Hydrogen embrittlement
Hydrogen embrittlement (HE) also known as hydrogen assisted cracking or hydrogen-induced cracking, describes the embrittlement of a metal by hydrogen. The essential facts about the nature of the hydrogen embrittlement of steels have now been known for 140 years. It is 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: Diffusible hydrogen can be introduced during manufacture from operations such as forming, coating, plating or cleaning. The most common 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 () 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. History. The hydrogen embrittlement phenomenon was first described by Johnson in 1875. The following conclusions can justifiably be reached from this 1875 paper: 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. Mechanisms. 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. Material susceptibility. Hydrogen embrittles a variety of metals 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 prone 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. Steels. 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. Copper alloys which contain oxygen can be embrittled if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts  with inclusions of CuO, forming HO (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. Fatigue. 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 hydrogen is released from moisture, such as in the coating of welding electrodes or from damp welding rods. 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 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. Prevention. 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. Testing. 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: