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1 Concept of hydrogen embrittlement: Hydrogen embrittlement is a phenomenon in which metal materials lose their ductility due to the absorption of hydrogen atoms. Hydrogen atoms are very small and can penetrate solid metals. Once absorbed by metals, hydrogen reduces the stress required for crack initiation and propagation in those metals, thereby leading to embrittlement. Hydrogen embrittlement occurs most significantly in steel, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminum, and stainless steel are less prone to hydrogen embrittlement. Since the 19th century, the basic facts regarding the nature of hydrogen embrittlement have been known. Hydrogen embrittlement in steel reaches its maximum near room temperature, while most metals are relatively unaffected by hydrogen embrittlement at temperatures above 150°C. Hydrogen embrittlement requires the presence of atomic hydrogen along with mechanical stress to induce crack propagation, although this stress may be applied or remain present. Hydrogen embrittlement increases at lower strain rates. Generally speaking, materials with higher strength are more prone to hydrogen embrittlement. Metals can be exposed to hydrogen from two types of sources: gaseous hydrogen and hydrogen chemically produced on the metal surface. Gaseous hydrogen is molecular hydrogen and does not cause embrittlement, although it does cause high-temperature hydrogen attack (HTHA). Atomic hydrogen from chemical erosion causes embrittlement, as it dissolves rapidly into the metal at room temperature. Gaseous hydrogen exists in pressure vessels and pipelines. The electrochemical sources of hydrogen include acids (which may be encountered during pickling, etching, or cleaning), corrosion (usually due to water corrosion or cathodic protection), and electroplating. During the welding process or when the metal is melted, hydrogen can be introduced into the metal during manufacturing due to the presence of moisture. The most common causes of failure in practice are poor electroplating control or wet welding electrodes. As a term, hydrogen embrittlement can be used specifically to refer to the embrittlement that occurs in steel and similar metals at relatively low hydrogen concentrations, or it can be used to encompass all embrittlement effects of hydrogen on metals. These broader embrittlement effects include hydride formation, which occurs in titanium and vanadium but not in steel, as well as hydrogen-induced bubbles, which occur only at high hydrogen concentrations without the need for stress. However, hydrogen embrittlement is almost always distinguished from high-temperature hydrogen corrosion; it occurs in steels at temperatures above 400°C and involves the formation of methane pockets. 2 Mechanism of hydrogen embrittlement Hydrogen embrittlement is a complex process involving many different micro-mechanisms, and not all of these micro-mechanisms need to be present. These mechanisms include the formation of brittle hydrides, which can lead to the creation of pores from high-pressure bubbles, enhanced depolymerization on the inner surface, and local plasticity at the crack tips that facilitates crack propagation. Various mechanisms have been proposed, and the reasons for brittleness after diffused hydrogen dissolves into metals have been studied. 1) Internal pressure: At high hydrogen concentrations, the absorbed hydrogen recombines in the voids to form hydrogen molecules (H2), generating pressure within the metal. This pressure can increase to the level at which cracks form, a phenomenon commonly known as hydrogen-induced cracking (HIC), as well as the formation of bubbles on the surface of the specimen, known as hydrogen embrittlement. These effects reduce ductility and tensile strength. 2) Hydrogen-enhanced local plasticity (HELP): Hydrogen increases the nucleation and movement of dislocations at the crack tip. HELP causes crack propagation through localized ductile failure at the crack tip, with minimal deformation occurring in the surrounding material, which makes the fracture appear brittle. 3) Hydrogen reduces dislocation emission: Molecular dynamics simulations reveal the transition from toughness to brittleness caused by dissolved hydrogen suppressing dislocation emission at crack tips. This prevents the crack tip from rounding, so sharp cracks lead to brittle cleavage failure. 4) Hydrogen-enhanced depolymerization (HEDE): Interstitial hydrogen reduces the stress required for metal atoms to break. HEDE occurs only when the local concentration of hydrogen is high, for example due to an increased solubility of hydrogen in the tensile stress field at crack tips, stress concentrations, or edge dislocations. 