Hydrogen embrittlement (HE) also known as hydrogen assisted cracking or hydrogen-induced cracking, describes the embrittlement of a metal by diffusible hydrogen. The essential facts about the nature of the hydrogen embrittlement of steels have now been known for 140 years. It is diffusible atomic hydrogen that is harmful to the toughness of iron and steel. It is a low temperature effect: most metals are relatively immune to hydrogen embrittlement above approximately 150°C.
In steels, diffusible hydrogen ions come from water that is typically introduced by a wet electrochemical process such as electroplating. It must be distinguished from the entirely different process high temperature hydrogen attack (HTHA) which is where steels operating at high temperatures above 400°C are attacked by hydrogen gas.
For hydrogen embrittlement to occur, a combination of three conditions are required:
- the presence and diffusion of hydrogen atoms or ions
- a susceptible material
Diffusible hydrogen can be introduced during manufacture from operations such as forming, coating, plating or cleaning. The commonest causes of failure in practice are poorly-controlled electroplating or bad welding practice with damp welding rods. Both of these introduce hydrogen ions which dissolve in the metal. Hydrogen may also be introduced over time (external embrittlement) through environmental exposure (soils and chemicals, including water), corrosion processes (especially galvanic corrosion) including corrosion of a coating and cathodic protection. Hydrogen atoms are very small and diffuse interstitially in steels. Almost uniquely amongst solute atoms they are mobile at room temperature and will diffuse away from the site of their introduction within minutes.
The hydrogen embrittlement phenomenon was first described by Johnson in 1875. The following conclusions can justifiably be reached from this 1875 paper:
- it is hydrogen that embrittles steel, not acid;
- that the hydrogen is nascent or diffusible, not molecular;
- it is diffusible hydrogen that embrittles, so the phenomenon is reversible;
- the effusion of diffusible hydrogen from the steel leads to frothing (bubbles);
- that stronger steel is more susceptible to embrittlement than softer versions.
It follows, therefore, that the harmful influence of diffusible hydrogen can be mitigated by preventing its entry into steel or by rendering it immobile once it penetrates the material.
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. There is a great variety of mechanisms that have been proposed: and investigated which cause brittleness once diffusible hydrogen has been dissolved into the metal. As the hydrogen is diffusible and mobile, brittleness can only occur when (a) it is captured in microscopic traps, and (b) these traps cause brittleness. In recent years, it has become widely accepted that HE is a complex, material and environmental dependent process so that no mechanism applies exclusively.
- Internal pressure: Adsorbed hydrogen species recombine to form hydrogen molecules, creating pressure from within the metal. This pressure can increase to levels where the metal has reduced ductility, toughness, and tensile strength, up to the point where it cracks open (hydrogen-induced cracking, or HIC).
- Hydrogen enhanced localised plasticity (HELP), where the generation and movement of dislocations is enhanced and results in localised deformation such as at the tip of a crack increasing the propagation of the crack with less deformation in surrounding material giving a brittle appearance to the fracture.
- 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.
- Hydrogen enhanced decohesion (HEDE), where the increased solubility of hydrogen in a tensile strength field, for instance on the tip of a crack or in areas with internal tensile strength or in the tension field of edge dislocations, reduces the yield stress locally.
- Metal hydride formation: The formation of brittle hydrides with the parent material allows cracks to propagate in a brittle fashion. This is particularly a problem with Vanadium alloys but most structural alloys do not easily form hydrides.
- Phase transformations: these occur for some materials when hydrogen is present and the new phase may be less ductile.
Hydrogen embrittles a variety of substances including steel, aluminium (at high temperatures only), and titanium. Austempered iron is also susceptible, though austempered steel (and possibly other austempered metals) display increased resistance to hydrogen embrittlement.
NASA reviewed which metals are susceptible to embrittlement and which only liable to hot hydrogen attack: nickel alloys, austenitic stainless steels, aluminium and alloys, copper (including alloys, e.g. beryllium copper). Sandia has also produced a comprehensive guide.
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.
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).
Vanadium, nickel, and titanium
A large number of alloys of vanadium, nickel, and titanium absorb significant amounts of hydrogen. This can lead to large volume expansion and damage to the crystal structure leading to the alloys becoming very brittle. This is a particular issue when looking for non-palladium based alloys for use in hydrogen separation membranes.
