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Ammonia corrosion resistance depends on the complete service, not on a single “ammonia-proof” material. Define whether the system contains anhydrous or aqueous ammonia, liquid or vapor, its concentration, temperature, pressure and impurities, then check the applicable pressure-equipment or installation code. Carbon steel is a common candidate, while low-temperature service may require specially qualified steels. Stainless steel can be compatible in some duties, but grade, temperature, code coverage and environment still matter. Copper, brass, zinc and galvanized parts should be excluded from anhydrous-ammonia refrigeration systems in accordance with U.S. EPA Region 7 guidance. Treat an internal material-selection decision separately from an external coating decision: a paint system that protects the outside of steel piping is not automatically suitable for an ammonia-wetted interior.
This guide gives a practical selection process, explains stress-corrosion cracking (SCC), and shows how insulation, moisture, fabrication and inspection change the risk.
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What must be defined before choosing an ammonia-resistant material?
The first step is to write a service description that a materials or corrosion engineer can review. The Health and Safety Executive (HSE) materials guidance states that construction materials for ammonia depend on operating temperature; its examples distinguish ambient-temperature mild steel from special steels needed at low temperatures. The same guidance warns that impurities in liquid ammonia, including air or carbon dioxide, can contribute to stress-corrosion cracking of mild steel.
Record these variables for every line, vessel, valve and fitting:
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- Ammonia form: anhydrous ammonia, aqueous ammonia or a mixture; liquid, vapor or two-phase flow.
- Concentration and purity: identify water, oxygen or air, carbon dioxide, chlorides, sulfates and any process contaminants.
- Temperature: normal, start-up, shutdown, upset and minimum design temperatures. A metal that is satisfactory at ambient temperature may lose toughness in refrigerated service.
- Pressure and cycling: include design pressure, vacuum, pressure fluctuations and thermal cycling.
- Stress state: account for operating stress, cold forming, weld residual stress, attachments and constrained thermal expansion.
- Exposure location: specify whether ammonia contacts the surface internally or the surface is exposed only to air, wash water, rain or wet insulation.
- Code and jurisdiction: identify the pressure-vessel, piping, refrigeration, marine or site code that governs the installation.
- Inspection and repair access: note insulation, supports, drains, low points and areas that will be difficult to examine after commissioning.
Do not approve a material from a generic compatibility chart alone. The design basis should connect the process specification, mechanical design, fabrication procedure and inspection plan.
Which metals are candidates for ammonia service?
There is no universal ranking. The following matrix shows how the main candidates should be treated during an engineering review.
| Material or component | When it may be considered | Important limitations |
|---|---|---|
| Carbon or mild steel | Common candidate for many ammonia pipes, tanks and refrigeration components when temperature, grade, stress and code requirements are satisfied. | Low-temperature toughness, impurities and weld residual stress can change the risk. General surface compatibility does not eliminate SCC. |
| Low-temperature or fine-grained carbon-manganese steel | Refrigerated or cryogenic duties where toughness and specified strength are controlled. | Use only within the code and design context for which the grade, weld procedure and impact properties are qualified. |
| Austenitic stainless steel such as 304 or 316 | Potentially useful where the design code permits it and the temperature, concentration and environment are suitable. The 2025 American Bureau of Shipping (ABS) advisory describes high chemical compatibility of liquid and gaseous ammonia with SS316. | Code coverage is not universal. The British Stainless Steel Association (BSSA) notes that the assumption of no corrosion risk for 304/316 in bulk ammonia is not substantiated by all published data. Chlorides, wet insulation and marine exposure can create external problems. |
| Copper, brass, zinc and galvanized components | Generally not acceptable for anhydrous-ammonia refrigeration wetted parts. | EPA Region 7 guidance says ammonia is highly corrosive to copper, brass and galvanized surfaces and materials and advises against copper, brass, zinc or galvanized components anywhere in an anhydrous-ammonia refrigeration system. |
| External primer and coating on steel | Protection of uninsulated external steel from atmospheric corrosion, provided the coating system is specified for the substrate and environment. | It is not evidence of internal ammonia compatibility and does not prevent SCC. Surface preparation, temperature, wet insulation and maintenance determine performance. |
Check every wetted item, not just the pipe wall. Valve bodies, trim, strainers, flexible connectors, instrument connections, gaskets, bolts and temporary construction components can introduce a prohibited alloy or a galvanic couple.
Is carbon steel suitable for ammonia?
