Hydrogen induced or hydrogen-assisted crack formation (often denoted hydrogen
embrittlement). In addition, a mechanism involving stepwise crack growth due to
fracture of a very thin corrosion product film has been proposed.

20. Contd..
• For some material–environment combinations it has been shown that accelerated
anodic dissolution of yielding metal is the significant mechanism. Example-
• austenitic stainless steels in acidic chloride solutions. In these steels, plastic
deformation is characterized by a dislocation pattern giving wide slip steps on the
surface.
• Scully has proposed a model for initiation and development of stress corrosion
cracks, which has been supported by other scientists
• The model in its simplest form is illustrated in Figure A necessary condition is that
the surface from the beginning is covered by a passivating film (A).

21. Contd..
• Plastic deformation gives a slip step with a surface that is metallically clean and very
active (B).
• The further development depends on the repassivation rate: a) In an intermediate
range of this rate, most of the new surface is passivated before significant corrosion
occurs, and the attack is concentrated on a narrow region where the dislocation density
is highest (C).
• This gives initiation and growth of cracks. b) If the repassivation rate is higher, the
critical region is passivated before significant corrosion occurs. c) If the repassivation
rate is low, corrosion is spread over a larger part of the new surface, which will result in
a kind of pit

22. • The crack growth rate is controlled by the rate of dissolution at the crack tip, then
the following equation can be used to estimate the crack growth rate.

23. Mechanism
METALLURGIACL
MECHANISM
-Dislocation
Coplanarity
-Stress Aging And
Microsegregation
-Adsorption
DISOLUTION MECHANISM
-Stress accltd Disolution
-Film formation at cracking
wall
- Noble Elment
enrichment
- Film Rupture
- Chloride ion migration
Hydrogen Mechanism
Mechanical
Mechanism

24. Metallurgical Mechanism
Dislocation Coplanarity- Resistance to cracking on accnt of disloctions.Pattern of
dislocations in SS are planar arrays where in other alloys its tangeled or cellular.
Stress Aging and Microsegregation- Microsegregation of solute atoms to dynamic
effects in crystal structure. Results in Transgranular SCC.Crack propagation is limited to
solute diffusion rate and electrochemical polarisation.
Adsorption-Surface Active species adsorb and interact with strained bonds at the crack
tip, causing reduction in bond strength and leading to crack propagation.

25. Dissolution Mechanism

26. Other Mechanism

27. Hydrogen Embrittlement

28. Hydrogen Embrittlement

31. Prevention
• Substitution Of alloys By using Ni and Mo
Because of there low hydrogen diffusion rates
• Coating

32. General Conditions
• Alloys that are normally very passive often require an agent in the environment
that can promote the breakdown of that passivity. An example is exposure of
316ss to chloride-containing fluids with dissolved oxygen
• Alloys that are normally very active often require an agent in the environment
that can promote the formation of passivity. An example is exposure of carbon
steel to more concentrated hydroxide-containing or nitrate-containing fluids
• The open circuit potential is often required to lie within a certain range that is
determined by the alloy-environment interaction. Determining that potential
range can be difficult. For example, for some alloys-environment combinations the
potential has to be in the region of hydrogen evolution, the absorption of which
by the alloy leads to modification of the structure and increased internal stress.
In other alloy-environment systems the potential needs to be above that potential
which causes localized corrosion to initiate.

33. Open Potential Condtn.

34. • The measured threshold stress is a stress above which stress corrosion cracking
will usually occur. This stress is a function of the alloy and environment. It is not a
property of the alloy. Cracking has been found at stress levels below the threshold
stress
• Alloy constituents can influence initiation of stress corrosion cracking. Specialized
heat treatment (tempering) can eliminate the problem. This characteristic is
especially true of aluminum alloys.
• Temperature is an important variable. If stress corrosion cracking is a possibility,
often the higher the temperature the greater the risk. But the threshold
temperature at which stress corrosion cracking occurs depends on alloy and
environment.
• Stress corrosion cracking usually occurs on materials that exhibit low corrosion
rates, i.e. are normally passive, in the environment. The implication is that in
many, but not all cases a passive surface layer, e.g. chromium-oxide rich surface
region on stainless steel, is required for stress corrosion cracking. An exception to
that rule is stress corrosion cracking of copper in certain non-oxidizing
environments

35. Prevention

36. Contd..

38. Inhibitors