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Metals can be exposed to hydrogen from two types of sources: gaseous hydrogen and hydrogen chemically generated at the metal surface. Metals used in [[pressure vessel]]s can be exposed to gaseous hydrogen. [[Electrochemistry|Electrochemical]] sources of hydrogen include exposure to [[acid]]s (as may occur during [[Pickling (metal)|pickling]], [[Chemical milling|etching]], or cleaning), [[corrosion]] (typically due to [[Corrosion|aqueous corrosion]] or [[cathodic protection]]), and [[electroplating]].<ref name=":0" /><ref name="nasa" /> Hydrogen can be introduced into the metal during manufacturing by the presence of moisture during [[welding]] or while the metal is [[Melting|molten]]. The most common causes of failure in practice are poorly-controlled electroplating or damp [[welding rods]]. |
Metals can be exposed to hydrogen from two types of sources: gaseous hydrogen and hydrogen chemically generated at the metal surface. Metals used in [[pressure vessel]]s can be exposed to gaseous hydrogen. [[Electrochemistry|Electrochemical]] sources of hydrogen include exposure to [[acid]]s (as may occur during [[Pickling (metal)|pickling]], [[Chemical milling|etching]], or cleaning), [[corrosion]] (typically due to [[Corrosion|aqueous corrosion]] or [[cathodic protection]]), and [[electroplating]].<ref name=":0" /><ref name="nasa" /> Hydrogen can be introduced into the metal during manufacturing by the presence of moisture during [[welding]] or while the metal is [[Melting|molten]]. The most common causes of failure in practice are poorly-controlled electroplating or damp [[welding rods]]. |
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Hydrogen embrittlement as a term can be used to refer specifically to the embrittlement that occurs in [[steel]] |
Hydrogen embrittlement as a term can be used to refer specifically to the embrittlement that occurs in [[steel]]s and similar metals at relatively low hydrogen [[concentration]]s, or it can be used to encompass all embrittling effects that hydrogen has on metals. These broader embrittling effects include [[hydride]] formation, which occurs in [[titanium]] and [[vanadium]] but not in steels, and hydrogen-induced [[blistering]], which only occurs at high hydrogen concentrations and does not require the presence of stress.<ref name=":1" /> However, hydrogen embrittlement is almost always distinguished from [[high temperature hydrogen attack]] (HTHA), which occurs in steels at temperatures above 400 °C and involves the formation of [[methane]] pockets.<ref name="twi-htha">{{cite web |last1=TWI – The Welding Institute |title=What is high temperature hydrogen attack (HTHA) / hot hydrogen attack? |url=https://www.twi-global.com/technical-knowledge/faqs/what-is-high-temperature-hydrogen-attack-htha-hot-hydrogen-attack |access-date=16 December 2020 |publisher=TWI - The Welding Institute}}</ref> The mechanism by which hydrogen causes embrittlement in steels is not fully understood and continues to be debated.<ref name=":0" /><ref name="Barnoush">{{cite web |last1=Barnoush |first1=Afrooz |title=Hydrogen embrittlement revisited by in situ electrochemical nanoindentations |url=http://www.uni-saarland.de/fak8/wwm/research/phd_barnoush/hydrogen.pdf |archive-url=https://web.archive.org/web/20110518222127/http://www.uni-saarland.de/fak8/wwm/research/phd_barnoush/hydrogen.pdf |access-date=18 December 2020 |archive-date=2011-05-18}}</ref><ref name="robertson15" /> |
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== Mechanisms == |
== Mechanisms == |
