Hydrogen Economy Faces Material Challenges Due to Embrittlement

Imagine a future without exhaust emissions, where blue skies are no longer obscured by factory smoke, replaced instead by clean, efficient hydrogen energy. This vision is captivating, seemingly within reach. However, hydrogen energy is not as docile as imagined—it imposes extremely stringent requirements on materials. The slightest oversight can lead to serious consequences. Today, we delve into the hidden "material killer" lurking in hydrogen energy applications: hydrogen embrittlement, and explore how to avoid this concealed trap on the path of energy transition.

In the global wave of energy transition, hydrogen energy is regarded as the "future star," carrying infinite aspirations for clean energy. It has diverse sources and can be produced through various methods such as water electrolysis, fossil fuel reforming, and biomass gasification. More importantly, hydrogen combustion produces only water, achieving true "zero emissions," making it an ideal fuel.

The application prospects of hydrogen energy are vast, spanning transportation, industrial production, building heating, power storage, and more. In transportation, hydrogen fuel cell vehicles are considered an ideal alternative to traditional fuel-powered cars, offering advantages like long range, short refueling time, and zero emissions. In industry, hydrogen can serve as a reducing agent or synthetic raw material for steel smelting and chemical production, reducing carbon emissions. In buildings, hydrogen fuel cells can provide electricity and heat, achieving energy self-sufficiency. In power storage, hydrogen can convert excess electricity into chemical energy for storage and then back into electricity when needed, addressing the intermittency of renewable energy.

However, to truly harness hydrogen energy and turn the "future star" into reality, we must overcome a series of technical challenges. Among them, hydrogen storage and transportation are critical links and among the most significant hurdles in hydrogen energy applications. Hydrogen has low volumetric energy density, is highly flammable and explosive, and prone to permeation, making its storage and transportation exceptionally difficult.

Currently, hydrogen storage methods mainly include high-pressure gaseous storage, cryogenic liquid storage, and solid-state storage. High-pressure gaseous storage is relatively mature but has low volumetric energy density, requires high-pressure containers, and poses safety risks. Cryogenic liquid storage significantly improves volumetric energy density but requires cooling hydrogen to extremely low temperatures (-253°C), consuming considerable energy. Solid-state storage offers high volumetric energy density and good safety but remains costly and technologically immature.

Hydrogen transportation methods mainly include pipeline transport, tanker trucks, and shipping. Pipeline transport is ideal for long-distance, large-scale hydrogen transportation but requires dedicated hydrogen pipelines with substantial investment. Tanker trucks are suitable for short-distance, small-scale transport but are inefficient. Shipping is suited for long-distance, large-scale transport but requires liquefaction or conversion of hydrogen into other forms before transportation.

Whether for storage or transportation, hydrogen imposes extremely stringent requirements on materials. Traditional metal materials are prone to hydrogen embrittlement in hydrogen environments, leading to equipment failure and even safety accidents. Therefore, developing high-performance materials suitable for hydrogen environments is a critical aspect of hydrogen energy applications.

Hydrogen, as the "number one element" on the periodic table, has the smallest atomic volume and lightest mass. These unique physical properties make it highly prone to permeating into the interior of metal materials, causing a series of issues. Particularly in practical hydrogen energy applications, hydrogen usually exists in liquid or gaseous forms, presenting additional challenges.

• Cryogenic Embrittlement: Liquid hydrogen reaches extremely low temperatures of -252.9°C (-423°F). At such temperatures, the ductility of metal materials significantly decreases, becoming more brittle and prone to fracture. This phenomenon is known as cryogenic embrittlement.
• Hydrogen Embrittlement: At room temperature, gaseous hydrogen decomposes into hydrogen atoms, which diffuse into the metal's interior, altering its microstructure and reducing its strength and toughness, making it more susceptible to fracture. This is what we commonly refer to as "hydrogen embrittlement."

