Tutorial: Hydrogen Damage


Description

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 :

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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 Stress Cracking
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 Cracking"), 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 industry.

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(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 2 forms:


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(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 Attack).

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

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Metal Hydride Formation [or 'Hydride Embrittlement']
In some alloys which form hydrides (e.g. Zirconium, Titanium, Tantalum) hydrogen form brittle hydride needles.

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Causes & Mechanisms

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General
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 corrosion process).

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Hydrogen Embrittlement and Hydrogen-Assisted Cracking / Hydrogen Stress Cracking (incl. Sulfide Stress Cracking)
Hydrogen Embrittlement
Typical Materials: 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
Failure Initiation: Surface and/or internal initiation; incubation period not observed

DESCRIPTION
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 of equipment).

Hydrogen-Assisted Cracking or Hydrogen Stress Cracking

Typical Materials: Carbon and low-alloy steels
Usual Hydrogen Source: Thermal processing, electrolysis, corrosion
Failure Initiation: Internal crack initiation

DESCRIPTION
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.

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(High-Temperature) Hydrogen Attack
Typical Materials: Carbon and low-alloy steels
Usual Hydrogen Source: Gaseous (above 200C)
Failure Initiation: Surface (decarburization); internal carbide interfaces (methane bubble formation)

DESCRIPTION
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 cracking.

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)
Usual Hydrogen Source: Water vapor reacting with molten steel
Failure Initiation: Internal defect
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(Hydrogen) Blistering
Typical Materials: Steels, copper, aluminum
Usual Hydrogen Source: Hydrogen sulfide corrosion, electrolytic charging, gaseous
Failure Initiation: Internal defect

DESCRIPTION
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

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Metal Hydride Formation
Typical Materials: V, Nb, Ta, Ti, Zr, U
Usual Hydrogen Source: Internal hydrogen from melting, corrosion, electrolyte charging, welding
Initiation: Internal defect

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Recognition

The appearance will depend on the mechanism involved. A number of examples are available in the accompanying Corrosion Atlas of this information system.

EXAMPLES from the MTI Atlas of Corrosion and Related Failures:
(use your browser's BACK button to return here after clicking)

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Examples 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)

Material: Titanium.
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.

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Prevention

Guidelines for Low-Temperature (Aqueous) Hydrogen Problems (e.g. embrittlement) :
-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 susceptible materials
-Use low-hydrogen welding electrodes for weldments
High-Temperature Service (e.g., Decarburization - "Hydrogen Attack")

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