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Hydrogen embrittlement is a complex process involving a number of distinct contributing micro-mechanisms, not all of which need to be present. The mechanisms include the formation of brittle [[hydride]]s, the creation of voids that can lead to high-pressure bubbles, enhanced [[Delamination|decohesion]] at internal surfaces and localised plasticity at crack tips that assist in the propagation of cracks.<ref name="robertson15">{{cite journal |title=Hydrogen Embrittlement Understood |first1=Ian M. |last1=Robertson |first2=P. |last2=Sofronis |first3=A. |last3=Nagao |first4=M. L. |last4=Martin |first5=S. |last5=Wang |first6=D. W. |last6=Gross |first7=K. E. |last7=Nygren |journal=Metallurgical and Materials Transactions A |volume=46A |date=2015 |issue=6 |pages=2323–2341|doi=10.1007/s11661-015-2836-1 |bibcode=2015MMTA...46.2323R |s2cid=136682331 |doi-access=free }}</ref> There is a great variety of mechanisms that have been proposed<ref name="robertson15"/> and investigated as to the cause of brittleness once [[Diffusion|diffusible]] hydrogen has been [[Solubility|dissolved]] into the metal.<ref name="HarryB">{{cite web |last1=Bhadhesia |first1=Harry |title=Prevention of Hydrogen Embrittlement in Steels |url=https://www.phase-trans.msm.cam.ac.uk/2016/preventing_hydrogen.pdf |access-date=17 December 2020 |website=Phase Transformations & Complex Properties Research Group, Cambridge University}}</ref> In recent years, it has become widely accepted that HE is a complex, material and environmental dependent process, so that no single mechanism applies exclusively.<ref name="HELP" /> |
Hydrogen embrittlement is a complex process involving a number of distinct contributing micro-mechanisms, not all of which need to be present. The mechanisms include the formation of brittle [[hydride]]s, the creation of voids that can lead to high-pressure bubbles, enhanced [[Delamination|decohesion]] at internal surfaces and localised plasticity at crack tips that assist in the propagation of cracks.<ref name="robertson15">{{cite journal |title=Hydrogen Embrittlement Understood |first1=Ian M. |last1=Robertson |first2=P. |last2=Sofronis |first3=A. |last3=Nagao |first4=M. L. |last4=Martin |first5=S. |last5=Wang |first6=D. W. |last6=Gross |first7=K. E. |last7=Nygren |journal=Metallurgical and Materials Transactions A |volume=46A |date=2015 |issue=6 |pages=2323–2341|doi=10.1007/s11661-015-2836-1 |bibcode=2015MMTA...46.2323R |s2cid=136682331 |doi-access=free }}</ref> There is a great variety of mechanisms that have been proposed<ref name="robertson15"/> and investigated as to the cause of brittleness once [[Diffusion|diffusible]] hydrogen has been [[Solubility|dissolved]] into the metal.<ref name="HarryB">{{cite web |last1=Bhadhesia |first1=Harry |title=Prevention of Hydrogen Embrittlement in Steels |url=https://www.phase-trans.msm.cam.ac.uk/2016/preventing_hydrogen.pdf |access-date=17 December 2020 |website=Phase Transformations & Complex Properties Research Group, Cambridge University}}</ref> In recent years, it has become widely accepted that HE is a complex, material and environmental dependent process, so that no single mechanism applies exclusively.<ref name="HELP" /> |
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* Internal pressure: At high hydrogen concentrations, absorbed hydrogen species recombine in voids to form hydrogen molecules (H<sub>2</sub>), creating pressure from within the metal. This pressure can increase to levels where cracks form, commonly designated hydrogen-induced cracking (HIC), as well as [[blister]] |
* Internal pressure: At high hydrogen concentrations, absorbed hydrogen species recombine in voids to form hydrogen molecules (H<sub>2</sub>), creating pressure from within the metal. This pressure can increase to levels where cracks form, commonly designated hydrogen-induced cracking (HIC), as well as [[blister]]s forming on the specimen surface, designated hydrogen-induced blistering. These effects can reduce [[ductility]] and [[Ultimate tensile strength|tensile strength]].<ref name="vergani">{{cite journal |last1=Vergani |first1=Laura |last2=Colombo |first2=Chiara |display-authors=etal |date=2014 |title=Hydrogen effect on fatigue behavior of a quenched and tempered steel |journal=Procedia Engineering |volume=74 |issue=XVII International Colloquium on Mechanical Fatigue of Metals (ICMFM17) |pages=468–71 |doi=10.1016/j.proeng.2014.06.299 |doi-access=free }}</ref> |
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* Hydrogen enhanced localised [[Plasticity (physics)|plasticity]] (HELP): Hydrogen increases the [[nucleation]] and [[Dislocation creep|movement]] of [[dislocation]] |
