Hydrogen embrittlement (HE) is a catastrophic failure process where a metal loses its ductility due to the presence of absorbed hydrogen atoms. The hydrogen atoms, being the smallest element, diffuse into the metal’s lattice structure, altering its mechanical properties and making it brittle. This phenomenon is insidious because it can cause components to fail suddenly and without warning under mechanical stress far below the material’s designed yield strength. This degradation is a serious concern in industries where material reliability is paramount, such as aerospace, high-pressure pipeline transport, and energy infrastructure.
How Hydrogen Causes Metal Failure
Hydrogen embrittlement requires the synergistic presence of both diffusible hydrogen within the metal and an applied tensile stress, which can be residual stress from manufacturing or external service stress. The tiny hydrogen atoms readily diffuse through the metal’s crystal lattice and migrate toward regions of high stress, such as crack tips, grain boundaries, and material defects, which act as “trap sites.” The concentration of hydrogen in these localized high-stress areas is what ultimately drives the mechanism of failure.
Scientists primarily explain the failure mechanism through two complementary theories, often acting together: Hydrogen Enhanced Decohesion (HEDE) and Hydrogen Enhanced Localized Plasticity (HELP). The HEDE mechanism proposes that concentrated hydrogen atoms weaken the cohesive metallic bonds holding the material’s atoms together. By reducing the energy required to separate the atoms, hydrogen effectively lowers the material’s fracture stress, making it easier for cracks to initiate and propagate in a brittle fashion. This process is most often observed at grain boundaries.
The HELP mechanism focuses on the effect of hydrogen on the movement of dislocations, which are defects that allow for plastic deformation in the metal’s crystal structure. Hydrogen atoms increase the mobility and velocity of these dislocations, leading to concentrated plastic flow in localized zones. This localized softening promotes intense shear deformation and the formation of micro-voids. The result is a type of failure that looks brittle on a macroscopic scale despite involving microscopic plasticity.
These two mechanisms, HEDE and HELP, often operate in tandem, with their dominance depending on the material’s composition, hydrogen concentration, and the stress state. For instance, in high-strength steels, the HELP mechanism may precede the decohesion mechanism by creating a pathway for hydrogen to accumulate at interfaces where HEDE then takes over.
Where Does Hydrogen Come From
The source of hydrogen must supply it in its atomic form (H) or as an H+ ion for it to diffuse into the metal lattice, as molecular hydrogen (H2) is generally too large to enter the metal at ambient temperatures. This atomic hydrogen often originates from industrial processes during manufacturing or from the environment during the component’s service life.
Processes such as electroplating and acid pickling or cleaning, which are common surface treatments, use electrochemical reactions that generate atomic hydrogen at the metal surface. This nascent hydrogen can then be absorbed into the material before it recombines into the molecular gas. Similarly, welding processes can introduce hydrogen if damp welding rods are used or if the surrounding atmosphere contains moisture, as the hydrogen is absorbed when the metal is molten.
Once a component is in service, environmental factors are the primary source of hydrogen ingress. Aqueous corrosion reactions, particularly in acidic or sulfide-containing environments, generate atomic hydrogen as a byproduct. Cathodic protection systems, designed to prevent corrosion, can inadvertently generate hydrogen ions at the surface, leading to absorption. High-pressure gaseous hydrogen environments, such as in storage tanks or pipelines, also cause embrittlement as the molecular gas dissociates into atomic hydrogen on the metal surface.
Metals and Alloys Vulnerable to Embrittlement
The susceptibility of a metal to hydrogen embrittlement is strongly tied to its microstructure and overall strength. High-strength steels, especially those with yield strengths exceeding 1,000 megapascals (or 150,000 pounds per square inch), are the most significantly affected group. Their high strength is typically achieved through heat treatments that create a very rigid, often martensitic, microstructure with numerous internal defects and trap sites where hydrogen can accumulate.
Materials with a body-centered cubic (BCC) crystal structure, such as ferritic steels, are generally more susceptible than those with a face-centered cubic (FCC) structure, like austenitic stainless steels. The more open BCC structure allows for faster hydrogen diffusion, and the microstructure is more prone to the mechanisms of embrittlement. Lower-strength, more ductile metals are generally less affected because they can tolerate more plastic deformation before fracturing.
Beyond high-strength steels, other alloy systems are known to be vulnerable to this degradation. Titanium alloys, commonly used in aerospace, are susceptible to hydrogen embrittlement, often through a mechanism involving the formation of brittle hydride compounds. Certain nickel-based alloys can also be embrittled, although many are selected for their comparatively high resistance in hydrogen-rich environments.
Strategies for Prevention and Mitigation
The most effective strategy against hydrogen embrittlement involves a multi-pronged approach that addresses material selection, manufacturing processes, and environmental control. Careful material selection is a primary defense, often involving the use of alloys inherently resistant to hydrogen uptake and damage. For example, austenitic stainless steels and specific aluminum or nickel alloys are often chosen for hydrogen service because their microstructures are less susceptible to the embrittling mechanisms.
During manufacturing, post-processing treatments are used to eliminate any hydrogen that may have been absorbed during surface finishing. A procedure known as “baking” or de-gassing involves heating the component immediately after processes like electroplating or acid cleaning. This thermal treatment increases the rate at which the trapped hydrogen atoms diffuse out of the metal before they can cause damage.
Environmental control measures focus on minimizing the metal’s exposure to hydrogen sources during its service life. This includes applying protective coatings, such as specific paints or metallic layers, that act as a barrier to hydrogen ingress. In corrosive environments, chemical inhibitors can slow the corrosion reaction, reducing the generation of atomic hydrogen. Reducing the external or residual stress on a component, through careful design or stress-relief heat treatments, can also mitigate the risk, as embrittlement requires the presence of stress to propagate cracking.