Selecting Stainless Steels for Processing Ammonia (NH₃): A Practical Guide

Ammonia (NH₃) is vital in industries like refrigeration, chemical processing, and power generation—particularly in heat exchangers, superheaters, boilers, and condensers. However, choosing the right material for ammonia systems isn’t as straightforward as it seems. The wrong stainless steel can lead to stress corrosion cracking (SCC), equipment failure, and costly downtime.

This blog explains everything you need to know about selecting the right stainless steel for ammonia environments, balancing performance, durability, and safety.

Why Material Selection Matters for Ammonia Systems?

Ammonia might seem harmless at first glance — after all, it’s widely used in industrial processes. But when it comes to material compatibility, it’s a different story.

Challenges of ammonia environments:

Stress Corrosion Cracking (SCC): Stainless steels, especially austenitic grades like 304 and 316, can suffer SCC when exposed to ammonia under stress — especially in moist conditions.
Pitting and Crevice Corrosion: Certain steels can corrode rapidly if moisture or contaminants (e.g., chlorides) mix with ammonia.
Hydrogen Embrittlement: Ammonia can cause hydrogen diffusion into the metal, weakening its structure over time.
Pressure and Temperature Exposure: Boilers, condensers, and superheaters often face high temperatures and pressures, intensifying corrosion and material fatigue.

Factors in Stainless Steel Selection for Ammonia

When choosing stainless steel for ammonia-handling equipment, prioritize these five factors:
Corrosion Resistance: The steel must withstand dry, wet, or chlorinated ammonia exposure without degrading.
SCC Resistance: Ammonia can trigger SCC, so stress-resistant alloys are essential.
Strength and Durability: Withstand pressure, temperature, and mechanical load over time.
Weldability: Good weldability ensures reliable joints, especially in heat exchangers and superheaters.
Maintenance and Longevity: The goal is to minimize downtime, replacements, and costly repairs.

Best Stainless Steel Grades for Ammonia Applications

1. Austenitic Stainless Steels

These are the most widely used stainless steels, known for their excellent corrosion resistance and ease of fabrication.
304/304L (1.4301/1.4307):
Affordable and easy to work with.
Suitable for dry ammonia environments.
Prone to SCC in wet ammonia or high-stress conditions.
Best for: Low-pressure ammonia systems and general refrigeration pipes.
316/316L (1.4401/1.4404):
Molybdenum content improves resistance to pitting and SCC.
Better performance in humid or wet ammonia environments.
Best for: Condensers, heat exchangers, and high-humidity ammonia systems.
321 (1.4541):
Titanium-stabilized to resist carbide precipitation.
Handles high-temperature ammonia exposure well.
Still vulnerable to SCC under chloride contamination.
Best for: High-temp superheaters and ammonia vapor systems.

2. Duplex Stainless Steels

Duplex stainless steels combine the strengths of austenitic and ferritic steels, offering higher strength and SCC resistance.
2205 Duplex (1.4462):
Outstanding SCC and pitting resistance.
Higher strength than austenitics — allows thinner walls and weight savings.
Suitable for both dry and wet ammonia environments — even with some chlorides.
Best for: High-pressure ammonia systems, boilers, and large heat exchangers.

3. Ferritic Stainless Steels

They are less common for ammonia but can be viable in specific low-stress applications.
430 (1.4016):
Low cost with reasonable corrosion resistance.
Poor strength and SCC resistance are limited to dry ammonia use only.
Best for: Low-cost, non-critical components like storage tanks.

Stainless Steels to Avoid for Ammonia Systems

Not all stainless steels are up to the task. Here’s what to avoid:
Martensitic Stainless Steels (e.g., 410, 420): Weak corrosion resistance and brittleness make these unsuitable.
High-carbon Austenitic Steels (e.g., 304H): Increased carbon content raises the risk of sensitization and intergranular corrosion.
Unstabilized Grades: Grades without stabilization (e.g., 304) risk weld decay and SCC after high-temp exposure.

Design and Maintenance Tips for Ammonia Systems

Even the best stainless steel without proper design and care won’t perform well. Here’s how to maximize longevity:
Weld Smart: To prevent weld corrosion, use low-carbon grades (like 304L or 316L) or stabilized grades (321).
Polish the Surface: Smooth, polished finishes reduce pitting and crevice corrosion risks.
Avoid Chlorides: Chlorides accelerate SCC — ensure the water in your system is low in chloride content.
Regular Inspections: Monitor for early signs of SCC, pitting, or mechanical wear — especially around welds and high-stress areas.

What’s the Best Stainless Steel for Ammonia?

Choosing the right stainless steel for ammonia handling boils down to the specific environment:
Dry ammonia: 304/304L is economical, but the 2205 duplex offers better strength and SCC resistance.
Wet ammonia: 316/316L holds up well, but 2205 is the more robust, long-term solution.
Chlorinated ammonia environments: 2205 is the clear winner — other grades will corrode faster and fail sooner.
If reliability, safety, and longevity matter — especially in high-stress, high-temperature, or high-chloride environments — 2205 Duplex is your best choice!

If you have Ammonia projects for stainless steel pipes, pipe fittings, flanges and plates, please feel free to contact us at [email protected] for a competitive quote!