* Hydrogen enhanced localised [[Plasticity (physics)|plasticity]] (HELP): Hydrogen increases the [[nucleation]] and [[Dislocation creep|movement]] of [[dislocation]]s at a crack tip. HELP results in crack propagation by localised [[ductile failure]] at the crack tip with less [[Deformation (engineering)|deformation]] occurring in the surrounding material, which gives a [[Brittleness|brittle]] appearance to the [[fracture]].<ref name="HELP">{{cite journal |last1=Haiyang Yu |title=Discrete dislocation plasticity HELPs understand hydrogen effects in bcc materials |journal=Journal of the Mechanics and Physics of Solids |date=February 2009 |volume=123 |pages=41–60 |doi=10.1016/j.jmps.2018.08.020 |s2cid=56081700 |url=https://www.sciencedirect.com/science/article/pii/S002250961830574X#bib0005 |access-date=18 December 2020|doi-access=free }}</ref><ref name="Barnoush" /> |
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* Hydrogen decreased dislocation emission: [[Molecular dynamics]] simulations reveal a [[ductile-to-brittle transition]] caused by the suppression of dislocation emission at the crack tip by dissolved hydrogen. This prevents the crack tip rounding-off, so the sharp crack then leads to brittle-cleavage failure.<ref name="song">{{cite journal |last1=Song |first1=Jun |title=Atomic mechanism and prediction of hydrogen embrittlement in iro |journal=Nature Materials |date=11 November 2012 |volume=12 |issue=2 |pages=145–151 |doi=10.1038/nmat3479 |pmid=23142843 |url=https://www.nature.com/articles/nmat3479 |access-date=22 December 2020}}</ref> |
* Hydrogen decreased dislocation emission: [[Molecular dynamics]] simulations reveal a [[ductile-to-brittle transition]] caused by the suppression of dislocation emission at the crack tip by dissolved hydrogen. This prevents the crack tip rounding-off, so the sharp crack then leads to brittle-cleavage failure.<ref name="song">{{cite journal |last1=Song |first1=Jun |title=Atomic mechanism and prediction of hydrogen embrittlement in iro |journal=Nature Materials |date=11 November 2012 |volume=12 |issue=2 |pages=145–151 |doi=10.1038/nmat3479 |pmid=23142843 |url=https://www.nature.com/articles/nmat3479 |access-date=22 December 2020}}</ref> |
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* Hydrogen enhanced decohesion (HEDE): Interstitial hydrogen lowers the stress required for metal atoms to fracture apart. HEDE can only occur when the local concentration of hydrogen is high, such as due to the increased hydrogen [[solubility]] in the [[tensile stress]] field at a crack tip, at stress concentrators, or in the tension field of [[Edge dislocation|edge dislocations]].<ref name="Barnoush" /> |
* Hydrogen enhanced decohesion (HEDE): Interstitial hydrogen lowers the stress required for metal atoms to fracture apart. HEDE can only occur when the local concentration of hydrogen is high, such as due to the increased hydrogen [[solubility]] in the [[tensile stress]] field at a crack tip, at stress concentrators, or in the tension field of [[Edge dislocation|edge dislocations]].<ref name="Barnoush" /> |
Mechanical failure modes |
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Hydrogen embrittlement (HE), also known as hydrogen-assisted crackingorhydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate through solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.[1][2][3][4]
The essential facts about the nature of hydrogen embrittlement have been known since the 19th century.[5][6] It is absorbed, diffusible, atomic hydrogen that causes embrittlement.[7] Hydrogen embrittlement is maximised at around room temperature in steels; most metals are relatively immune to hydrogen embrittlement at temperatures above 150 °C.[8] Hydrogen embrittlement requires the presence of a mechanical stress, whether that stress is applied or residual.[2][9][10] Hydrogen embrittlement increases at lower strain rates.[1][2][11] In general, higher-strength materials are more susceptible to hydrogen embrittlement.