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. In general hydrogen embrittlement has a strong effect on high-stress, low-cycle fatigue and very little effect on high-cycle fatigue.
Sources of hydrogen
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. 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.
A common case of embrittlement during manufacture is poor arc welding practice, in which the hydrogen is released from moisture, such as in the coating of welding electrodes or from damp welding rods. To minimize this, special low-hydrogen electrodes are used for welding high-strength steels.
Apart from arc welding, the most common problems are from the chemical or electrochemical processes which generate hydrogen ions at the surface which rapidly dissolve in the metal. One of these chemical reactions involves hydrogen sulfide in sulfide stress cracking (SSC), a significant problem for the oil and gas industries.
After a manufacturing process or treatment which may cause hydrogen ingress, the component should be baked to remove or immobilise the hydrogen.
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. 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 chrome/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. 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:
- ASTM B577 is the Standard Test Methods for Detection of Cuprous Oxide (Hydrogen Embrittlement Susceptibility) in Copper. The test focuses on hydrogen embrittlement of copper alloys, including a metallographic evaluation (method A), testing in a hydrogen charged chamber followed by metallography (method B), and method C is the same as B but includes a bend test.
- ASTM B839 is the Standard Test Method for Residual Embrittlement in Metallic Coated, Externally Threaded Articles, Fasteners, and Rod-Inclined Wedge Method.
- ASTM F519 is the Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments. There are 7 different samples designs and the two most commons tests are (1) the rapid test, the Rising Step Load (RSL) test per ASTM F1624 and (2) the sustained load test, which takes 200 hours. The sustained load test is still included in many legacy standards, but the RSL method is increasingly being adopted due to speed, repeatability, and the quantitative nature of the test. The RSL method provides an accurate ranking of the effect of hydrogen from both internal and external sources.
- ASTM F1459 is the Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE) Test. The test uses a diaphragm loaded with a differential pressure.
- ASTM G142 is the Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both. The test uses a cylindrical tensile specimen tested into an enclosure pressurized with hydrogen or helium.
- ASTM F1624 is the Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique. The test uses the incremental step loading (ISL) or rising step load (RSL) method 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). F1624 provides a rapid, quantitative measure of the effects of hydrogen both from internal sources and external sources (which is accomplished by applying a selected voltage in an electrochemical cell). The F1624 test is performed by comparing a standard fast-fracture tensile strength to the fracture strength from a rising step load test where the load is held for hour(s) at each step. In many cases it can be performed in 30 hours or less.
- ASTM F1940 is the Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners. While the title now explicitly includes the word fasteners, F1940 was not originally intended for these purposes. F1940 is based on the F1624 method and is similar to F519 but with different root radius and stress concentration factors. When specimens exhibit a threshold cracking of 75% of the net fracture strength, the plating bath is considered to be 'non-embrittling'.
There are many other related standards for hydrogen embrittlement:
- NACE TM0284-2003 (NACE International) Resistance to Hydrogen-Induced Cracking
- ISO 11114-4:2005 (ISO)Test methods for selecting metallic materials resistant to hydrogen embrittlement.
- Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
Notable failures from hydrogen embrittlement
- In 2013, six months prior to opening, the East Span of the Oakland Bay Bridge failed during testing. Catastrophic failures occurred in shear bolts in the span, after only two weeks of service, with the failure attributed to embrittlement, possibly from the environment.
- In the City of London, 122 Leadenhall Street, generally known as 'the Cheesegrater', suffered from hydrogen embrittlement in steel bolts, with three bolts failing in 2014 and 2015. Most of the 3,000 bolts were replaced at a cost of £6m.
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- Barnoush, Afrooz. "Hydrogen embrittlement revisited by in situ electrochemical nanoindentations" (PDF). Archived from the original (PDF) on 2011-05-18. Retrieved 18 December 2020.
- Federal Engineering and Design Support. "Embrittlement" (PDF). Fastenal. Fastenal Company Engineering Department. Retrieved 9 May 2015.
