Chemical Engineering August 2021 - 39

not sufficiently meet the needs for
ensuring a heightened level of integrity
in the design, construction, examination,
inspection and testing of
a gaseous hydrogen piping system.
Hydrogen embrittlement
Before touching on high-temperature
hydrogen attack (HTHA), please
refer to Table 1. In this table are a
list of codes, standards, and regulations
that include six documents
on the topic of hydrogen embrittlement
(HE), a generalized corrosion
anomaly that includes HTHA. HE
can occur at low temperatures and
at high temperatures. Considered a
form of corrosion, it can be initiated
in a number of ways while also affecting
a wide range of materials.
HE comes in many forms that
can be characterized as hydrogenassisted
cracking (HAC), hydrogen
stress cracking (HSC), stress corrosion
cracking (SCC), hydrogenassisted
corrosion cracking (HACC),
hydrogen blistering (HB), hydrogen
induced cracking (HIC), stress oriented
hydrogen-induced
cracking
(SOHIC),
step-wise cracking (SWC),
sulfide stress cracking (SSC), soft
zone cracking (SZC), and high temperature
hydrogen attack (HTHA).
Taken to its simplest form, hydrogen
embrittlement is the mechanics
of failure at the grain boundary of
metals, causing a reduction of ductility
as a result of the permeation
of atomic hydrogen. The means
by which this occurs is varied and
somewhat identified by the respective
titles, such as HTHA, in which
the coincident high temperature
and high pressure of hydrogen are
the requisites for embrittlement,
and SSC, in which atomic hydrogen
is produced as an off-gas resulting
from acid corrosion with the hydrogen
then permeating the metal containment
to potentially cause embrittlement.
But the general result is the
same with all of the above hydrogen
embrittlement cases in which the
strength of a metal is reduced below
its allowable stress range through
embrittlement,
in
turn setting
the
stage for a possible catastrophic
event, given the volatility of the fluid.
With regard to material selection
for gaseous H2 service, there are
two main considerations, beyond
that of wall thickness and mechanical
joint ratings: 1. high-temperature
hydrogen attack (HTHA) and
2. the deep concern regarding leak
potential. Both of these topics are
now discussed.
HTHA
Atomic hydrogen, as opposed to
molecular hydrogen, can propagate,
among other ways, by means of hydrogen
being put under elevated pressures
at elevated temperatures, which
sets the stage for potential HTHA. At
these conditions, atomic hydrogen is
able to diffuse into carbon-steel piping
material or equipment, where it
reacts with the carbon in solution with
the metal to form methane gas at the
grain boundaries. Unable to escape,
the gas expands to create fissures
and cracks in the pipe or vessel wall
- this is HTHA. You can see clearly
the results of HTHA in Figure 2, where
the fissures and cracks are apparent
in the wall of the 8-in. nominal pipe
size (NPS) section of piping that failed
under these
conditions.
Carbon steel
is acceptable
for use in hydrogen
service
when operating
temperatures
remain
below 500°F.
HTHA occurs,
as mentioned
above, when
hydrogen is
contained
under high
partial pressure
in combination
with
high temperatures.
When
the partial
pressure of hydrogen
is expected
to be
approximately
3,000 psig, at
coincidental
temperatures
above approximately
450°F,
which is what
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37
Garlock Sealing Technologies
FIGURE 4. Kammprofile gaskets come with a
metal core bonded with a soft filler material on
both sides. After installation, the soft non-metallic
filler gets pushed into the metal core serrated
grooves to provide good sealing
the conditions were for the Figure 2
catastrophe, then carbon steel is not
recommended.
As can be seen by the modified
Nelson diagram in Figure 3, as
taken in part from API 941, elevated
temperatures have the greatest ef
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Chemical Engineering August 2021

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