Stress Corrosion Cracking IN METALS-MUKESH SUNDARARAJAN

1. STRESS CORROSION CRACKING
- MUKESH SUNDARARAJAN

2. • Stress corrosion cracking is a failure mechanism that is
caused by environment, susceptible material, and tensile
stress. Temperature is a significant environmental factor
affecting cracking.
“OR”
A structure under static tensile stress, much below the
yield stress, in contact with corrosive environment may fail
due to SCC. Three conditions must be present
simultaneously to produce SCC: a critical corrosive
environment, a susceptible alloy and some component of
tensile stress.
Stress corrosion cracking

3. NOW THE PHENOMENA OF
“SCC” IS SHOWN IN THE
FOLLOWING FIGURES

4. Cont’d
• SCC leads to the formation of a crack which would not
have developed by the action of the stress or environment
alone. It is the unexpected sudden failure of normally
ductile metals subjected to a tensile stress in a corrosive
environment.
• The required tensile stresses may be in the form of
directly applied stresses or in the form of residual stresses.

6. Causes of SCC
• There are three main causes for SCC as given
follows:
• Corrosive Environments
• Tensile Stresses Above Threshold
• A Susceptible Material

7. Cont’d
•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
3: Concentration of stresses at the root of pre-existing
cracks or notches

8. Stages Involved In SCC
• The process of SCC consists of three
stages:
• Crack Initiation
• Crack Propagation
• Brittle Fracture

9. 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.

11. 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).

12. 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.

13. Examples
• Many materials, particularly high strength materials, are
susceptible to stress corrosion cracking when exposed to a
specific environment. For example, cold worked brass,
which is found in ammunition cartridges, is susceptible to
stress corrosion cracking when exposed to an environment
containing ammonia. In chloride containing environments,
titanium alloys, aluminum alloys, and high
strength stainless steels are susceptible and specific alloys,
which are resistant to stress corrosion cracking, should be
used. The stresses required to initiate and propagate
cracking are often low and many failures occur due to
residual stresses rather than applied stress.

15. Ways Of Propagation Of SCC
• SCC can proceed in either of two ways :
• Intergranular Stress Corrosion Cracking
• Transgranular Stress Corrosion Cracking

16. Intergranular SCC
• The cracks which propagate along the grain boundaries
called as intergranular stress corrosion cracking as
shown in the following figures.

17. Transgranular SCC
• The cracks which run through the individual grains
called as transgranular stress corrosion cracking as
shown in the following figures.

18. IdentificationOf SCC
• Visual identification prior to failure is difficult
due to the typical tightness of stress-corrosion
cracks.
• A low-power hand lens will greatly aid
determination. Detection of cracks can also be
enhanced with ultrasonic, radiographic, or
acoustic emission techniques.

19. Preventive Measures
• There are a number of different ways to control SCC. The method used depends
on the application and may involve changing the mechanical, metallurgical and
environmental conditions.
• Mechanical
• Avoid stress concentrators
• Relieve fabrication stresses
• Introduce surface compressive stresses
• Reduce operating stresses
• Metallurgical
• Change alloy composition
• Change alloy structure
• Use metallic conversion coating
• Environmental
• Add inhibitor
• Modify temperature

20. THANK YOU