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- Robertson, Ian M.; Sofronis, P.; Nagao, A.; Martin, M. L.; Wang, S.; Gross, D. W.; Nygren, K. E. (2015). "Hydrogen Embrittlement Understood". Metallurgical and Materials Transactions A. 46A (6): 2323–2341. Bibcode:2015MMTA...46.2323R. doi:10.1007/s11661-015-2836-1. S2CID 136682331.
- Fernandez-Sousa, Rebeca (2020). "Analysis of the influence of microstructural traps on hydrogen assisted fatigue". Acta Materialia. 199: 253. arXiv:2008.05452. Bibcode:2020AcMat.199..253F. doi:10.1016/j.actamat.2020.08.030. S2CID 221103811.
- Haiyang Yu (February 2009). "Discrete dislocation plasticity HELPs understand hydrogen effects in bcc materials". Journal of the Mechanics and Physics of Solids. 123: 41–60. doi:10.1016/j.jmps.2018.08.020. S2CID 56081700. Retrieved 18 December 2020.
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- Dolan, Michael D.; Kochanek, Mark A.; Munnings, Christopher N.; McLennan, Keith G.; Viano, David M. (February 2015). "Hydride phase equilibria in V–Ti–Ni alloy membranes". Journal of Alloys and Compounds. 622: 276–281. doi:10.1016/j.jallcom.2014.10.081.
- Djukic, M.B.; et al. (2014). "Hydrogen embrittlement of low carbon structural steel". Procedia Materials Science. 3 (20th European Conference on Fracture): 1167–1172. doi:10.1016/j.mspro.2014.06.190.
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- Tartaglia, John; Lazzari, Kristen; et al. (March 2008). "A Comparison of Mechanical Properties and Hydrogen Embrittlement Resistance of Austempered vs Quenched and Tempered 4340 Steel". Metallurgical and Materials Transactions A. 39 (3): 559–76. Bibcode:2008MMTA...39..559T. doi:10.1007/s11661-007-9451-8. ISSN 1073-5623. S2CID 136866718.
- NASA (2016). "Hydrogen Embrittlement" (PDF). Retrieved 18 December 2020. Cite journal requires
- Marchi, C. San (2012). "Technical Reference for Hydrogen Compatibility of Materials" (PDF).
- Morlet, J. G. (1958). "A new concept in hydrogen embrittlement in steels". The Journal of the Iron and Steel Institute. 189: 37.
- Francis, Rob. "A Failure Analysis of Hydrogen Embrittlement in Bridge Fasteners". Corrosionpedia. Corrosionpedia. Retrieved 18 December 2020.
- M. TERESA FERRAZ, MANUELA OLIVEIRA. "STEEL FASTENERS FAILURE BY HYDROGEN EMBRITTLEMENT" (PDF). Ciência e Tecnologia dos Materiais, Vol. 20, n.º 1/2, 2008 M. Teresa Ferraz and Manuela Oliveira Hydrogen embrittlement 128 Ciência e Tecnologia dos Materiais, Vol. 20, n.º 1/2, 2008. Retrieved 18 December 2020.
- Yun Chung (2 December 2014). "Validity of Caltrans' Environmental Hydrogen Embrittlement Test on Grade BD Anchor Rods in the SAS Span" (PDF).
- Weman, Klas (2011). Welding Processes Handbook. Elsevier. p. 115. ISBN 978-0-85709-518-3.
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- "ASTM F1459 - 06(2012): Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE)". Astm.org. Retrieved 2015-02-24.
- "ASTM G142 - 98(2011) Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both". Astm.org. Retrieved 2015-02-24.
- ASTM STP 543, "Hydrogen Embrittlement Testing"
- Raymond L (1974). Hydrogen Embrittlement Testing. ASTM International. ISBN 978-0-8031-0373-3.
- "ASTM F1940 - 07a(2014) Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners". Astm.org. Retrieved 2015-02-24.
- "ASTM F519 - 17a Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments". www.astm.org. Retrieved 21 April 2018.
- Mair, Lucy (14 January 2015). "British Land to replace 'a number of bolts' on Leadenhall Building". constructionnews.co.uk. Retrieved 21 April 2018.
- "Cheesegrater bolts to cost Severfield £6m after Leadenhall building loses five". cityam. Retrieved 22 December 2020.