thumb|upright=1.5|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 in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are generally less susceptible. Similar hydrogen-assisted fracture and embrittlement phenomena in steels have also been discussed within the framework of hydrogen-enhanced localized plasticity mechanisms.

Hydrogen effects in steels

Hydrogen exposure may also influence the mechanical and tribological behaviour of alloyed steels during machining and wear processes. Studies on high-nickel steels have indicated that hydrogen absorption can modify the properties of the surface layer, affect plastic deformation during cutting, and influence the formation of wear products and surface damage under lubricated friction conditions.

Pipeline steels

For pipeline steels such as API 5L X70 and X80, hydrogen uptake and environment-assisted cracking are major integrity concerns; recent reviews synthesize hydrogen behaviour, embrittlement mechanisms, and mitigation strategies specific to line-pipe grades.) forms brittle zirconium hydride () platelets, which induce high tensile stresses at the platelet edges that lead to crack propagation when under thermomechanical stress. Recent imaging studies reveal potential mitigating mechanisms such as reducing radial hydride connectivity through hydride reorientation treatments.

Fatigue

While many failures in practice involve delayed or fast fracture, there is experimental evidence that hydrogen also affects the fatigue properties of steels.

Surface coatings

Coatings act as a barrier between the metal substrate and the surrounding environment, hindering the ingress of hydrogen atoms. Various techniques can be used to apply coatings, such as electroplating, chemical conversion coatings, or organic coatings. The choice of coating depends on the type of metal, the operating environment, and the application requirements.

Electroplating is commonly used to deposit a protective layer onto the metal surface. This process involves immersing the metal substrate in 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 improve corrosion resistance and reduce susceptibility to hydrogen embrittlement.

Chemical conversion coatings are another effective method for surface protection. These coatings are formed through chemical reactions between the metal substrate and a chemical solution, resulting in a thin, tightly adhering protective layer. Examples include chromate, phosphate, and oxide coatings.

Organic coatings, such as paints or polymer coatings, provide additional protection by forming a physical barrier between the metal surface and the environment.

Thermally sprayed coatings can also help reduce hydrogen ingress because coating materials such as ceramics or cermets may have low hydrogen permeability.

Testing

Most analytical methods for hydrogen embrittlement involve evaluating the effects of internal hydrogen from production and/or external hydrogen from service environments such as cathodic protection. For steels, it is important to test specimens in the laboratory that are at least as hard as the final parts.

There are numerous ASTM standards for testing hydrogen embrittlement:

  • ASTM B577 – Standard Test Methods for Detection of Cuprous Oxide (Hydrogen Embrittlement Susceptibility) in Copper.
  • ASTM B839 – Standard Test Method for Residual Embrittlement in Metallic Coated, Externally Threaded Articles, Fasteners, and Rods.
  • ASTM F519 – Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments.
  • ASTM F1459 – Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE) Test.
  • Resources on hydrogen embrittlement, University of Cambridge
  • Hydrogen embrittlement revisited by in situ electrochemical nanoindentations
  • A Sandia National Laboratories technical reference manual