Metals can be exposed to hydrogen from two types of sources: gaseous hydrogen and hydrogen chemically generated at the metal surface. Metals used in pressure vessels can be exposed to gaseous hydrogen. Electrochemical sources of hydrogen include exposure to acids (as may occur during pickling, etching, or cleaning), corrosion (typically due to aqueous corrosionorcathodic protection), and electroplating.[1][2] Hydrogen can be introduced into the metal during manufacturing by the presence of moisture during welding or while the metal is molten. The most common causes of failure in practice are poorly-controlled electroplating or damp welding rods.
Hydrogen embrittlement as a term can be used to refer specifically to the embrittlement that occurs in steels and similar metals at relatively low hydrogen concentrations, or it can be used to encompass all embrittling effects that hydrogen has on metals. These broader embrittling effects include hydride formation, which occurs in titanium and vanadium but not in steels, and hydrogen-induced blistering, which only occurs at high hydrogen concentrations and does not require the presence of stress.[11] However, hydrogen embrittlement is almost always distinguished from high temperature hydrogen attack (HTHA), which occurs in steels at temperatures above 400 °C and involves the formation of methane pockets.[12] The mechanism by which hydrogen causes embrittlement in steels is not fully understood and continues to be debated.[1][13][14]
Hydrogen embrittlement is a complex process involving a number of distinct contributing micro-mechanisms, not all of which need to be present. The mechanisms include the formation of brittle hydrides, the creation of voids that can lead to high-pressure bubbles, enhanced decohesion at internal surfaces and localised plasticity at crack tips that assist in the propagation of cracks.[14] There is a great variety of mechanisms that have been proposed[14] and investigated as to the cause of brittleness once diffusible hydrogen has been dissolved into the metal.[6] In recent years, it has become widely accepted that HE is a complex, material and environmental dependent process, so that no single mechanism applies exclusively.[15]
Hydrogen embrittles a variety of metals including steel,[19][20] aluminium (at high temperatures only[21]), and titanium.[22] Austempered iron is also susceptible, though austempered steel (and possibly other austempered metals) displays increased resistance to hydrogen embrittlement.[23] NASA has reviewed which metals are susceptible to embrittlement and which only prone to hot hydrogen attack: nickel alloys, austenitic stainless steels, aluminium and alloys, copper (including alloys, e.g. beryllium copper).[2] Sandia has also produced a comprehensive guide.[24]
Steel with an ultimate tensile strength of less than 1000 MPa (~145,000 psi) or hardness of less than 32 HRC is not generally considered susceptible to hydrogen embrittlement. As an example of severe hydrogen embrittlement, the elongation at failure of 17-4PH precipitation hardened stainless steel was measured to drop from 17% to only 1.7% when smooth specimens were exposed to high-pressure hydrogen.[citation needed]
As the strength of steels increases, the fracture toughness decreases, so the likelihood that hydrogen embrittlement will lead to fracture increases. In high-strength steels, anything above a hardness of HRC 32 may be susceptible to early hydrogen cracking after plating processes that introduce hydrogen. They may also experience long-term failures anytime from weeks to decades after being placed in service due to accumulation of hydrogen over time from cathodic protection and other sources. Numerous failures have been reported in the hardness range from HRC 32-36 and more above; therefore, parts in this range should be checked during quality control to ensure they are not susceptible.
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), 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).
