Stainless steel has phenomenal resistance to uniform corrosion but it seriously prone to SCC. WHY??? Lets see

1. Stress Corrosion Cracking phenomena
in Austenitic grade Stainless Steel
By
KOUSHIK KOSANAM
CSUN ID-201437281

2. CONTENTS
Austenitic stainless steels
Stress corrosion cracking
SCC mechanisms
Types- chlorine induced, caustic, high temp, high
pressure, residual stress during machining ,
operations, welding, and irradiation
Conclusions
Case studies

3. Austenitic grade stainless steel
Minimum of 16-25% Cr.
FCC crystal structure.
Austenitic steels are non-magnetic in the annealed condition
good formability and weldability, as well as excellent toughness, particularly at low or
cryogenic temperatures.
Austenitic grades also have a low yield stress and relatively high tensile strength.
The fatigue endurance limit is only about 30% of the tensile strength and better
thermal resistance when compared to ferrite stainless steels.

4. Different grades of Austenitic stainless
steel

5. Stress corrosion cracking
Stress corrosion cracking (SCC) is the cracking induced from the combined
influence of tensile stress and a corrosive environment.
Austenitic stainless steels are more susceptible to SCC.

6. Different materials and environments leads
to SCC

7. SCC mechanisms
Tensile de-cohesion of the atomic bounds due
to the presence of adsorbed ions in the crystal
lattice ahead of the crack tip
Slip Step Dissolution
Due to the application of tensile stress, the
passive film breaks and exposed to solution.
The failure occurs at slip steps and occurs
mostly in FCC materials.
The presence of interstitial atoms in this
lattice decreases its shear modulus and the
repulsion forces between edge dislocations,
consequently reducing the distance between
these dislocations.
Film rupture
Corrosive medium breaks the film and
propagation happens through grain
boundaries.
Transgranular failure

8. Stress corrosion by halides
 The most corrosive environment for austenitic grade
stainless steels.
 Chloride induced SCC is known to be typically critical
when compared to other. Bromide and iodide are heavier
ions, hence not well known for SCC
 Fluoride ions are smaller in size than chloride ions but
tend to form strong metal fluoride bonds tending to cause
more of a heavy uniform dissolution than localized attack

9. CASE STUDY -1 (SCC by halides)
 304 SS is tested under different conditions for SCC.
 Generally solutions of high alkalinity (but not actually
caustic) are less likely to give rise to SCC.
 Temperature is a very important feature and in this series
of tests, no SCC was observed for tests carried out at
temperatures below 60°C.
 Na > Mg >Ca >Zn.
Temperature 20-800C
PH 2-12
Types of chloride solutions Na, Mg, Ca, Zn
Time 13500 hrs

11. Case study-2(SCC in chlorine
environment)
Two different tests were conducted related to
chloride Stress Corrosion Cracking of 316
Austenitic Stainless Steels in 3.5wt% &
9.35wt% NaCl solution in a room temperature
environment.
After 838 hrs, cracking started in 9.35wt%NaCl
and 1288 hrs in 3.5 wt% Nacl.

12. CASE STUDY-3(Effect of nitrate on SCC)
• 316L suffers intergranular stress corrosion cracking
(SCC) at strain rates of 3.7 × 10−7 and 5.3 × 10−7 s −1,
and no SCC at higher strain rates.
• Additions of NO2 − (1400–5600 ppm) accelerate
susceptibility to chloride SCC. (c) With increasing NO2 −
content, SCC susceptibility increases in the order.

14. CASE STUDY-4(SCC in BWR)
 This is happened in boiling water reactors (BWRs),
typically at 288C.
 The presence of dissolved oxygen in this high purity
water makes it more oxidizing, hence makes austenitic
stainless steels more prone to SCC.
 Hydrogen gas is passed to reduce the excess amount of
oxygen.
 The localized enhanced oxidation of crack tips is the main
feature in SCC in high temperature aqueous environment
 The exposure to neutron irradiation increases the
susceptibility of austenitic stainless steels (SSs) to stress
corrosion cracking (SCC).
 it was found that sulfur (S) content below 0.002 wt. %
provides the best resistance to IASCC.

