Neutron and X-ray scattering
A new way to look at hydrogen embrittlement
Speaker: Matthew Connolly, Ph.D.
Fatigue and Fracture Group
Applied Chemicals and Materials Division
National Institute of Standards and Technology
Abstract
The world is continuing a push toward a clean-energy economy, however one of the
challenges of a clean-energy economy is the localized nature of renewable energy
sources and a delocalized nature of energy demand. There are places with sufficient
solar, wind, hydro, or geothermal energy production resources, however energy
demand does not necessarily correlate well spatially with these sources. Hydrogen is
a carbon-free fuel which can be used for the efficient transportation of energy from
areas with clean-energy production to areas with high energy demand. However, much
of the world’s existing fuel transportation infrastructure (i.e. pipelines and pressure
vessels) are made of steels which have been known to become brittle and have a
decreased lifetime in the presence of hydrogen. Although embrittlement of steels by
hydrogen has been observed for over a hundred years, the exact mechanism of
hydrogen embrittlement is still not known. Elucidating the mechanisms of hydrogen
embrittlement of steels is complicated by the fact that multiple mechanisms may be
activated at once or may even require a synergistic co-existence for activation. Some
leading proposed mechanisms of hydrogen embrittlement include hydrogen-enhanced
decohesion (HEDE), the hydrogen-enhanced localized plasticity (HELP) mechanism,
and the Nano-Void Coalescence Mechanism (NVC). In HEDE, accumulation of hydrogen
at locations of high triaxial stresses lead to the weakening of Fe-Fe bonds once the
hydrogen concentration reaches a critical concentration. In HELP, the introduction of
hydrogen gas creates areas of extended dislocations in the Fe lattice and enhances
dislocation mobility in the steel framework. In NVC, hydrogen is predicted to lead to
the stabilization and promotion of vacancy (“nano-scale void”) agglomeration. A full
understanding of these mechanisms, their relationship to fatigue properties, and their
interaction with each other requires a measurement capable of probing all three
mechanisms at once. I will present a new way of looking at hydrogen embrittlement:
neutron imaging and simultaneous High Energy X-ray Diffraction (HEXRD) and Small-
Angle X-ray Scattering (SAXS) measurements during fatiguing of a steel crack in
hydrogen.
Speaker Biography
Dr. Matthew Connolly is a staff scientist in the Fatigue and Fracture group of the
Applied Chemicals and Materials Division at the National Institute of Standards and
Technology (NIST). He received his B.S. in Physics from the University of Northern
Iowa and a Ph.D in Physics from the University of Missouri. His graduate work was
supported by a National Science Foundation (NSF) Integrative Graduate Education
and Research Traineeship (IGERT), which formally trained him in neutron scattering
techniques at the University of Missouri Research Reactor. His Ph.D. thesis included
both experimental and computational studies of hydrogen storage in carbon-based
adsorbent materials. He joined NIST in late 2014 under a National Research Council
(NRC) fellowship, during which he applied neutron and x-ray scattering techniques to
the study of hydrogen embrittlement in pipeline steels. Matthew remained at NIST as
a staff scientist starting in 2016 and continues to study hydrogen embrittlement as well
as cryogenic fracture and the development of a new measurement technique, neutron
Transmission Bragg Edge Strain Tensor Tomography (TBESTT). He was awarded a
Bronze Medal from the Department of Commerce for the development of a novel
experimental apparatus which allowed for first-ever in situ measurements related to
hydrogen embrittlement.