Ammonia is not broadly corrosive to most metals in the conventional sense of eating away at surfaces. Corrosion tests exposing a wide range of metals and alloys to liquid ammonia for up to eight months found very low weight losses, and sometimes weight gains, leading researchers to conclude that most metals are essentially inert in liquid ammonia when it comes to general surface corrosion.1CORROSION. Corrosion Performance of Some Metals and Alloys in Liquid Ammonia That said, the story is far more complicated than “no, ammonia is safe for metals.” Copper and its alloys are genuinely attacked and dissolved by ammonia, and steel can crack catastrophically under certain conditions without losing any visible surface metal at all. Whether ammonia damages a given metal depends on the metal itself, the contaminants dissolved in the ammonia, the temperature, and whether the metal is under mechanical stress.
Why Ammonia Looks Harmless on Paper
Ammonia and ammonium hydroxide are not particularly corrosive in themselves. Corrosion problems arise primarily with specific materials and when contaminants like oxygen or air are present.2ASM International. Corrosion by Ammonia Pure, dry anhydrous ammonia sits in carbon steel tanks across the world without incident for years. The reason is that many metals form thin protective films on their surface when exposed to ammonia, and as long as those films remain intact, the metal underneath stays sound. Steel in ammonia contaminated with small amounts of oxygen, for instance, forms a passive film that shifts its electrochemical potential in a direction indicating protection.3CORROSION. The Electrochemistry and Mechanism of Stress Corrosion Cracking of Constructional Steels in Liquid Ammonia
This is why people who work with ammonia day-to-day sometimes think of it as benign toward metals. If you measure corrosion by how much metal dissolves into the liquid, ammonia barely registers for most engineering alloys. The danger lies in a different kind of failure altogether.
Copper and Brass Are the Big Exception
If there is one metal family you should keep away from ammonia, it is copper and anything made from copper alloys, including brass and bronze. Ammonia attacks copper through a chemical process that forms soluble copper-ammonia complexes. In ammonia water containing dissolved oxygen, copper loses electrons and combines with ammonia molecules to form these complexes, which dissolve into the liquid and carry the metal away. Copper oxides that form on the surface also react with ammonia to create more of these complexes, accelerating the process rather than offering any protection.4Arabian Journal of Chemistry. Corrosion behavior and mechanism of copper soaked in ammonia water
Brass, an alloy of copper and zinc, faces an even worse outcome because it is vulnerable to stress corrosion cracking in ammonia. This is an old and well-documented problem sometimes called “season cracking,” named after ammunition cartridge cases that cracked during monsoon seasons in tropical climates where ammonia from decaying organic matter was in the air. In laboratory testing, brass specimens under stress and exposed to aqueous ammonia solution develop intergranular cracking, meaning the cracks run along the boundaries between metal grains, weakening the material from within.5Metallurgical Transactions A. Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis
This is why plumbing codes and refrigeration guidelines are strict about keeping ammonia away from copper tubing and fittings. Even trace amounts of ammonia in a building’s air, from cleaning products or a small refrigerant leak, can over time attack exposed copper surfaces. If you have copper pipes or brass valves anywhere near an ammonia source, the risk is real and not hypothetical.
Stress Corrosion Cracking in Steel
Carbon steel is the workhorse metal for ammonia storage tanks, pipelines, and process equipment. It resists general corrosion from ammonia well, but it has an Achilles’ heel: stress corrosion cracking, or SCC. This is a failure mode where a metal that looks perfectly fine on the outside suddenly cracks, sometimes through the full wall thickness of a pressure vessel, without any visible thinning or pitting beforehand. The cracks form when three conditions overlap: a susceptible material, a corrosive environment, and tensile stress in the metal.
SCC in carbon steel ammonia tanks was identified as a serious industrial problem decades ago when agricultural ammonia storage vessels began failing unexpectedly. Investigations determined that stress corrosion cracking was the cause of these failures, and subsequent research focused on understanding which contaminants in the ammonia promoted or inhibited the problem.6CORROSION Journal. Stress-Corrosion Cracking of Steels In Agricultural Ammonia
The two contaminants that matter most are oxygen and water, and they push in opposite directions.
Oxygen Promotes Cracking, Water Prevents It
Oxygen is the primary driver of ammonia SCC in steel. Research has consistently shown that oxygen contamination is necessary for stress corrosion cracking of mild steel, with levels as low as one part per million sufficient to trigger cracking when water is absent.7National Board of Boiler and Pressure Vessel Inspectors. Stress Corrosion Cracking of Steel in Liquefied Ammonia Service – A Recapitulation At higher concentrations, around 200 parts per million, the effect becomes even more pronounced. However, there is an interesting twist: very high oxygen levels can actually inhibit cracking by forming a thicker, more robust iron oxide film on the steel surface.8International Journal of Hydrogen Energy. Stress corrosion cracking of steels in liquid ammonia: A comprehensive literature review In practice, though, the oxygen levels found in real-world ammonia systems tend to fall in the dangerous middle range rather than at the protective extreme.