Carbon steel is often suitable for ammonia equipment, but “carbon steel” is not a complete specification. Grade, yield strength, toughness, weld condition, impurities and design temperature must all be verified against the governing code.
For a code-specific example, the 2025 ABS advisory discusses all-welded carbon-manganese steel tank walls under the IGC Code. In that tank-construction context, the cited provisions specify fine-grained steel, a specified minimum yield strength not exceeding 355 MPa and an actual yield strength not exceeding 440 MPa. Those figures are not general limits for every ammonia pipe or vessel; they illustrate why the applicable code and equipment category must be named before a strength value is used.
At low temperature, ask for impact-toughness data and the minimum design metal temperature, not only tensile strength. HSE says special steels may be required at low temperatures to avoid embrittlement. Review weld procedure qualifications, heat-affected-zone toughness, post-weld treatment where required, and the effect of attachments that restrain thermal movement.
For ambient service, mild steel may be a practical choice when the ammonia specification is controlled and SCC safeguards are in place. For refrigerated service, select the grade and weld details through the applicable pressure, refrigeration or marine code. Never substitute a higher-strength grade simply to reduce wall thickness without checking whether the increased strength and residual stress raise cracking susceptibility.
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Can stainless steel be used with ammonia?
Some stainless steels can provide good ammonia compatibility, but stainless is not an automatic solution. ABS reports high chemical compatibility between liquid or gaseous ammonia and SS316 in its containment-systems advisory, while also noting that SS316 is outside the IGC Code coverage discussed there. Code acceptance, temperature, concentration, stress and external environment still have to be checked.
The BSSA’s discussion of stainless steels for ammonia handling is a useful warning against blanket statements about 304 or 316. Published data do not substantiate a general assumption that these grades have no corrosion risk in bulk ammonia. In particular, inspect for chloride-bearing moisture, deposits, crevices, weld heat tint and wet insulation. A stainless alloy can be appropriate for an internal process surface yet suffer external corrosion in a coastal or contaminated atmosphere.
For insulated tanks that can become wet and absorb chlorides, BSSA discusses painting or an aluminium-foil coating on the outside face of 304. It also says 316 should be considered for exposed parts at coastal or marine locations. These are external-surface recommendations for the described environment, not a universal approval for ammonia-contact components.
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For anhydrous-ammonia refrigeration, EPA Region 7’s instruction is direct: do not use copper, brass, zinc or galvanized components in any part of the system. The restriction includes fittings, valves and small instrument connections that are easy to overlook during procurement or maintenance.
The HSE likewise states that “ammonia is highly corrosive towards copper and zinc.” Treat that statement as a material-screening rule, then verify the exact alloy and plating on every component. Do not assume that a galvanized exterior is safe because the process fluid is inside a steel pipe; ammonia leaks, condensate and wash water can expose the coating or create contaminated pockets.
This guidance is specifically important for anhydrous-ammonia refrigeration. Aqueous ammonia, dilute solutions and contaminated services can have different chemistry. The service specification and applicable code should control any exception rather than transferring a rule from one ammonia form to another.
What is ammonia stress-corrosion cracking and how can it be reduced?
Stress-corrosion cracking is localized cracking that occurs when a susceptible material, a compatible cracking environment and tensile stress act together. It can threaten containment without producing a large area of uniform wall loss, so a sound-looking surface is not proof that SCC is absent.
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HSE identifies air or carbon dioxide impurities in liquid ammonia as concerns for mild steel. The 2023 ISGINTT safety guide lists material construction, residual stress, cargo temperature, pressure and impurities as contributing factors and notes that the reaction proceeds faster at higher temperatures. The Singapore maritime safety analysis also highlights weld-related integrity issues in carbon-steel ammonia equipment.
Reduce SCC risk by controlling all three sides of the triangle:
- Material susceptibility: select a grade, strength level, toughness and weld condition qualified for the service and code. Avoid unapproved high-strength substitutions.
- Stress: design out unnecessary concentrations, qualify forming and welding, control fit-up and residual stress, and examine welds and attachments in the inspection plan.
- Environment: control ammonia purity, prevent air ingress where the design requires it, manage temperature and pressure excursions, and keep contamination out of the process.
ISGINTT lists refrigerated storage below −30°C, low-yield-strength steels, thermal stress relief of tank welds and adding 0.2% water among risk-reduction measures in its cargo guidance. These are context-specific measures, not a general operating recipe. Do not change ammonia composition or impose a temperature limit without confirming the current code, ammonia specification and engineering design.
