Stress
Corrosion Cracking |
Stress corrosion cracking (SCC) is a progressive fracture
mechanism in metals that is a result of the simultaneous
interaction of a corrodent and a sustained tensile stress.
Structural failure due to SCC is often sudden and unpredictable,
occurring after as little as a few hours of exposure,
or after months or even years of satisfactory service.
Metal components frequently experience SCC in the absence
of any other obvious kinds of corrosive attack. Virtually
all alloy systems are susceptible to SCC by a specific
corrodent under a specific set of conditions.
The tensile stresses necessary for SCC are "static",
and they may be residual and/or applied (see chart below).
| Sources
of Stress for SCC |
| Residual
|
Applied
|
- Welding
- Shearing, Punching,
Cutting
- Bending, Crimping,
Riveting
- Machining
(Lathe-Mill-Drill)
- Heat Treating
- EDM, Laser/Wire Cutting
- Grinding
|
- Quenching
- Thermal Cycling
- Thermal Expansion
- Vibration
- Rotation
- Bolting
- Pressure
- Dead Load
|
Progressive cracking due to "cyclic" stresses in a corrosive
environment is termed "corrosion-fatigue". The boundary
between SCC and corrosion-fatigue is sometimes vague.
However, because the environments that cause SCC and
corrosion-fatigue are not the same, the two are treated
as separate and distinct metal fracture mechanisms.
Compressive residual stresses, such as those induced
in the surface layers of a structure by controlled shot
peening, could prevent or delay SCC and corrosion-fatigue.
Intergranular
Corrosion
Photomicrographs
of peened and unpeened type 304 stainless steel
plate surfaces (sensitized at 1200°F - 1 hour
and tested for Intergranular Corrosion in NH)3-HF
peened with ceramic beads). |
|
It
was discovered at Atomics International that intergranular
corrosion can be prevented in austenitic stainless
steels by shot peening prior to exposure to sensitizing
temperatures. For this purpose, the surfaces must
be severely cold worked by the shot peening to
break up surface grains and grain boundaries.
When exposed to sensitizing temperatures, carbides
will precipitate on the multitude of nucleation
sites (i.e., slip planes, dislocations) formed
within grains rather than preferentially along
continuous grain boundaries to support intergranular
attack in a corrosive medium.
LEFT - Peened ~~ RIGHT
- Unpeened
|
MIC
has published a technical report of applications which
is available upon request. Please contact
us for more information.
|