Water, on the other hand, is ammonia’s safety net. Repeated studies have shown that as little as 0.1 percent water in liquefied ammonia inhibits stress corrosion cracking in the carbon steels used for pressure vessels.7National Board of Boiler and Pressure Vessel Inspectors. Stress Corrosion Cracking of Steel in Liquefied Ammonia Service – A Recapitulation For agricultural ammonia service, the recommended minimum water content is 0.20 percent, along with eliminating air from the system and relieving residual stresses from welding.6CORROSION Journal. Stress-Corrosion Cracking of Steels In Agricultural Ammonia
This might seem counterintuitive. Water causes rust on steel exposed to air, so why would it protect steel inside an ammonia tank? The mechanism involves water’s effect on the electrochemistry at the steel surface. In simplified terms, water helps maintain a stable protective film on the steel that prevents the ammonia and oxygen from attacking the metal at vulnerable spots like grain boundaries and weld zones. Without enough water, those protective films break down in localized areas, and cracks initiate.
The Vapor Space Problem
One of the more insidious findings in ammonia corrosion research involves what happens above the liquid line in a storage tank. Even when liquid ammonia contains enough water to inhibit cracking, the protective benefit does not automatically carry over to the vapor space above it. European reports documented stress corrosion cracking in the vapor space of large ammonia storage spheres that contained water-inhibited liquid ammonia below.7National Board of Boiler and Pressure Vessel Inspectors. Stress Corrosion Cracking of Steel in Liquefied Ammonia Service – A Recapitulation
The reason is physics. When ammonia evaporates from the liquid, oxygen and nitrogen partition heavily into the vapor phase, while water stays behind in the liquid. The partition coefficients are dramatic: oxygen concentrates in the vapor roughly 587 times more than in the liquid, and nitrogen about 603 times more. Meanwhile, calculations and measurements confirmed that the vapor over liquid ammonia is remarkably dry compared to the liquid itself. So when ammonia vapor condenses on a cool steel surface, such as the top of a tank on a cool night, the condensed liquid carries far more oxygen and far less water than the bulk liquid below. That condensate hits the steel under exactly the conditions most favorable for stress corrosion cracking: high oxygen, low water.
This discovery changed how the industry designs and operates ammonia storage vessels. It means that simply adding water to the bulk ammonia is not sufficient protection for every part of the tank. Vapor-space surfaces, particularly in large vessels that experience temperature swings, need additional protection through stress relief of welds, material selection, or other engineering controls.
Which Metals Hold Up Best
For anyone choosing materials for ammonia service, the hierarchy is fairly clear. Stainless steels generally perform well, resisting both general corrosion and stress corrosion cracking far better than plain carbon steel. Nickel-based alloys are among the most resistant, particularly in hot ammonia environments where the gas can decompose and release nitrogen that attacks other metals. Testing across a range of steels and nickel alloys in ammonia-bearing atmospheres of varying aggressiveness showed that nickel-base alloys in the iron-nickel-chromium family were substantially more resistant to nitrogen attack than alloys richer in iron and chromium.9CORROSION. Behavior of Stainless Steels and Other Engineering Alloys In Hot Ammonia Atmospheres
Aluminum alloys also perform well in ammonia service, showing minimal general corrosion. Titanium, zirconium, and tantalum are all highly resistant, though their cost limits them to specialized applications. The ASM International handbook on ammonia corrosion covers the full range of these materials, confirming that the selection of resistant alloys is well established for industrial practice.2ASM International. Corrosion by Ammonia
The metals to avoid are copper and its alloys (for the dissolution and cracking reasons covered above), zinc (which corrodes in ammonia), and plain carbon steel in any service where oxygen contamination cannot be controlled and water content cannot be guaranteed. Carbon steel remains the standard for ammonia tanks and pipelines because it is economical and performs well when contaminant levels are managed, but it demands ongoing attention to ammonia purity and stress conditions that more expensive alloys do not.
