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Stress corrosion cracking (SCC) is a form of localized damage that
refers to cracking under the combined influence of tensile stress and a corrosive
environment. The macroscopic fracture appearance tends to be of the "brittle"
type, even if the metal/alloy is of a mechanically ductile variety. Stresses that can
contribute to SCC include the applied, residual and thermal varieties, and also those
generated by the build-up of corrosion products. Welding, heat treating, fitting and
forming operations can produce significant residual stress levels, in some cases
approaching the yield strength (often surprising the unsuspecting).
Stresses to consider include:
- Thermally induced (temperature gradients, differential thermal
expansion and contraction)
- Induced by build-up of (voluminous) corrosion products
- Assembly
- poor fit up (tolerancing problems)
- torqueing
- press and shrink fits
- fastener interference
- joining (see below)
- Residual, from manufacturing processes
- joining (welding, brazing, soldering)
- casting
- surface treatment (plating, mechanical cleaning, etc.)
- heat treatment (e.g. quenching, phase changes)
- forming and shaping
- machining
- cutting and shearing
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| Mechanisms: Mechanisms proposed for
stress corrosion cracking include the following:
Active Path: Localized preferential corrosion (dissolution) at the
crack tip, along a susceptible path, with the bulk of the material remaining in a more
passive state. Note that the rate of metal dissolution can be several orders of magnitude
higher when an alloy is in its active state, compared to its passive condition - as, for
example, indicated by potentiodynamic polarization curves.
Hydrogen Embrittlement: It has been postulated that harmful hydrogen
concentrates in highly stressed regions associated with the crack tip or other notches,
leading to localized embrittlement.
Brittle Film-Induced Cleavage: Cracks initiated in a brittle surface
film may propagate (over a microscopic distance) into underlying more ductile material,
before being arrested by ductile blunting of the crack tip. If the brittle film reforms
over the blunted crack tip (under the influence of corrosion processes), such a process
could be repeated.
Source: National Physical Laboratory: "Stress Corrosion Cracking"
(Guides to Good Practice in Corrosion Control), Middlesex (UK), 2000. |
Stress corrosion cracking is generally considered to be the most complex
of all corrosion types. Cracking can have a transgranular or intergranular morphology.
Multiple variables affect stress corrosion cracking phenomena, such as stress level, alloy
composition, microstructure, concentration of corrosive species, surface finish,
micro-environmental surface effects, temperature, electrochemical potential, etc. Further
complications are initiation and propagation phases, and the observation that in some
cases cracks initiate at the base of corrosion pits.
| The detection and monitoring of SCC and data
interpretation tend to be highly challenging. Even in closely controlled laboratory
experiments, it is very difficult to obtain reproducible results. Some stress corrosion
cracks are so fine, that metallographic examination is required for their identification.
Stressed coupons have been used for monitoring purposes. Electrochemical noise has been
employed as an on-line monitoring technique in selected applications but it provides no
direct information on the rate of crack growth. The use of in-line inspection tools is an
approach related to pipeline inspection. |
References/Literature:
R.W. Staehle: "Lifetime Prediction of Materials in
Environments", in Uhlig's Corrosion Handbook 2nd Edition (R.W. Revie Ed.), Wiley, New
York, 2000.
R.N. Parkins: "Stress Corrosion Cracking", in Uhlig's Corrosion
Handbook 2nd Edition (R.W. Revie Ed.), Wiley, New York, 2000.
H.J. DeBruyn, K. Lawson and E.E. Heaver: "On-Line Monitoring Using
Electrochemical Noise measurement in CO-CO2-H2O Systems, in ASTM STP
1277 "Electrochemical Noise Measurement for Corrosion Applications", American
Society for Testing and Materials, West Conshohocken (PA), 1996.
National Physical Laboratory: "Stress Corrosion Cracking"
(Guides to Good Practice in Corrosion Control), Middlesex (UK), 2000.
Links:
Report on In-line Inspection Technologies for pipelines through
http://ops.dot.gov/pipetech.htm
Visit the National Energy Board of Canada for more
information on pipeline SCC (various items are available by conducting a search for stress
corrosion cracking)
http://www.neb.gc.ca/
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