Hydrogen-induced cracking (HIC)

Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate 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] Hydrogen embrittlement is maximised at around room temperature in steels, and most metals are relatively immune to hydrogen embrittlement at temperatures above 150 °C.[7] Hydrogen embrittlement requires the presence of both atomic ("diffusible") hydrogen and a mechanical stress to induce crack growth, although that stress may be applied or residual.[2][8][9] Hydrogen embrittlement increases at lower strain rates.[1][2][10] In general, higher-strength steels are more susceptible to hydrogen embrittlement than mid-strength steels.[11]

Metals can be exposed to hydrogen from two types of sources: gaseous hydrogen and hydrogen chemically generated at the metal surface. Gaseous hydrogen is molecular hydrogen and does not cause embrittlement though it can cause hot hydrogen attack (see below). It is the atomic hydrogen from chemical attack which causes embrittlement because the atomic hydrogen dissolves quickly into the metal at room temperature.[6] Gaseous hydrogen is found in pressure vessels and pipelines. Electrochemical sources of hydrogen include acids (as may be encountered during pickling, etching, or cleaning), corrosion (typically due to aqueous corrosion or cathodic 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.[10] 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 mechanisms (there are many) by which hydrogen causes embrittlement in steels are not comprehensively understood and continue to be explored and studied.[1][13][14]

Mechanisms

Crack in a hardened steel due to hydrogen, observed by scanning electron microscopy (SEM).

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]

Material susceptibility

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]

Steels

Steels were embrittled with hydrogen through cathodic charging. Heat treatment (baking) was used to reduce hydrogen content. Lower bake times resulted in quicker fracture times due to higher hydrogen content.[25]

Steel with an ultimate tensile strength of less than 1000 MPa (~145,000 psi) or hardness of less than HRC 32 on the Hardness Rockwell Scale 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[2]

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.

Testing the fracture toughness of hydrogen-charged, embrittled specimens is complicated by the need to keep charged specimens very cold, in liquid nitrogen, to prevent the hydrogen diffusing away.[26]

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 Cu
2
O
, forming 2 metallic Cu atoms and 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 directly produced inside the copper crystal lattice, not because exposure of copper to external steam causes the problem).

Vanadium, nickel, and titanium

Alloys of vanadium, nickel, and titanium have a 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 (because metallic hydrides are fragile ceramic materials). This is a particular issue when looking for non-palladium-based alloys for use in hydrogen separation membranes.[18]

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.[27][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]

Environmental embrittlement

Hydrogen embrittlement is a volume effect: it affects the volume of the material. Environmental embrittlement[2] is a surface effect where molecules from the atmosphere surrounding the material under test are adsorbed onto the fresh crack surface. This is most clearly seen from fatigue measurements where the measured crack growth rates[24] can be an order of magnitude higher in hydrogen than in air. That this effect is due to adsorption, which saturates when the crack surface is completely covered, is understood from the weak dependence of the effect on the pressure of hydrogen.[24]

Environmental embrittlement is also observed to reduce fracture toughness in fast fracture tests, but the severity is much reduced compared with the same effect in fatigue[24]

Hydrogen embrittlement is the effect where a previously embrittled material has low fracture toughness whatever atmosphere it is tested in. Environmental embrittlement is the effect when the low fracture toughness is only observed when the testing happens in that atmosphere.

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.[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.[28][29][30]

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][31] 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 duration before use. 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
2
S
) in sulfide stress cracking (SSC), a significant problem for the oil and gas industries.[32]

After a manufacturing process or treatment which may cause hydrogen ingress, the component should be baked to remove or immobilize the hydrogen.[29]

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.

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.[33] 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.

Surface Coatings

Coatings act as a barrier between the metal substrate and the surrounding environment, hindering the ingress of hydrogen atoms. These coatings can be applied through various techniques such as electroplating, chemical conversion coatings, or organic coatings. The choice of coating depends on factors such as the type of metal, the operating environment, and the specific requirements of the application.

Electroplating is a commonly used method to deposit a protective layer onto the metal surface. This process involves immersing the metal substrate into an electrolyte solution containing metal ions. By applying an electric current, the metal ions are reduced and form a metallic coating on the substrate. Electroplating can provide an excellent protective layer that enhances corrosion resistance and reduces the susceptibility to hydrogen embrittlement.

Chemical conversion coatings are another effective method for surface protection. These coatings are typically formed through chemical reactions between the metal substrate and a chemical solution. The conversion coating chemically reacts with the metal surface, resulting in a thin, tightly adhering protective layer. Examples of conversion coatings include chromate, phosphate, and oxide coatings. These coatings not only provide a barrier against hydrogen diffusion but also enhance the corrosion resistance of the metal.

Organic coatings, such as paints or polymer coatings, offer additional protection against hydrogen embrittlement. These coatings form a physical barrier between the metal surface and the environment. They provide excellent adhesion, flexibility, and resistance to environmental factors. Organic coatings can be applied through various methods, including spray coating, dip coating, or powder coating. They can be formulated with additives to further enhance their resistance to hydrogen ingress.

Thermally sprayed coatings offer several advantages in the context of hydrogen embrittlement prevention. The coating materials used in this process are often composed of materials with excellent resistance to hydrogen diffusion, such as ceramics or cermet alloys. These materials have a low permeability to hydrogen, creating a robust barrier against hydrogen ingress into the metal substrate.[34]

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:

Notable failures from hydrogen embrittlement

See also

References

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