Progress: Addressed, Not Adequately
DOT Relevance:192.51 192.65
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.
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.
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.
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.