5) Formation of metal hydrides: The formation of brittle hydrides and the base material causes cracks to propagate in a brittle manner. This is particularly a problem for vanadium alloys, but most structural alloys do not readily form hydrides. 6) Phase transition: Hydrogen can induce phase transitions in certain materials, and the ductility of the new phases may be poor. 3 Hydrogen embrittlement characteristics of common metal materials – Hydrogen can embrittle various metals, including steel, aluminum (only at high temperatures), and titanium. Isotropically quenched iron is also highly susceptible, although isotropically quenched steel (and possibly other isotropically quenched metals) exhibits greater resistance to hydrogen embrittlement. 1) The hydrogen embrittlement problem of steel occurs when the steel becomes brittle due to hydrogen as a result of cathodic charging. Heat treatment (baking) is used to reduce the hydrogen content. The higher hydrogen content and shorter baking time result in a faster break time. Steels with a ultimate tensile strength of less than 1000 MPa (~145,000 psi) or a Rockwell hardness rating of less than HRC 32 are generally considered to be less prone to hydrogen embrittlement. As an example of severe hydrogen embrittlement, when smooth specimens are exposed to high-pressure hydrogen, the elongation at break of 17-4PH precipitation-hardening stainless steel decreases from 17% to only 1.7%. As the strength of steel increases, its fracture toughness decreases, thereby increasing the likelihood of fracture caused by hydrogen embrittlement. In high-strength steels, any material with a hardness higher than HRC32 may develop premature hydrogen cracking after an electroplating process that introduces hydrogen. Due to the accumulation of hydrogen from cathodic protection and other sources over time, long-term failures may also occur at any time, from a few weeks to several decades after operation begins. Many failures have been reported in the hardness range of HRC32-36 and above ； Therefore, parts within this range should be inspected during quality control to ensure they are not affected. 2) The hydrogen embrittlement problem of copper: When exposed to hot hydrogen, copper alloys containing oxygen become brittle. Hydrogen diffuses through copper and reacts with the impurities in copper to form 2 metal Cu atoms and H2O (water), which then create compressed bubbles at the grain boundaries. This process causes the grains to be literally forced apart from one another, and it is known as steam embrittlement (because the steam is generated directly within the copper lattice, rather than the problem arising from the copper being exposed to external steam). 3) Hydrogen embrittlement in vanadium, nickel, and titanium Alloys of vanadium, nickel, and titanium have a high hydrogen solubility, allowing them to absorb large amounts of hydrogen. This leads to the formation of hydrides, resulting in irregular volume expansion and reduced ductility (since metal hydrides are brittle ceramic materials). This is a special issue when searching for non-palladium-based alloys for hydrogen separation membranes. 4 Methods to prevent pre-hydrogen embrittlement: Hydrogen embrittlement can be prevented through various approaches, all of which aim to minimize contact between metal and hydrogen, especially during manufacturing and the electrolysis of water. Crisping processes such as those using acids should be avoided, as well as increased contact with elements such as sulfur and phosphates. Using appropriate plating solutions and procedures also helps to prevent hydrogen embrittlement. If the metal has not yet started to crack, hydrogen embrittlement can be reversed by removing the hydrogen source and allowing the hydrogen in the metal to diffuse out through heat treatment. This debinding process, known as low-hydrogen annealing or baking, is used to overcome the weakness introduced into metals by methods such as electroplating due to hydrogen, but it is not always completely effective because sufficient time and temperature must be achieved. In welding, metals are usually preheated and post-heated to allow hydrogen to dissipate before it can cause any damage. This is particularly applicable to high-strength steels and low-alloy steels, such as chromium/molybdenum/vanadium alloys. Since it takes time for hydrogen atoms to recombine into hydrogen molecules, welding-induced hydrogen cracks may occur within 24 hours after the welding operation is completed. Another way to prevent this problem is through material selection. This will impart inherent resistance to the process and reduce the need for post-treatment or continuous fault monitoring. Certain metals or alloys are highly susceptible to this problem; therefore, choosing materials that are least affected while still maintaining the desired properties will provide the best solution.