There are numerous examples of damage associated with hydrogen which are
contained under the collective term "hydrogen damage", sometimes also
coined "hydrogen-assisted cracking" (HAC) if fracture is involved.
Similar phenomena may even be known under different terms.
Common terms and forms of hydrogen damage are :
- Hydrogen Embrittlement and Hydrogen-Assisted Cracking/
Hydrogen Stress Cracking (incl. Sulfide Stress Cracking)
- Hydrogen Embrittlement
(HE) - is the loss of ductility and tensile
strength in a metal, as a result of penetration (absorption) of hydrogen into
it. Hydrogen embrittlement requires a susceptible metal (hardness, metallurgical
condition, composition) and a fabrication method or process condition capable of
promoting the entry of atomic hydrogen into the steel. Common examples of the
latter are: electroplating, acid pickling, high-pressure hydrogen environments,
and corrosion with cathodic liberation of hydrogen.
The susceptibility of
most metals to HE increases with: (1) increasing strength level (hardness), (2)
increasing amounts of cold-work (plastic deformation), and (3) increasing
residual or applied stress.
Hydrogen-Assisted Cracking (HAC) or Hydrogen
This cracking process [occasionally still called 'Hydrogen-Assisted Stress-Corrosion
Cracking' - HSCC] results from the presence of hydrogen in a metal in
combination with tensile stress. It is a cathodic process in which, regardless
of the bulk environment, the specific aggressive species is atomic hydrogen (in
contrast to Stress Corrosion Cracking). Characteristically, it is less likely at
moderately elevated temperatures, e.g. above 70-80°C, than is SCC. The
morphology tends to be intergranular in nature.
More: The term HAC (or HSCC) is used to distinguish those cracking
failures (often of hardened steel alloys) which occur primarily as a result of
the cathodic hydrogen charged into the steel rather than the corrosion reaction,
per se (whereas in conventional SCC the anodic reaction is said to be the
essential part of the cracking process - although this distinction might be very
artificial according to some SCC theories).
This mode of failure is merely a special case of (localized) hydrogen embrittlement where the nascent
atomic hydrogen is supplied to the metal as a byproduct of a corrosion reaction.
It occurs most frequently with high-strength alloys, often enhanced by
the presence of sulfides ( mostly indicated as "Sulfide Stress Cracking"
- SSC), or by other "poisons" that promote the entry of hydrogen in
metals, e.g. cyanide (" Cyanide Stress
arsenic, antimony, or selenide ions. The phenomenon of Sulfide Stress Cracking,
which is a major type of HAC, is of particular importance in the Oil & Gas
(High-Temperature) Hydrogen Attack on steel
- A loss of strength and ductility of steel by high-temperature reaction of
absorbed hydrogen with carbides in the steel, resulting in decarburization
and/or internal fissuring or blister formation. So, the Hydrogen Attack can take
(Hydrogen) Blistering [or 'Cold Hydrogen Attack']
- Surface bulges, resulting from subsurface voids produced in a metal by
hydrogen absorption in (usually) low-strength alloys. This type of blistering
may occur at lower temperatures than the Methane Blistering (see Hydrogen
The term Hydrogen Induced
Cracking (HIC) is
-normally- reserved for a form of hydrogen blistering in which stepwise
internal cracks are created that can affect the integrity of the metal.
Illustration of Hydrogen Blistering
Hydride Formation [or 'Hydride Embrittlement']
- In some alloys which form hydrides (e.g. Zirconium, Titanium, Tantalum)
hydrogen form brittle hydride needles.
- All forms of hydrogen damage are caused by the (local) presence of, or the
interaction with, hydrogen. Hydrogen blistering and hydrogen embrittlement are
caused by penetration of atomic hydrogen into metal. Decarburization
(high-temperature attack) is caused by reaction with hydrogen at high
temperatures. The origin of hydrogen can often be found in the cleaning,
pickling, cathodic protection, welding, treatment and operation (and even in a
Hydrogen Embrittlement and Hydrogen-Assisted Cracking / Hydrogen Stress Cracking
(incl. Sulfide Stress Cracking)
- Hydrogen Embrittlement
||Plain carbon and low-alloy steels; martensitic,
precipitation-hardening, and (cold-worked) ferritic and austenitic
stainless steels; (nickel-base alloys - Alloy 400 / cold worked); titanium
alloys; Be-Cu bronze; Weldments (with higher C) !|
|Usual Hydrogen Source:
||Gaseous hydrogen, internal hydrogen from electrochemical charging |
||Surface and/or internal initiation; incubation period not observed
Atomic hydrogen dissolved in steel can interfere
with the normal process of plastic flow (dislocation movement). For this
interference to occur, the hydrogen atoms must have time to diffuse to the site
of dislocation movement (plastic strain). Therefore, hydrogen embrittlement is
most likely to occur at slow strain-rates and at temperatures of -100°C to
+120°C (values valid for steels).
At high strain-rates, as in impact loading (or in some hardness
measurements), the dissolved hydrogen has no effect on the behavior of the steel
because dislocation movement is too rapid for hydrogen diffusion and
interaction. At temperatures below ca. -100°C, the hydrogen diffusion rate
is too low for interaction with dislocation movement. At temperature above ca.
