Ammonia has emerged as a promising hydrogen carrier due to its high hydrogen density, ease of liquefaction, and established transport infrastructure. However, its storage and transportation require careful material selection to address challenges such as corrosion, hydrogen embrittlement, and chemical compatibility. The following discussion details material considerations for ammonia infrastructure, focusing on storage tanks, pipelines, and auxiliary components.

**Storage Tanks**
Ammonia is typically stored in liquid form at moderate pressures (10-15 bar) or refrigerated at -33°C at ambient pressure. Material selection for storage tanks prioritizes resistance to stress corrosion cracking (SCC) and general corrosion.

- **Carbon Steel**: The most widely used material due to its cost-effectiveness and adequate resistance to anhydrous ammonia. ASTM A516 Grade 70 is commonly specified for pressure vessels. However, impurities like water or oxygen can accelerate SCC, necessitating strict purity controls.
- **Stainless Steel**: Austenitic grades such as 304L and 316L offer superior corrosion resistance, particularly in systems where contamination risks are high. Their use is limited by higher costs but justified in critical applications.
- **Nickel Alloys**: Alloys like Inconel 625 provide exceptional resistance to both ammonia and potential contaminants, making them suitable for high-pressure or high-purity systems.

**Pipelines and Transport Vessels**
Ammonia pipelines and transport vessels must withstand cyclic loading and environmental exposure.

- **Carbon Steel Pipelines**: API 5L X52 or X60 grades are standard for ammonia pipelines. Post-weld heat treatment (PWHT) is often applied to reduce residual stresses and mitigate SCC risks.
- **Composite Materials**: Fiber-reinforced polymers (FRPs) are being explored for their corrosion resistance and lightweight properties, though long-term durability under ammonia exposure requires further validation.
- **Cryogenic Transport**: For refrigerated ammonia, stainless steel (304 or 316) is preferred due to its toughness at low temperatures. Aluminum alloys like AA5083 are also used in marine tanks for their weight savings and corrosion resistance.

**Valves, Fittings, and Seals**
Secondary components must maintain integrity under ammonia exposure.

- **Valve Bodies**: Duplex stainless steels (e.g., UNS S31803) combine strength and corrosion resistance, suitable for high-pressure ammonia service.
- **Seals and Gaskets**: PTFE (Teflon) and perfluoroelastomers (FFKM) are chemically inert to ammonia, ensuring leak-free performance. Buna-N (nitrile rubber) is avoided due to degradation risks.
- **Bolting Materials**: ASTM A193 B7 (chromium-molybdenum steel) is commonly used, but stress-relieved grades are recommended to prevent SCC.

**Hydrogen Embrittlement Considerations**
Ammonia decomposition can release atomic hydrogen, leading to embrittlement in susceptible materials.

- **Low-Strength Steels**: Less prone to hydrogen embrittlement; hence, carbon steels with yield strengths below 620 MPa are preferred.
- **Avoidance of High-Strength Alloys**: Quenched and tempered steels or martensitic stainless steels are generally avoided unless proven resistant through testing.
- **Hydrogen Trapping Mitigation**: Microalloying with vanadium or titanium can reduce hydrogen diffusion rates in steels.

**Compatibility with Impurities**
Real-world ammonia may contain contaminants like water, air, or oils, influencing material performance.

- **Wet Ammonia**: Increases corrosion rates in carbon steel. Stainless steels or nickel alloys are recommended for wet service.
- **Oxygen Contamination**: Accelerates SCC in carbon steel. Deaeration or oxygen scavengers are employed to mitigate this risk.

**Regulatory and Standards Compliance**
Materials must comply with industry standards to ensure safety and reliability.

- **ASME Boiler and Pressure Vessel Code**: Governs ammonia storage tanks, specifying acceptable materials and fabrication practices.
- **ISO 21010**: Provides guidelines for materials in ammonia refrigeration systems.
- **DOT Regulations (49 CFR)**: Mandate material and design requirements for ammonia transport vessels in the U.S.

**Operational Best Practices**
Beyond material selection, operational measures enhance longevity.

- **Regular Inspection**: Ultrasonic testing (UT) and phased array inspections detect SCC or hydrogen-induced cracking early.
- **Protective Coatings**: Epoxy or phenolic linings can extend the life of carbon steel tanks in corrosive environments.
- **Passivation**: Nitriding or oxidizing treatments on stainless steel surfaces improve resistance to ammonia attack.

**Future Material Developments**
Research is ongoing to improve ammonia infrastructure materials.

- **Advanced Composites**: Carbon-fiber-reinforced tanks may offer weight savings for mobile applications.
- **Ceramic Coatings**: Alumina or zirconia coatings could provide additional barriers against hydrogen permeation.
- **High-Entropy Alloys**: Experimental alloys show promise in resisting both corrosion and embrittlement.

In summary, the selection of materials for ammonia storage and transport hinges on balancing cost, corrosion resistance, and mechanical integrity. Carbon steel remains the workhorse for large-scale infrastructure, while stainless steels and nickel alloys address more demanding conditions. Operational vigilance and adherence to standards further ensure the safe and efficient use of ammonia as a hydrogen carrier.