16. CASE STUDY-5(SCC in chlorine
environment)
The effect of salt loading on chloride-induced
stress corrosion cracking in 304L was studied at
atmospheric pressure.
Stress relieved samples were uniaxially pre-
strained to 5% and were loaded with nine levels
of MgCl2, investigating Cldeposition levels
from 1.7 x 10-4 to 3.1 x 10-2 g.cm-2.
Samples were subject to 60 MPa stress, 90°C
at 70% relative humidity, for 480 hours.

17. There appears to be a direct positive linear correlation between the level of salt deposited
onto a sample, the visibly corroded area and the crack number density for salt loadings
between.
At very low salt loadings cracks were shorter and finer than cracks seen at higher salt
loadings

18. Material susceptibility
 The SCC also depends on the materials characteristics like alloying
elements, stacking fault energy (SFE), grain boundary etc.
Stalking fault energy and role of alloying elements
 Stacking fault is dependent on the composition and temperature of the
materials which affects the deformation rate in austenitic stainless steels.
 Materials with lower SFE values tend to be more susceptible to SCC. For
instance, austenitic stainless steels have SFE of 20J/m^2, which prone
them to chlorine induced stress corrosion cracking.
 Increase in percentage of Nickel can help in increasing resistance to SCC,
high alloy austenitic stainless steels with 27% Cr,31% Ni and 3.5% Mo had
 This is the one of main reason behind better resistance to SCC 316 over
304 grade, especially in caustic and chlorine environments, because 316
have higher percentage of Ni and Mo better resistance to SCC when
compared to other grades of austenitic stainless steel.

19. Plasticity of stainless steel during cold working led to the formation of partial conversion
of austenitic to martensitic phase. Since two phases has different potentials, setting up a
corrosion battery with austenitic as anode and martensite as cathode
fatigue fractures are caused by simultaneous action of cyclic stress, tensile stress and
plastic strain.
CASE STUDY-6 (Phase induced SCC)

20. Residual stresses and machining errors causing SCC
The surface states can compromise corrosion
resistance (pitting corrosion and SCC) of
stainless steels.
These processes affect the electrochemical
and mechanical stabilities of passive film and
that of the near-surface layers, by changing the
surface reactivity and altering the near-surface
residual stress/strain state.
Roughness has a significant effect on stress-
corrosion crack initiation.
The metal on the surface layers is plastically
deformed during machining/grinding, slip
bands and deformation twins exist throughout
the layer adjacent to the surface

21. Conclusions
 Minimum amount of stress or chloride level to prevent
material from SCC is not found.
 Material selection is critical.
 Avoid material defects, machining errors, and maintaining
observation conditions.
 The adhesion issue is tackled by having an intermediate
100–300 mm thick layer of NiCrAlY in case of BWR.
 Cathode protection is one of the effective option.
 Residual stresses measurements and following standard
heat treatment conditions.

22. Typical References
 V. KAIN, Bhabha Atomic Research Centre, Stress corrosion cracking (SCC) in
stainless steels.
 Yi Xie et.al (2015), Chloride-Induced Stress Corrosion Cracking of Used
Nuclear Fuel Welded Stainless Steel Canisters: A Review
 J. E. TRUMAN, THE INFLUENCE OF CHLORIDE CONTENT, pH AND
TEMPERATURE OF TEST SOLUTION ON THE OCCURRENCE OF
STRESS CORROSION CRACKING WITH AUSTENITIC STAINLESS
STEEL, Corrosion Science, 1977, Vol. 17, pp. 737 to 746.
 Samir Milad Elsariti et.al, Behaviour of Stress Corrosion Cracking of
Austenitic Stainless Steels in Sodium Chloride Solutions, Procedia Engineering
53 ( 2013 ) 650 – 654 .
 R. K. Singh Raman et.al, (2014) ,Stress Corrosion Cracking of an Austenitic
Stainless Steel in Nitrite-Containing Chloride Solutions, pg no-7799-7808.
 G.G. Scatigno et.al, (2019) The effect of salt loading on chloride-induced stress
corrosion cracking of 304L austenitic stainless steel under atmospheric
conditions.

23. THANK YOU