The mechanical properties of all metals are detrimentally affected by hydrogen. The magnitude of the deterioration depends on the type of metal, properties of the specific metal (e.g., strength), the environment (e.g., hydrogen pressure and temperature), and the mechanical loading. Exposure of metals to hydrogen can lead to embrittlement, which can be manifested as significant losses in tensile strength, ductility, and fracture toughness as well as accelerated fatigue crack growth. This can result in failure of pressure containing components.
Hydrogen Interactions with Metal Components
Hydrogen interacts with metal components in a five-step mechanism:
- Hydrogen gas consists of diatomic hydrogen molecules (HH) that are in continuous motion
- Diatomic hydrogen (HH) undergoes adsorption at metal surface
- Diatomic hydrogen (HH) dissociates to monatomic hydrogen (H)
- Monatomic hydrogen (H) absorbs into the metal
- Monatomic hydrogen (H) diffuses through interstitial spacings in the metal
Hydrogen can react with the metal or pass all the way through to the outside surface, effectively resulting in a very small hydrogen leak.
Hydrogen in the metal lattice degrades the mechanical properties of the metal (e.g., fatigue and fracture resistance). This phenomenon is known as hydrogen embrittlement (HE). Some facts about HE:
- The HE mechanism involves monatomic hydrogen (H) interacting with metallurgical features (e.g., grain boundaries).
- Small amounts of hydrogen in the metal (parts per million by weight) can have substantial effects on its fatigue and fracture resistance
- HE can result in both crack nucleation and propagation of a crack through the wall of a structure.
- The mechanism of HE occurs at temperatures under 200°C (400°F). Other hydrogen-related degradation mechanisms operate at higher temperatures.
For liquid hydrogen service, consideration of low temperature embrittlement is also required.
Hydrogen Embrittlement Enables Crack Propagation:
- Hydrogen (H) absorbs into metal following steps described above
- Hydrogen (H) that diffuses into the metal concentrates at defects (e.g., manufacturing flaws on component surfaces)
- Hydrogen (H) interacts with metallurgical features causing embrittlement and inducing crack propagation
The mechanisms of hydrogen embrittlement are not yet fully understood and are still being researched. One broadly accepted HE mechanism is Hydrogen Enhanced Decohesion (H.E.D.E). This theory posits that H atoms gathering at locations of high triaxial stress will lead to the weakening of the bonds of metal atoms and cause fracture.
Variables Affecting Hydrogen Embrittlement
Hydrogen embrittlement depends not only on properties of the material but also on characteristics of mechanical loading and environment. The intersection of material, mechanical, and environmental variables dictates activation and severity of hydrogen embrittlement.
Examples of material variables that affect hydrogen embrittlement (HE) of metals in hydrogen gas containment components:
- Nickel content in austenitic stainless steels
- Material strength
Primary mechanical variables that affect hydrogen embrittlement (HE) of metals in hydrogen gas containment components are:
- Constant stress vs. cyclic stress
- Magnitude of constant stress or cyclic stress amplitude
Examples of environmental variables that affect hydrogen embrittlement (HE) of metals in hydrogen gas containment components:
- Hydrogen gas pressure