Hydrogen embrittlement is a form of hydrogen corrosion that reduces a metal's fatigue and fracture resistance. This process can be simply understood as follows: hydrogen molecules decompose into hydrogen atoms, which permeate the metal's interior and accumulate at stress concentration points, such as crack tips, grain boundaries, inclusions, or precipitates. In some cases, hydrogen atoms may even recombine into hydrogen molecules, further exacerbating hydrogen embrittlement.

The occurrence of hydrogen embrittlement is a complex physicochemical process involving multiple steps: hydrogen adsorption, absorption, diffusion, and accumulation. Hydrogen atoms first adsorb onto the metal surface, then decompose into hydrogen ions, which enter the metal's interior, diffuse to stress concentration points, interact with metal atoms, alter the metal's microstructure, and ultimately degrade the material's mechanical properties.

The hazards of hydrogen embrittlement cannot be underestimated. It can lead to premature equipment failure, increased maintenance costs, and even safety accidents, hindering the widespread adoption of hydrogen energy. For example, hydrogen pipeline fractures due to embrittlement can cause leaks, leading to fires or explosions. Hydrogen fuel cell vehicle storage tanks failing due to embrittlement can render vehicles inoperable or even cause accidents.

Therefore, material selection is crucial in the design and construction of hydrogen energy systems. Only materials with excellent resistance to hydrogen embrittlement can ensure the safe and reliable operation of hydrogen energy systems.

So, how can we build robust and reliable hydrogen energy equipment? The answer lies in selecting the right materials! Particularly, specially formulated, high-quality stainless steels can better withstand the challenges posed by hydrogen.

Stainless steel is an alloy steel primarily composed of iron, with alloying elements such as chromium, nickel, and molybdenum. Chromium forms a dense oxide film on the stainless steel surface, preventing corrosion. Nickel enhances strength and toughness, while molybdenum improves corrosion resistance.

Different types of stainless steel exhibit varying resistance to hydrogen embrittlement. Generally, austenitic stainless steels perform well against hydrogen embrittlement, while ferritic and martensitic stainless steels are more susceptible.

Austenitic stainless steels, primarily composed of austenite, offer good plasticity, toughness, and weldability, making them widely used in chemical, petroleum, and food industries. Common grades include 304, 316, and 321. Ferritic stainless steels, primarily composed of ferrite, have higher strength and corrosion resistance but poorer plasticity and toughness, making them prone to cryogenic embrittlement and hydrogen embrittlement. Common grades include 430 and 446. Martensitic stainless steels can be strengthened through heat treatment, offering high strength and hardness but poorer plasticity and toughness, making them susceptible to hydrogen embrittlement. Common grades include 410 and 420.

Therefore, austenitic stainless steels should be prioritized in hydrogen energy system design and construction. If ferritic or martensitic stainless steels must be used, appropriate protective measures—such as surface coatings or cathodic protection—should be implemented to mitigate hydrogen embrittlement risks.

Beyond stainless steels, other materials like aluminum alloys, titanium alloys, and nickel-based alloys also exhibit good resistance to hydrogen embrittlement. These materials are costlier but offer superior performance, making them suitable for high-safety applications.

Below, we address several common questions to explore key considerations in hydrogen energy material selection.

A: Cryogenic embrittlement refers to the decline in a metal's ductility, toughness, and fatigue and fracture resistance as temperatures drop. Simply put, metals become more brittle and prone to fracture at low temperatures.

Different types of stainless steels exhibit varying resistance to cryogenic embrittlement. Austenitic stainless steels maintain good toughness at low temperatures, while ferritic stainless steels (including low-alloy steels, ferritic, or duplex stainless steels) are more prone to cryogenic embrittlement. Thus, austenitic stainless steels are the preferred choice for liquid hydrogen systems. If your application is highly sensitive to cryogenic embrittlement, austenitic stainless steels are the way to go.

Austenitic stainless steels resist cryogenic embrittlement due to their face-centered cubic (FCC) crystal structure, which maintains good plasticity and toughness at low temperatures, preventing brittle fractures. Additionally, some aluminum alloys, titanium alloys, and nickel-based alloys also resist cryogenic embrittlement, making them suitable for cryogenic environments like liquid hydrogen storage and transport.