Ammonia as a Future Fuel and Its Pipeline Challenges
Ammonia is gaining attention as a potential carrier for hydrogen energy, which means existing natural gas pipelines are being evaluated for possible reuse in ammonia service. This has pushed ammonia corrosion research back into the spotlight. The primary concern for carbon steel pipelines carrying liquid ammonia remains stress corrosion cracking driven by trace impurities and local stress, not general wall thinning. External corrosion of carbon steel pipelines can be managed through standard coatings and cathodic protection, but internal degradation in liquid ammonia service is dominated by SCC that is highly sensitive to trace impurities and local stress concentrations.10International Journal of Hydrogen Energy. Pipeline transport of ammonia as a hydrogen carrier: Opportunities and challenges
The challenge for pipeline operators is that natural gas pipelines were not designed with ammonia SCC in mind. Weld procedures, post-weld heat treatments, and impurity specifications that work perfectly for natural gas may not be adequate for ammonia. Residual stresses from welding are a particular concern, since ammonia SCC tends to initiate at sites of high tensile stress. Converting a pipeline from gas to ammonia service is not just a matter of switching fluids; it requires careful evaluation of the entire system’s metallurgical condition.
Household and Commercial Ammonia Exposure
Most people encounter ammonia not as a pressurized industrial chemical but as a dilute cleaning solution, typically around 5 to 10 percent ammonium hydroxide in water. At these concentrations, the risk to most household metals is low. Stainless steel sinks, aluminum cookware, and steel tools can generally tolerate brief contact with household ammonia cleaners without damage. The water content alone provides significant protection against the stress cracking mechanisms that dominate industrial failures.
The metal you should genuinely worry about in a household context is copper, along with anything plated or alloyed with copper. Brass faucets, copper decorative items, bronze hardware, and copper-bottomed cookware can all be damaged by ammonia-based cleaners. The same dissolution mechanism that threatens industrial copper applies at the kitchen sink: ammonia pulls copper ions into solution, forming those blue-tinted copper-ammonia complexes that represent your metal leaving permanently.4Arabian Journal of Chemistry. Corrosion behavior and mechanism of copper soaked in ammonia water If you have ever seen a brass fixture turn dull and slightly blue-green after exposure to an ammonia cleaner, that is active corrosion happening in real time.
Jewelry is another practical concern. Gold and silver themselves resist ammonia, and jewelers sometimes use dilute ammonia solutions to clean gold pieces. But many jewelry findings, clasps, and plated items contain copper or brass underneath, and prolonged soaking in ammonia can attack those base metals even if the surface layer appears fine. The same applies to electronics: printed circuit boards use copper traces extensively, and ammonia exposure from cleaning or industrial processes can cause corrosion at copper surfaces and solder joints.
High-Temperature Ammonia and Nitriding
At elevated temperatures, ammonia’s relationship with metals changes character. Instead of the dissolution or stress cracking seen at or near room temperature, hot ammonia breaks down into nitrogen and hydrogen. The nitrogen can then diffuse into the metal surface, forming hard, brittle nitride compounds in a process called nitriding. This is actually exploited deliberately in some manufacturing processes to harden steel surfaces, but when it happens unintentionally in ammonia-processing equipment, it embrittles the metal and leads to cracking.
Nickel-based alloys resist nitriding far better than iron-chromium steels, which is why they are preferred for high-temperature ammonia converters and cracking reactors.9CORROSION. Behavior of Stainless Steels and Other Engineering Alloys In Hot Ammonia Atmospheres The nickel in these alloys is simply less reactive with nitrogen than iron or chromium, so the damaging nitride layer forms much more slowly, if at all. For anyone designing or maintaining equipment that handles ammonia at temperatures above roughly 300°C, material selection shifts from worrying about SCC to worrying about nitriding, and the alloy choices change accordingly.
Common Misconceptions About Ammonia and Metal
One persistent misunderstanding is that “corrosion-resistant” means “immune.” Stainless steel resists ammonia well under most conditions, but it is not immune to all forms of ammonia-related damage. Certain stainless grades can experience chloride-assisted stress corrosion cracking when ammonia contains chloride contaminants, and even resistant grades will nitride at high enough temperatures. No single metal is universally safe in every ammonia condition.
Another common error is assuming that because a tank has held ammonia for years without problems, it will continue to do so indefinitely. Stress corrosion cracking is time-dependent and often invisible until the moment of failure. A tank can develop cracks that propagate slowly through the wall over years before suddenly running to failure. The absence of visible corrosion products or wall thinning does not mean the metal is healthy. This is why ammonia storage vessels in industrial and agricultural service require periodic inspection using methods that can detect cracks, not just wall thickness measurements.
A third misconception applies specifically to anyone using ammonia around the home or workshop: the belief that brief exposure does not matter. For steel and aluminum, this is mostly true. For copper and brass, even short contact with ammonia cleaners can initiate surface damage, and repeated short exposures accumulate over time. If you routinely clean a kitchen with ammonia-based products and your faucet hardware is brass, the damage is cumulative even if each individual exposure seems harmless.