Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, become brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

 and fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

 following exposure to hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. Hydrogen embrittlement is often the result of unintentional introduction of hydrogen into susceptible metals during forming or finishing operations.

Hydrogen embrittlement is also used to describe the formation of zircaloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance...

 hydride. Use of the term in this context is common in the nuclear industry.


The mechanism starts with lone hydrogen atoms diffusing
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC). High-strength and low-alloy steels, nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

 and titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 alloys are most susceptible. Austempered
Austempering is an isothermal heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a lower bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is...

 iron is also susceptible. Steel with an ultimate tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 of less than 1000 MPa or hardness of less than 30 HRC
Hardness comparison
There are a large number of hardness testing methods available . Although it is impossible in many cases to give an exact conversion, it is possible to give an approximate material-specific comparison table e.g...

 are not generally considered susceptible to hydrogen embrittlement. Jewett et al. reports the results of tensile tests carried out on several structural metals under high-pressure molecular hydrogen environment. These tests have shown that austenitic
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...

 stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

s, aluminum (including alloys), copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 (including alloys, e.g. beryllium copper
Beryllium copper
Beryllium copper , also known as copper beryllium, beryllium bronze and spring copper, is a copper alloy with 0.5—3% beryllium and sometimes with other alloying elements. Beryllium copper combines high strength with non-magnetic and non-sparking qualities. It has excellent metalworking, forming...

) are not susceptible to hydrogen embrittlement along with few other metals. For example of a severe embrittlement measured by Jewett, the elongation at failure of 17-4PH precipitation hardened stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

 was measured to drop from 17% to only 1.7% when smooth specimens were exposed to high-pressure hydrogen.

Hydrogen embrittlement can occur during various manufacturing operations or operational use - anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to this include cathodic protection
Cathodic protection
Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the...

, phosphating, pickling
Pickling (metal)
Tarnision is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called Tarnision liquor, which contains strong acids, is used to remove the surface impurities...

, and electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...

. A special case is arc welding
Arc welding
Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes...

, in which the hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this, special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 in sulfide stress cracking, or SSC, a process of importance for the oil and gas industries).


If the metal has not yet started to crack, the condition can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out, possibly at elevated temperatures. Susceptible alloys, after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat treatment to remove absorbed hydrogen. There is a 4-hour time limit for baking out entrapped hydrogen after acid treating the parts. This is the time between the end of acid exposure and the beginning of the heating cycle in the baking furnace. This per SAE AMS 2759/9 Section which calls out the correct procedure for eliminating entrapped hydrogen.

In the case of welding, often pre- and post-heating the metal is applied to allow the hydrogen to diffuse out before it can cause any damage. This is specifically done with high-strength steels and low alloy steel
Low alloy steel
Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels...

 such as the chrome/molybdenum/vanadium alloys. Due to the time needed to re-combine hydrogen atoms into the harmful hydrogen molecules, hydrogen cracking due to welding can occur over 24 hours after the welding operation is completed.

Products such as ferrosilicates can be used to treat surfaces normally subject to hydrogen embrittlement in order to prevent it from taking place.

Related phenomena

If steel is exposed to hydrogen at high temperatures, hydrogen will diffuse into the alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

 and combine with carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 to form tiny pockets of methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 at internal surfaces like grain boundaries and voids. This methane does not diffuse out of the metal, and collects in the voids at high pressure and initiates cracks in the steel. This selective leaching
Selective leaching
Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by...

 process is known as hydrogen attack
Hydrogen damage
Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen.Hydrogen is present practically everywhere, in the atmosphere, several kilometres above the earth and inside the earth. Engineering materials are exposed to hydrogen and they...

, or high temperature hydrogen attack and leads to decarburization of the steel and loss of strength and ductility.

Copper alloys which contain oxygen can be embrittled if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts with inclusions of Cu2O, forming H2O (water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

), which then forms pressurized bubbles at the grain boundaries. This process can cause the grains to literally be forced away from each other, and is known as steam embrittlement (because steam is produced, not because exposure to steam causes the problem).


There are two ASTM standards for testing embrittlement due to hydrogen gas. The standard ASTM F1459-06 Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE) Test uses a diaphragm loaded with a differential pressure. The test ASTM G142-98(2004) Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both uses a cylindrical tensile specimen tested into an enclosure pressurized with hydrogen or helium.

Another ASTM standard exists for quantitatively testing for the Hydrogen Embrittlement threshold stress for the onset of Hydrogen-Induced Cracking due to platings and coatings from Internal Hydrogen Embrittlement (IHE) and Environmental Hydrogen Embrittlement (EHE) http://www.astm.org/cgi-bin/SoftCart.exe/STORE/filtrexx40.cgi?U+mystore+yvst4574+-L+ASTMF1624:06+/usr6/htdocs/astm.org/DATABASE.CART/REDLINE_PAGES/F1624.htm - F1624-06 Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique. References: ASTM STP 543,"Hydrogen Embrittlement Testing" http://www.astm.org/cgi-bin/SoftCart.exe/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP543.htm?L+mystore+hsjb1846+1193986997 and ASTM STP 962,"Hydrogen Embrittlement: Prevention and Control." http://www.astm.org/cgi-bin/SoftCart.exe/BOOKSTORE/PUBS/652.htm?E+mystore
  • NACE TM0284-2003 (NACE International
    NACE International
    NACE International is a professional organization for the corrosion control industry established in 1943. As of 2010, there are approximately 22,312 NACE members in over 100 countries. NACE International is involved in every industry and area of corrosion prevention and control, from chemical...

    ) Resistance to Hydrogen-Induced Cracking
  • ISO 11114-4:2005 (ISO
    International Organization for Standardization
    The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

    )Test methods for selecting metallic materials resistant to hydrogen embrittlement http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=41281&ICS1=23&ICS2=20&ICS3=30.
  • ASTM F1940-07a- Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners
  • ASTM F519-06e2-Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments

See also

  • Hydrogen analyzer
    Hydrogen analyzer
    A Hydrogen Analyzer is a device used to measure the hydrogen concentration in steels and alloys. It also has industrial applications for Corrosion Monitoring.-See also:* Hydrogen embrittlement* Hydrogen sensor* Hydrogen leak testing...

  • Hydrogen damage
    Hydrogen damage
    Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen.Hydrogen is present practically everywhere, in the atmosphere, several kilometres above the earth and inside the earth. Engineering materials are exposed to hydrogen and they...

  • Hydrogen piping
    Hydrogen piping
    Hydrogen piping, in industrial settings, is a system of pipes used to move hydrogen. Due to issues with hydrogen embrittlement, and corrosion, materials for hydrogen pipes must be carefully selected...

  • Hydrogen safety
    Hydrogen safety
    Hydrogen safety covers the safe use and handling of hydrogen. Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be accounted for to ensure safe operation...

  • Low hydrogen annealing
    Low hydrogen annealing
    Low hydrogen annealing is a heat treatment in metallurgy for the reduction or elimination of hydrogen in a material to prevent hydrogen embrittlement.-Process description:...

  • Nascent hydrogen
    Nascent hydrogen
    Atomic hydrogen is the species denoted by H , contrasted with dihydrogen, the usual 'hydrogen' commonly involved in chemical reactions. It is claimed to exist transiently but long enough to effect chemical reactions...

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.