Hydrogen Damage

Hydrogen damage is a generic term used to describe a wide range of metal degradation processes that occur due to the interaction of hydrogen with metallic materials. It arises primarily from atomic or ionic hydrogen entering the metal lattice, rather than from molecular gaseous hydrogen, which by itself is largely harmless. Once hydrogen atoms dissolve into metals, they can diffuse rapidly, interact with microstructural features, and initiate mechanical and chemical damage. Hydrogen damage is of major concern in industries such as petrochemicals, power generation, welding, nuclear engineering, and metallurgy, where metals are frequently exposed to hydrogen during processing or service.

Fundamental Mechanism of Hydrogen Damage

Molecular hydrogen does not significantly affect metals because it cannot readily penetrate the metallic lattice. In contrast, atomic hydrogen, generated during processes such as corrosion, welding, electroplating, chemical cleaning, or exposure to high-temperature hydrogen environments, can enter metals in solid solution. Once inside, hydrogen migrates to lattice defects such as dislocations, grain boundaries, inclusions, voids, and interfaces.
The accumulation of hydrogen at these sites alters local stress states, weakens atomic bonding, and promotes defect formation. Depending on temperature, pressure, material composition, and microstructure, hydrogen can lead to embrittlement, internal cracking, blistering, or chemical degradation of the metal.

Creation of Internal Defects

One of the most severe forms of hydrogen damage is high-temperature hydrogen attack (HTHA), which predominantly affects carbon and low-alloy steels exposed to hydrogen at elevated temperatures and pressures. In this process, hydrogen reacts with carbon present in steel to form methane. Since methane cannot diffuse through the metal lattice, it accumulates at grain boundaries and internal voids, generating high internal pressures.
This reaction leads to internal decarburisation, resulting in a significant loss of strength, creep resistance, and toughness. Over time, the formation of methane-filled cavities causes fissuring and intergranular cracking, eventually leading to catastrophic failure. This phenomenon was extensively documented by G. A. Nelson and forms the basis of the widely used Nelson curves for assessing steel suitability in hydrogen service.

Blistering and Hydrogen-Induced Blister Cracking

Hydrogen blistering occurs when atomic hydrogen diffuses through the metal and recombines to form molecular hydrogen at internal defects such as inclusions, laminations, or voids. Because molecular hydrogen occupies a much larger volume and cannot diffuse away, pressure builds up locally.
Continued hydrogen absorption increases this pressure, leading to the formation of surface or subsurface blisters. These blisters may grow over time and eventually rupture, causing surface damage and internal cracking known as hydrogen-induced blister cracking (HIBC).
Blistering has been observed in a wide range of materials, including carbon steels, aluminium alloys, titanium alloys, and nuclear structural materials. Metals with low hydrogen solubility, such as tungsten, are particularly susceptible to blister formation because hydrogen is more likely to accumulate at defects rather than remain dissolved. In contrast, metals with high hydrogen solubility, such as vanadium, tend to form stable metal hydrides instead of bubbles or blisters.

Shatter Cracks, Flakes, Fisheyes, and Microperforations

Hydrogen can also cause various internal and surface cracking phenomena, particularly in steels.
Flakes and shatter cracks are internal fissures commonly observed in large steel forgings. Hydrogen absorbed during melting and casting segregates at internal discontinuities such as voids or inclusions. During subsequent forging or cooling, the trapped hydrogen generates internal stresses that lead to crack formation, often appearing hours or days after processing.
Fisheyes are distinctive bright, circular regions seen on fracture surfaces, especially in weldments. Hydrogen enters the metal during fusion welding and becomes trapped at inclusions or defects. Under subsequent mechanical loading, these regions act as crack initiation sites, leading to delayed fracture.
Microperforations can occur in steel containment vessels exposed to extremely high hydrogen pressures. In such cases, hydrogen-induced fissures propagate through the metal thickness, forming microscopic leakage paths through which gases or fluids may escape, compromising pressure integrity and safety.

Loss of Tensile Ductility

A common and widely observed effect of hydrogen is the reduction in tensile ductility. Hydrogen lowers the ability of metals to undergo plastic deformation, often without significantly affecting yield or tensile strength.
In highly ductile materials such as austenitic stainless steels and aluminium alloys, hydrogen may not cause classical brittle fracture. However, it can produce a marked reduction in elongation and reduction in area during tensile testing. This loss of ductility increases the risk of crack initiation under service loads and reduces tolerance to stress concentrations.
The reduction in ductility is particularly dangerous because it may not be detected through conventional strength-based assessments, yet it significantly impairs structural reliability.

Control of Hydrogen Damage

The most effective method of controlling hydrogen damage is limiting the contact between hydrogen and the metal. Since hydrogen can be introduced at many stages of a material’s lifecycle, preventive measures must be applied during both manufacturing and service.
Key control strategies include:

• Reducing hydrogen uptake during melting, casting, rolling, forging, and welding
• Using low-hydrogen welding consumables and proper preheating and post-weld heat treatment
• Controlling surface preparation processes such as pickling, electroplating, and chemical cleaning
• Preventing corrosion reactions that generate atomic hydrogen during service

Two broad approaches are used to mitigate hydrogen damage. The first is environmental control, which involves managing temperature, pressure, chemical exposure, and moisture to minimise hydrogen generation. The second is metallurgical control, which focuses on material selection, alloying, heat treatment, and microstructural optimisation to reduce hydrogen susceptibility.

Detection and Monitoring of Hydrogen Damage

Early detection of hydrogen damage is essential because many hydrogen-related defects develop internally and may not be visible at the surface. A range of non-destructive testing (NDT) techniques is used to identify and monitor hydrogen damage.
Ultrasonic methods are particularly effective and include:

• Ultrasonic echo attenuation
• Amplitude-based backscatter techniques
• Velocity ratio measurements
• Creeping wave time-of-flight measurements
• Pitch-catch mode and shear wave velocity analysis
• Advanced ultrasonic backscatter techniques (AUBT)

For detecting microcracks and macrocracks, time-of-flight diffraction (TOFD) is widely used due to its high sensitivity and accuracy. Additional techniques such as ultrasonic thickness mapping and in-situ metallography using replicas allow monitoring of damage progression without removing components from service.
The Australian Institute for Non-Destructive Testing (AINDT) has highlighted the use of backscatter techniques to locate hydrogen-affected regions, with velocity ratio measurements used to confirm findings. Combining multiple NDT methods improves reliability and reduces the risk of undetected damage.