Hangbo Alloy Group
Shanghai · Est. 2012
Anhydrous ammonia and ammonia crackers challenge carbon steel and many stainless grades. See which nickel alloys resist SCC and how Hangbo supplies them.
Ammonia is made at industrial scale by the Haber-Bosch reaction (N2 + 3H2 → 2NH3) at roughly 400–500°C and 150–300 bar, then stored and shipped as anhydrous liquid, aqueous solution or, increasingly, as 'green ammonia' made from renewable hydrogen. Both the synthesis loop and everything downstream bring classic material headaches: low-temperature embrittlement of carbon steel, stress-corrosion cracking of some austenitic stainless grades in ammoniacal service, and nitriding of susceptible alloys inside the converter. Choosing the right nickel alloy is what keeps an ammonia plant available and, just as important, safe. As demand for ammonia as both fertilizer and energy carrier grows, the material question moves from a back-room concern to a board-level availability risk.
Dry anhydrous ammonia at moderate temperature is benign to carbon steel, which is why much storage is built from it — but carbon steel turns brittle when the metal gets cold, and it is the wrong choice for aqueous or ammoniacal solutions. Ordinary 300-series stainless, meanwhile, is prone to stress-corrosion cracking in polythionic and ammoniacal environments, and some grades nitride in the hot converter. Nickel-rich alloys sidestep both failure modes: they resist nitriding and they are far less SCC-sensitive in ammonia. The embrittlement side of the problem is covered in our hydrogen embrittlement notes, which matter directly for cracked-ammonia hydrogen.
Two shutdown-related mechanisms deserve special attention. Polythionic acid forms when sulfur-bearing equipment is shut down and then exposed to air and moisture, attacking sensitized stainless — a reason ammonia plants with any sulfur history avoid 300-series in wet lines. And the ammonium carbamate formed in any urea co-production is aggressively corrosive, pushing the boundary duty toward C-276 rather than 825.
For low-temperature liquid-ammonia storage the cold-brittleness issue is the same one faced in LNG service, which is why the cryogenic grading in our cryogenic nickel-alloy guide applies directly.
Green ammonia is produced by feeding renewable hydrogen into the Haber-Bosch loop, and it is also being positioned as a hydrogen carrier: an ammonia cracker reverses the reaction (2NH3 → N2 + 3H2) at 400–600°C over a catalyst inside nickel-alloy reactor tubing. That cracker duty is a hydrogen-embrittlement and high-temperature nitriding problem at the same time, which is exactly why nickel-rich alloys win over stainless. The failure modes are documented in our hydrogen embrittlement article.
Cracker tubing runs at 400–600°C under an ammonia/hydrogen/nitrogen mix, so the tube material must resist both nitriding and embrittlement at once; Incoloy 800H or 625 tubes with controlled carbon are the usual answer. The purified hydrogen that emerges then feeds fuel cells or e-fuels, which means the alloy choice in the cracker quietly sets the quality ceiling for the whole downstream chain.
Specify low-carbon grades (Nickel 201, not 200) wherever the metal runs hot, keep welds clean and fully fused to avoid crevices that invite SCC, and verify every heat with EN 10204 3.1 certification and positive material identification. Because ammonia is both toxic and under pressure, traceability is not paperwork — it is the difference between a safe plant and an incident.
Inspection backs the metallurgy. Baseline ultrasonic thickness and positive-material-identification mapping at commissioning, then targeted re-checks at each turnaround, catch sulfidation thinning or mixed-material errors before they leak. Given ammonia's toxicity under pressure, that discipline is what turns a good alloy specification into a genuinely safe plant.
For dry anhydrous ammonia at moderate temperature, yes — but carbon steel goes brittle when it gets cold and is unsuitable for aqueous or ammoniacal service. Where duty mixes temperatures or phases, a nickel alloy is the safer call; the low-temperature angle is covered in our cryogenic nickel-alloy guide.
Nickel-rich alloys — Monel 400, Alloy 600/625 and Nickel 200/201 — are far more resistant to SCC in ammoniacal service than standard austenitic stainless. Monel 400 in particular is the long-standing choice for aqueous ammonia; see the Monel 400 guide.
Nickel 201 caps carbon at 0.02% so it does not carburize or nitride-embrittle at the high temperatures inside an ammonia converter, where Nickel 200 would gradually lose ductility. For cooler anhydrous service Nickel 200 is adequate.
As a dedicated nickel alloy manufacturer serving chemical processing, marine, oil & gas, power generation sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.