Detailed Analysis of the Mechanism of Hydrogen Embrittlement
Detailed Analysis of the Mechanism of Hydrogen Embrittlement
Hydrogen embrittlement (HE) is a phenomenon in which metals become brittle and ultimately fail due to the absorption of hydrogen. This is not just a simple material degradation process but a complex interplay of physical and chemical mechanisms occurring within the material. Below is a more detailed explanation of the mechanisms behind hydrogen embrittlement.
1. Hydrogen Ingress and Diffusion
The first stage of hydrogen embrittlement involves the ingress and diffusion of hydrogen into the metal.
① Mechanisms of Hydrogen Ingress
Hydrogen enters metals in atomic or molecular form through the following four primary pathways:
Electrochemical Ingress
Hydrogen ions (H⁺) generated during electroplating, electrolysis, or corrosion gain electrons at the metal surface and are absorbed as atomic hydrogen (H).
In acidic environments, corrosion reactions (e.g., acid pickling) produce hydrogen, making ingress easier.
Gas Absorption
In high-pressure hydrogen environments (such as hydrogen fuel cells and hydrogen storage tanks), molecular hydrogen (H₂) dissociates into atomic hydrogen and penetrates the metal.
Hydrogen Absorption During Welding
During welding, moisture and organic materials decompose, generating hydrogen, which then infiltrates the metal.
Mechanical Absorption
Under high-stress conditions, surface fractures allow preferential hydrogen ingress.
② Hydrogen Diffusion
Once inside, hydrogen moves through the metal lattice and accumulates at grain boundaries and defects.
The diffusion rate depends on temperature, crystal structure, and the presence of lattice defects.
At low temperatures (near room temperature), hydrogen primarily moves through interstitial diffusion (diffusion through gaps in the metal lattice).
At higher temperatures, dislocation diffusion (movement along defects and dislocations) becomes dominant.
2. Major Mechanisms of Hydrogen-Induced Failure
Hydrogen embrittlement occurs through various failure mechanisms influenced by hydrogen behavior within the metal.
① Hydrogen-Induced Cracking (HIC)
When hydrogen accumulates within the metal at high concentrations, cracks can form even in the absence of external stress.
This typically appears as layered cracks along the rolling direction in steels.
② Stress-Induced Hydrogen Embrittlement (SHE)
The interaction between external tensile stress and hydrogen weakens the metal, promoting crack initiation and propagation.
Hydrogen reduces dislocation mobility while accumulating in voids and defects, leading to brittle failure.
③ Hydrogen Pressure Theory
Hydrogen accumulates in voids, dislocations, and microcavities, creating high local pressure that internally pushes the material apart, causing cracks.
This mechanism is especially dominant in high-pressure hydrogen environments.
④ Hydrogen-Enhanced Localized Plasticity (HELP)
Hydrogen increases the mobility of dislocations, promoting localized deformation while reducing the plasticity of the surrounding areas.
This leads to a reduction in overall ductility and premature failure.
⑤ Hydrogen-Induced Phase Transformation
When hydrogen dissolves into the metal, it can cause localized phase transformations, leading to structural embrittlement.
For example, in austenitic stainless steels, hydrogen absorption can induce martensitic transformation, making the material more brittle.
3. Conditions Favoring Hydrogen Embrittlement
Hydrogen embrittlement is particularly likely to occur under the following conditions:
① High-Strength Materials
High-strength steels and ultra-high-strength aluminum alloys have a small interatomic spacing and low hydrogen solubility, leading to localized hydrogen accumulation and embrittlement.
② High-Stress Environments
Hydrogen embrittlement is more severe under tensile stress, particularly in stress concentration areas and welded joints.
③ Low-Temperature Conditions
While hydrogen diffusion increases with temperature, hydrogen embrittlement is more pronounced at lower temperatures (especially around room temperature).
This is because hydrogen is less mobile at low temperatures, leading to local high-concentration zones.
④ Presence of Hydrogen Sources
Corrosive environments, hydrogen storage materials, and high-pressure hydrogen gas increase the risk of hydrogen embrittlement by continuously supplying hydrogen.
4. Summary
Hydrogen embrittlement progresses through a complex series of processes, including hydrogen ingress, diffusion, accumulation, and failure. Key points include:
Hydrogen Ingress Pathways
Electrochemical reactions, gas absorption, welding, and mechanical absorption.
Hydrogen Diffusion
Hydrogen moves through lattice defects and grain boundaries.
Failure Mechanisms
Hydrogen-induced cracking (HIC), stress-induced hydrogen embrittlement (SHE), hydrogen pressure theory, HELP, and hydrogen-induced phase transformations.
Susceptible Conditions
High-strength materials, high-stress environments, low temperatures, and continuous hydrogen supply.
As the hydrogen economy advances, the need for effective hydrogen embrittlement mitigation strategies is becoming increasingly critical. Future research must focus on developing hydrogen-resistant materials, advanced surface treatments, and optimized heat treatment techniques to ensure structural integrity in hydrogen-rich environments.