120°C, the hydrogen is also ineffective in causing embrittlement, perhaps
because the hydrogen escapes from the metal as fast as it enters. However,
steels charged with atomic hydrogen at temperatures above 120°C can show
embrittlement if subsequently stressed at lower temperatures (e.g. on shutdown
Hydrogen-Assisted Cracking or Hydrogen Stress
Typical Materials: Carbon and low-alloy steels
Usual Hydrogen Source: Thermal processing, electrolysis, corrosion
Failure Initiation: Internal crack initiation
HAC is merely a special case of localized hydrogen
embrittlement where the nascent atomic hydrogen is supplied to the steel as a
by-product of the corrosion reaction.
Acid corrosion of steel in the presence of a "poison" such as
sulfide (Sulfide Stess Cracking), cyanide, arsenic, antimony, or selenide ions,
is a prime example.
The nascent atomic hydrogen charged into a metal by a corrosion reaction
behaves identically to that charged into a metal by high-pressure process
hydrogen or by cathodic plating out of hydrogen in an electroplating operation.
That is, the metal (steel) containing the atomic hydrogen will show the same
reduced notch-ductility in the temperature range of -20°C to 120°C.
(High-Temperature) Hydrogen Attack
- Typical Materials: Carbon and low-alloy steels
Usual Hydrogen Source: Gaseous (above 200°C)
Surface (decarburization); internal carbide interfaces (methane bubble
While not a corrosion phenomenon in the usual
sense, hydrogen attack (on steels) at elevated temperatures is potentially a
very serious problem, e.g. in hydrotreating, reforming, and hydrocracking units
of oil refineries at above roughly 260°C (500°F) and hydrogen partial
pressures of above ca. 690 kPa (100 psia).
Under these conditions molecular hydrogen (H2) dissociates at the steel
surface to atomic hydrogen (H) which readily diffuses into the steel. At grain
boundaries, dislocations, inclusions, gross discontinuities, laminations, and
other internal voids, atomic hydrogen will react with carbon to form methane
(More>>>). The large size of its molecule precludes methane
diffusion. As a result, internal methane pressures become sufficiently high to
blister the steel or cause intergranular
If temperatures are high enough, dissolved carbon diffuses to the steel
surface and combines with hydrogen to evolve methane. Hydrogen attack now takes
the form of decarburization rather than
blistering or cracking.
Blistering may also occur as a result of the internal recombination of
atomic hydrogen and the formation of molecular hydrogen (gas), without the
involvement of methane formation (see Hydrogen Blistering - below).
NOTE ... A special kind of hydrogen attack is the
formation of shatter cracks, flakes, or fisheyes in steelmaking or the
fabrication of components:
Shatter Cracks, Flakes, Fisheyes
Typical Materials: Steels (forgings and castings)
Hydrogen Source: Water vapor reacting with molten steel
Initiation: Internal defect
- Typical Materials: Steels, copper, aluminum
Usual Hydrogen Source: Hydrogen sulfide corrosion, electrolytic charging, gaseous
Failure Initiation: Internal defect
Nascent atomic hydrogen adsorbs on the metal surface,
then enters a defect or void where it can recombine into molecular hydrogen.
Imperfectly bonded areas found in laminations or gross inclusions are examples
of such defects.
As the molecular hydrogen forms in the defect area, the pressure increases
causing growth and further separation of the flaw. The internal flaw can
eventually result in an externally evident "blister".
Illustration of Hydrogen Blistering
- Typical Materials: V, Nb, Ta, Ti, Zr, U
Usual Hydrogen Source: Internal hydrogen from melting, corrosion, electrolyte charging, welding
Initiation: Internal defect
The appearance will depend on the mechanism involved. A number of examples
are available in the accompanying Corrosion Atlas of this information
EXAMPLES from the MTI Atlas of Corrosion and Related Failures:
(use your browser's BACK button to return here after
from the Corrosion Atlas
- - unprotected carbon steels in steam systems
- Case 01.01.20.01 (click on photo to zoom)
Material: Carbon steel.
System: High pressure boiler.
Part: Evaporator tube.
Phenomenon: Hydrogen damage (intergranular hydrogen stress cracking).
Appearance: Longitudinal cracks in cold-bent bend.
Time to Failure: Over 10 years.
- - noble and reactive metals in process installations
- Case 09.11.20.01 (click on photo to zoom)
System: Ammonium carbamate condenser.
Part: Pipe of pipe bundle.
Phenomenon: Hydrogen damage (hydride embrittlement).
Appearance: Cracking and uniform corrosion, the wall thickness being reduced from 3 mm to 1 mm.
Time to Failure: 5 years.
Guidelines for Low-Temperature (Aqueous) Hydrogen Problems (e.g.
-Reduce hydrogen evolution by reducing corrosion rate
-Control hydrogen pick-up during plating by controlling plating conditions
-Remove absorbed hydrogen by baking
-Substitute alternative, less
-Use low-hydrogen welding electrodes for weldments
High-Temperature Service (e.g., Decarburization -