A: Hydrogen embrittlement is a form of hydrogen corrosion that reduces a metal's fatigue and fracture resistance. The process can be simplified as follows: hydrogen molecules decompose into atoms that permeate the metal's interior and accumulate at stress concentration points like crack tips, grain boundaries, inclusions, or precipitates. In some cases, hydrogen atoms may recombine into molecules, exacerbating embrittlement.

Hydrogen embrittlement involves multiple steps: hydrogen adsorption, decomposition, absorption, diffusion, accumulation, and ultimately brittle fracture. Its occurrence depends on factors like hydrogen pressure, temperature, purity, metal type, composition, microstructure, and stress magnitude/frequency.

A: Materials prone to hydrogen embrittlement significantly increase system failure risks. Generally, higher-strength materials are more susceptible. Austenitic stainless steels, with their FCC crystal structure, moderate strength, and good ductility, are typically better suited for hydrogen environments. However, different grades vary in resistance. Ferritic and martensitic stainless steels, due to their crystal structures and compositions, are more prone to embrittlement. High-strength steels and titanium alloys are also vulnerable in hydrogen environments.

Using unsuitable materials can lead to premature equipment failure, higher maintenance costs, and safety hazards. Potential consequences include:

• Safety risks: Hydrogen leaks may cause fires or explosions.
• Downtime: Equipment repairs or replacements disrupt operations.
• Increased replacement frequency: Components require more frequent changes, raising costs.
• Environmental impact: Hydrogen leaks waste energy and may harm the environment.
• Higher operational costs: Overall maintenance and operational expenses rise.

A: High-quality stainless steels with higher nickel content are better suited for hydrogen environments, especially for long-term service. ASTM standards require 316 stainless steel to contain at least 10% nickel, but for hydrogen applications, 12% nickel content performs better. Nickel stabilizes the microstructure, enhancing resistance to embrittlement. Tests show 316 stainless steel with 12% nickel retains tensile ductility well under hydrogen exposure.

Nickel stabilizes austenite, improving plasticity and toughness, thereby enhancing embrittlement resistance. It also slows hydrogen diffusion, reducing atomic accumulation and embrittlement risks. Beyond nickel content, factors like purity, grain size, and processing also affect performance.

A: In the rapidly evolving hydrogen energy sector, material selection is complex but critical. The right choices directly impact hydrogen's reputation as a reliable, sustainable fuel. Poor selections could jeopardize its future. Seek suppliers with deep materials science expertise and successful hydrogen application cases. Consult experts to identify optimal solutions for specific needs.

• Mechanical properties: Strength, toughness, plasticity, fatigue resistance.
• Corrosion resistance: Resistance to hydrogen and electrochemical corrosion.
• Hydrogen embrittlement resistance: The most crucial performance metric.
• Cost: Balance performance and affordability.
• Processability: Ease of shaping into components.
• Weldability: Compatibility with welding processes.

Additionally, consider operational environments (temperature, pressure, media) to select materials with suitable performance under specific conditions.

As hydrogen energy technology advances, material requirements grow stricter. Future hydrogen energy materials will develop in these directions:

• High strength and toughness: Enhances system safety and reliability.
• Hydrogen corrosion resistance: Extends system lifespan.
• Low cost: Promotes widespread adoption.
• Lightweight: Improves fuel cell vehicle range.
• Smart materials: Self-healing, diagnostic capabilities boost safety.

Researchers are exploring novel materials like high-entropy alloys, nanomaterials, and intermetallics, which show promise for future hydrogen applications.

Hydrogen energy, as a clean, efficient, and sustainable power source, holds vast potential. However, achieving large-scale adoption requires overcoming technical challenges, with material selection being pivotal. Only by choosing suitable materials can we ensure safe, reliable hydrogen systems, unlocking their full potential for a cleaner future. Let us work together to solve material challenges and advance hydrogen energy's global adoption.