A coating cannot remove SCC susceptibility from the metal beneath it. If cracking is suspected, isolate the equipment safely and obtain a qualified inspection and materials-engineering assessment; do not rely on repainting or a visual check.
What coating should be used on ammonia piping?
For uninsulated external steel piping, EPA Region 7 recommends cleaning, priming and painting with an appropriate corrosion-protective coating that remains consistent with the site’s color-coding scheme. The source supports an external coating system, not one universal chemistry or product.
Specify the coating only after answering these questions:
- What is the substrate and its existing condition: bare carbon steel, stainless steel, aged paint or partially rusted metal?
- What surface preparation is possible, and what cleanliness and profile does the coating manufacturer require?
- What temperatures will the coating see during operation, shutdown and application?
- Will the surface be exposed to rain, salt spray, wash water, chemicals or ultraviolet light?
- Will insulation cover the pipe, and can water enter through damaged jacketing or a failed vapor barrier?
- What dry-film thickness, primer/intermediate/topcoat combination, curing time, inspection and repair procedure does the site specification require?
- Will the color and markings remain legible and compatible with emergency response requirements?
ASHRAE’s refrigerant-piping insulation guidance explains that chlorides and sulfates from insulation manufacture or external sources can hydrolyze in water and form corrosive acids. It points to coating-system options for carbon steel and recommends consulting coating or pipe manufacturers for system details. Select a coating that is qualified for the actual substrate, preparation, temperature, wet-insulation scenario and external environment.
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Do not apply an external pipe coating to an ammonia-wetted interior unless the coating manufacturer and the governing engineer have specifically qualified that product for direct ammonia contact, immersion, temperature, pressure and cure conditions. EPA’s recommendation for painted uninsulated piping does not constitute approval of an internal lining.
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How should insulation and moisture change the decision?
Insulation can reduce heat gain yet conceal external corrosion. A failed jacket, vapor barrier or seal can allow water to remain against the pipe; dissolved chlorides and sulfates can then create an aggressive environment. Supports, penetrations, low points and damaged cladding deserve particular attention because they are common moisture pathways.
For stainless storage tanks and piping in wet or marine locations, use the environment-specific advice from BSSA rather than assuming the alloy solves every problem. For carbon steel, the coating and insulation system must be designed together: a coating that performs in dry air may fail when trapped moisture remains beneath insulation.
Keep drainage and inspection access in the design. Avoid details that create stagnant pockets, and make it possible to remove or open insulation at representative high-risk locations without damaging the system unnecessarily.
What is a practical material-and-coating selection workflow?
Use the following sequence for a new installation, modification or repair.
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- Identify the governing code. Confirm the jurisdiction and whether the item is refrigeration piping, process piping, a pressure vessel, storage tank, marine containment or another regulated category.
- Separate wetted and non-wetted surfaces. Create two material lists: every item that contacts ammonia and every surface exposed to atmosphere, insulation or wash water.
- Screen out prohibited alloys. For anhydrous-ammonia refrigeration, remove copper, brass, zinc and galvanized components from the bill of materials, then check valves, instruments, fasteners and temporary items.
- Shortlist grades by temperature and stress. Compare carbon steel, low-temperature steels and stainless options using code-approved grades, toughness, strength, weldability and SCC susceptibility.
- Review fabrication. Require qualified welding and forming procedures, control residual stress and specify nondestructive examination appropriate to the consequence of a leak.
- Specify external protection. For uninsulated steel, select a complete primer/coating system for the prepared substrate and environment. For insulated systems, address vapor barriers, jacketing, seals and water drainage as part of the corrosion-control design.
- Obtain compatibility evidence. Request written manufacturer data for ammonia concentration, temperature, immersion or splash exposure, cure requirements and any restrictions. Treat generic “chemical resistant” language as insufficient.
- Plan inspection before commissioning. Establish baseline thickness or condition records where appropriate, accessible inspection points, insulation-removal locations, coating checks and acceptance criteria.
- Approve through a qualified review. Have a corrosion/materials engineer and the responsible code engineer sign off the final selection, especially for low-temperature, high-pressure, marine or contaminated service.
How should ammonia equipment be inspected and maintained?
Inspection is an integrity control, not an optional check after painting. EPA Region 7 advises periodic inspection of refrigeration piping for failed insulation or vapor barriers, rust and corrosion, including careful inspection beneath failed insulation. Damaged or deteriorated piping should be replaced rather than concealed with a new coat.
