Embrittlement (1.1)
Criticality: High
Score: 20
Progress: Addressed, Not Adequately
DOT Relevance:§192.51 – §192.65
Description of Key Area
Internal hydrogen embrittlement is when atomic hydrogen diffuses
into and supersaturates the metal structure. The hydrogen acts to lower
fracture resistance under applied stress and the concentration of hydrogen in
the metal can increase over time. Environmental hydrogen embrittlement occurs
with simultaneous hydrogen exposure and applied stress. Atomic hydrogen
diffuses into the near-surface volume of metals and facilitates, over time, the
propagation of surface defects propagation of surface flaws. The rate of
sub-critical crack growth can be governed by hydrogen diffusion.
Increased crack propagation susceptibility degrades properties as
ductility and fracture toughness. Impurities in the metal can affect the
resistance of the metal to hydrogen-assisted fracture. Metals can be processed
to have a wide range of strengths and resistance to hydrogen-assisted fracture
generally decreases as the strength of the alloy increases.
The susceptibility to hydrogen-assisted fracture generally
increases as hydrogen pressure increases. Temperature effects are not as clear.
Some metals such as austenitic stainless steels exhibit a local maximum in
hydrogen-assisted fracture susceptibility as a function of temperature.
Although not well understood, trace gases mixed with the hydrogen
gas can also affect hydrogen-assisted fracture. Moisture, for example, may be
detrimental to aluminum alloys since wet oxidation produces high-fugacity
hydrogen, while in some steels moisture is believed to improve resistance to
hydrogen-assisted fracture by producing surface films that serve as kinetic
barriers to hydrogen uptake. An inverse strain rate effect is generally
observed in the presence of hydrogen; in other words, metals are less
susceptible to hydrogen-assisted fracture at high strain rates.
Sections 192.51 to 192.65 of Subpart B (Materials) of 49 CFR 192
“prescribes the minimum requirements for the selection and qualification of
pipe and components for use in pipelines.” Section 192.55 relates to steel pipe
in particular.
Discussion of Criticality
Hydrogen pipelines have operated safely for years using X42 or
X52 steels at pressures less than 6.9 MPa (1000 psi) with low cycling. A
pipeline system built to serve a much larger market for hydrogen might operate
at increased loads and pressure cycles. There is particular concern related to
the heat affected zones of welds.
Discussion of Progress
Embrittlement studies have been performed and are currently
ongoing. UIUC is currently investigating embrittlement issues and is
coordinating with related work at SECAT, Inc., ORNL, and SRNL. A study of
embrittlement of high strength fasteners for use on hydrogen systems has been
completed by the Hendrix Group in 1998.
One ASTM standard does exist on this topic. It is ASTM F519, the
Hydrogen Embrittlement Test.
Recommendations
Further research is needed on this key topic, especially related
to embrittlement of base metal and of welds. A national database of
embrittlement problems and incidents should be developed and maintained. A
comprehensive listing of metals commonly used in piping should be created. The
sequence of the listing should be in order of increasing tendency for
embrittlement.
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