Alloysofvanadium, nickel, and titanium have high hydrogen solubility, and can therefore absorb significant amounts of hydrogen. This can lead to hydride formation, resulting in irregular volume expansion and reduced ductility. This is a particular issue when looking for non-palladium-based alloys for use in hydrogen separation membranes.[18]
While most failures in practice have been through fast failure, there is experimental evidence that hydrogen also affects the fatigue properties of steels. This is entirely expected given the nature of the embrittlement mechanisms proposed for fast fracture.[26][16] In general hydrogen embrittlement has a strong effect on high-stress, low-cycle fatigue and very little effect on high-cycle fatigue.[2][24]
During manufacture, hydrogen can be dissolved into the component by processes such as phosphating, pickling, electroplating, casting, carbonizing, surface cleaning, electrochemical machining, welding, hot roll forming, and heat treatments.
During service use, hydrogen can be dissolved into the metal from wet corrosion or through misapplication of protection measures such as cathodic protection.[2] In one case of failure during construction of the San Francisco–Oakland Bay Bridge galvanized (i.e. zinc-plated) rods were left wet for 5 years before being tensioned. The reaction of the zinc with water introduced hydrogen into the steel.[27][28][29]
A common case of embrittlement during manufacture is poor arc welding practice, in which hydrogen is released from moisture, such as in the coating of welding electrodes or from damp welding rods.[22][30] To avoid atomic hydrogen formation in the high temperature plasma of the arc, welding rods have to be perfectly dried in an oven at the appropriate temperature and time before to be used. Another way to minimize the formation of hydrogen is to use special low-hydrogen electrodes for welding high-strength steels.
Apart from arc welding, the most common problems are from chemical or electrochemical processes which, by reduction of hydrogen ions or water, generate hydrogen atoms at the surface, which rapidly dissolve in the metal. One of these chemical reactions involves hydrogen sulfide (H
2S) in sulfide stress cracking (SSC), a significant problem for the oil and gas industries.[31]
After a manufacturing process or treatment which may cause hydrogen ingress, the component should be baked to remove or immobilize the hydrogen.[28]
Hydrogen embrittlement can be prevented through several methods, all of which are centered on minimizing contact between the metal and hydrogen, particularly during fabrication and the electrolysis of water. Embrittling procedures such as acid pickling should be avoided, as should increased contact with elements such as sulfur and phosphate. The use of proper electroplating solution and procedures can also help to prevent hydrogen embrittlement.
If the metal has not yet started to crack, hydrogen embrittlement can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out through heat treatment. This de-embrittlement process, known as low hydrogen annealing or "baking", is used to overcome the weaknesses of methods such as electroplating which introduce hydrogen to the metal, but is not always entirely effective because a sufficient time and temperature must be reached.[7] Tests such as ASTM F1624 can be used to rapidly identify the minimum baking time (by testing using design of experiments, a relatively low number of samples can be used to pinpoint this value). Then the same test can be used as a quality control check to evaluate if baking was sufficient on a per-batch basis.
In the case of welding, often pre-heating 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 steels such as the chromium/molybdenum/vanadium alloys. Due to the time needed to re-combine hydrogen atoms into the hydrogen molecules, hydrogen cracking due to welding can occur over 24 hours after the welding operation is completed.
Another way of preventing this problem is through materials selection. This will build an inherent resistance to this process and reduce the need of post processing or constant monitoring for failure. Certain metals or alloys are highly susceptible to this issue, so choosing a material that is minimally affected while retaining the desired properties would also provide an optimal solution. Much research has been done to catalog the compatibility of certain metals with hydrogen.[24] Tests such as ASTM F1624 can also be used to rank alloys and coatings during materials selection to ensure (for instance) that the threshold of cracking is below the threshold for hydrogen-assisted stress corrosion cracking. Similar tests can also be used during quality control to more effectively qualify materials being produced in a rapid and comparable manner.
Most analytical methods for hydrogen embrittlement involve evaluating the effects of (1) internal hydrogen from production and/or (2) external sources of hydrogen such as cathodic protection. For steels, it is important to test specimens in the lab that are at least as hard (or harder) than the final parts will be. Ideally, specimens should be made of the final material or the nearest possible representative, as fabrication can have a profound impact on resistance to hydrogen-assisted cracking.
There are numerous ASTM standards for testing for hydrogen embrittlement:
There are many other related standards for hydrogen embrittlement:
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