Build inspection routes around likely damage locations:
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- under insulation, jacketing laps and vapor-barrier terminations;
- pipe supports, clamps, guides and points where water can collect;
- welds, attachments, dead legs, drains and low points;
- coating holidays, blisters, delamination, impact damage and exposed steel;
- valves, flanges, instrument take-offs and dissimilar-metal connections;
- external surfaces exposed to marine salt, chlorides, sulfates or repeated wash-down.
The Singapore report identifies external corrosion and weld-related issues as important causes of integrity loss in carbon-steel ammonia equipment, with likely corrosion locations on wet external surfaces beneath insulation. Record findings, repair the moisture source, restore the coating or insulation system to its specification, and reassess hidden metal before returning equipment to service.
What common selection mistakes should be avoided?
- Calling a metal “ammonia-proof.” Compatibility changes with phase, concentration, temperature, impurities and stress.
- Using a stainless grade without checking the code. A material may be chemically compatible yet not permitted for the equipment category or temperature.
- Confusing external paint with internal compatibility. A primer protects atmospheric steel; it does not qualify a wetted lining.
- Ignoring small components. One brass valve, zinc-plated fitting or galvanized accessory can violate the materials specification.
- Assuming no visible rust means no SCC. SCC can be localized and concealed; welds and stressed areas require appropriate examination.
- Painting over contamination or wet insulation. Inadequate preparation traps moisture and leaves the corrosion mechanism active.
- Applying cargo guidance universally. The −30°C and 0.2% water figures from ISGINTT belong to its specific cargo guidance and require engineering and code verification.
- Substituting high-strength steel to save weight. Strength, residual stress and SCC susceptibility must be evaluated together.
- Designing without future access. If insulation, supports or jacketing prevent examination, the integrity program will be weaker than the material datasheet suggests.
When is a materials engineer’s review essential?
Obtain a qualified corrosion/materials review for any new ammonia service, low-temperature or high-pressure equipment, change in ammonia purity, repair after cracking, replacement with a different alloy, coating on a wetted surface, or installation in a marine or chloride-bearing environment. The reviewer should reconcile the process specification, code, material certificates, weld procedures, coating data and inspection plan.
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For design and repair decisions, verify the current jurisdictional requirements rather than relying on an old drawing or a supplier’s generic compatibility statement. The sources cited here provide decision principles and context-specific guidance; they do not replace the code calculations or approval required for a particular installation.
Bottom line: how do you choose?
Choose the material first from the defined ammonia service and governing code, then control SCC through grade, toughness, strength, fabrication, stress and impurity management. Use carbon steel where its grade and temperature range are qualified; use low-temperature steels or stainless only when their code coverage and environmental limits are demonstrated. Exclude copper, brass, zinc and galvanized components from anhydrous-ammonia refrigeration systems. Finally, specify external coatings as part of a complete surface-preparation, insulation, moisture-control and inspection program—and never treat paint as proof that an ammonia-wetted surface is compatible.
Frequently Asked Questions
Does ammonia corrode ordinary steel?
Ordinary or mild steel can be suitable in some ambient-temperature ammonia duties, but impurities, tensile or weld residual stress and low temperature can create cracking or embrittlement risks. The grade, weld condition and service specification must be checked against the governing code before use.
Is 316 stainless steel always safe for ammonia?
No. Published guidance describes high chemical compatibility of SS316 with liquid and gaseous ammonia in a particular containment context, but code coverage and external conditions still apply. Chlorides, wet insulation, stress and marine exposure can introduce corrosion risks.
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Only if a coating manufacturer and the responsible engineer have specifically qualified the lining for the exact ammonia concentration, temperature, pressure, immersion and curing conditions. An EPA recommendation for painting uninsulated external piping does not approve an internal coating.
How do I detect corrosion hidden under ammonia-pipe insulation?
Inspect failed jacketing, vapor barriers, supports, penetrations and low points, then open insulation at representative high-risk locations using the site’s safe-work procedure. Look for wetness, rust, coating damage and weld-related indications; replace deteriorated piping rather than simply covering it.
Why are galvanized fittings prohibited in ammonia refrigeration?
EPA Region 7 identifies ammonia as highly corrosive to copper, brass and galvanized surfaces and advises against copper, brass, zinc or galvanized components anywhere in an anhydrous-ammonia refrigeration system. The check must include valves, instruments, fasteners and other small accessories, not only the main pipe.
