Stress corrosion cracking is explained with respect to its causes, mechanisms and remedies.

1. JABIN MATHEW BENJAMIN
13MY04
M.E. INDUSTRIAL METALLURGY
STRESS CORROSION CRACKING
(SCC)

2. Flixborough explosion, UK : June 1974
 28 fatalities as well as the near complete destruction of the
NYPRO plant in north Lincolnshire by blast and then fire
 The failure of a bypass assembly introduced into a train of six
cyclohexane oxidation reactors after one of the reactors was
removed owing to the development of a leak.
 Resulting from the presence of nitrates that had contaminated
river water being used to cool a leaking flange.
 Mild steel exposed to hot nitrate solution and to stresses which
in extreme cases may be well below the yield strength, and may
remain in the structure from the fabrication procedure or may
derive from operating stresses  developed intergranular
cracks.

3. WHAT IS SCC?
 Failure of metal resulting from joint action of
 Stress
 Chemical attack
 Initiation and propagation of cracks without any
outside evidence of corrosion
 Sudden and unpredictable failure: Catastrophic

4. Requirements for SCC
Susceptible
metal
Specific
environment
Tensile
stress

5. METAL-ENVIRONMENT FOR SCC
 Cu alloys in ammoniacal solution
 Austenitic Steels in oxide and chloride environment
at high temperature
 Carbon steels in hydroxide and nitrate environment
 Very high temperature water is aggressive to wide
variety of materials

6. Stresses!!
 SCC is caused by
 Static stresses
 Residual : Cold working, welding, heat treatment, machining
 Thermal stresses
 Applied stresses
 Build-up of corrosion products in confined spaces
 Compressive residual stress
 Used to prevent or delay onset of SCC

7. Stress Corrosion Cracking
Crack produced by SCC in an aircraft part

8. SCC sites
 SCC initiates at any pre-existing flaws
 Surface discontinuities
 Grooves, cracks
 Corrosion pits
 Grain boundaries
 Intergranular cracking: sensitization of impurities at grain
boundaries making them very reactive

9. Mechanisms of SCC
 Pre existing active path mechanism
 Suggests that there are pre existing paths that are susceptible
to anodic dissolution
 For example
 A grain boundary precipitate anodic to the grain boundary
 Segregated impurities that behave anodic.
 Area adjacent to depleted grain boundary will be preferentially
attacked.
 Creates localized galvanic cells, segregate being generally
anodic
 Predominant : SCC governed by electrochemical factors than
stress.

10.  Grain boundary precipitation
 E.g.: formation of chromium carbide
on the grain boundary and depletion
of Cr adjacent to grain boundary in
stainless steels.
 Precipitation of carbide in Ni alloys
and Mg2Si in Al alloys.
 Grain boundary segregation
 Impurities, such as P, S, C and Si, segregate at the grain boundary and
cracking

11. Mechanisms of SCC
 Strain generated active path mechanism
 Strain is the controlling influence
 Initially a passivating film is formed on the metal surface
 Film is ruptured by plastic
strain, exposing bare metal.
 Film forms again and
process continues
 Crack propagates by successive dissolution of crack tip in the
process
 Emergence of slip steps also considered to cause film rupture

12. Formed brass product
with SCC caused cracks
Inside of Cu pipe showing crack
Intergranular cracks in Cu

13. PREVENTION
 Choosing a material that is not susceptible to SCC in the service
environment
 Residual stresses can be relieved by stress-relief annealing
 Similarly shot-peening or grit-blasting tend to introduce a surface
compressive stress.
 Neutralizing the environment whenever possible, e.g. reducing chloride
ion concentration.

14. Reference
 Zaki Ahmad, Principles of corrosion engineering
and corrosion control, Sept 2006, Elsevier science
and technology books.
 www.corrosion-doctors.org/forms-scc

15. THANK YOU