Hangbo Alloy Group
Shanghai · Est. 2012
Urea runs ammonium carbamate at 180-220 C and 200 bar, among the most corrosive services. See which nickel alloys survive strippers, condensers and pumps.
Urea (carbamide) is the world's most widely produced nitrogen fertilizer, and one of the harshest corrosion environments in the entire chemical industry. Modern large-capacity urea plants, rated from 1,500 to over 4,000 tonnes per day, run their strippers at 180-200 bar and 180-220 C in a medium of liquid ammonium carbamate saturated with carbon dioxide and ammonia. Very few alloys survive that combination of high pressure, high temperature, and aggressive chemistry without selective corrosion. Specifying the right nickel alloy is the difference between a plant that runs for a decade and one that is stripped out for relining within a couple of years.
Urea synthesis passes directly through ammonium carbamate, a melt that aggressively attacks stainless steels by dissolving their passive chromium oxide film. Even conventional 316L stainless develops intergranular attack and localized penetration in a matter of months when exposed to hot, concentrated carbamate. The corrosion rate in this service is governed by three variables: carbamate concentration, temperature, and the level of passivating oxygen held in the loop.
A small upset in passivator injection can swing a stable plant into severe corrosion within hours, which is why material selection is treated so conservatively in urea duty. The cost of an unplanned stripper outage - lost production plus a full reline - dwarfs any saving made by down-specifying the alloy.
The chemistry is worth understanding before any grade is chosen. Carbon dioxide and ammonia first form ammonium carbamate, which then dehydrates to urea and water: CO2 + 2NH3 = NH2COONH4 (carbamate) = NH2CONH2 + H2O. The carbamate intermediate - not the urea product - is the corrosive species. It attacks by stripping chromium from the surface, and once the chromium-depleted layer forms, attack accelerates.
The classic defense is a tightly controlled corrosion allowance combined with a high-alloy material that holds its passive film under the passivated regime. Urea-grade 316L (low silicon, low carbon) is used only in the lowest-risk zones, while the high-pressure loop demands purpose-designed nickel-containing grades.
The workhorse of the urea loop is urea-grade 25-22-2 (Sandvik 2RE69 type, 00Cr25Ni22Mo2 with very low silicon). It is the standard for stripper tubes and liners in the majority of plants worldwide. Where the duty is more severe, or where operators want a wider safe operating window, the higher-nickel grades take over.
Titanium and zirconium appear at the bottom of some strippers, but nickel-containing grades dominate the carbamate loop because they combine fabricability with the required corrosion margin.
Material is matched to duty, not applied uniformly. A sensible baseline specification for a modern large plant looks like this:
Every heat should be supplied with EN 10204 3.1 certification and verified chemistry, because silicon and carbon control are exactly what make a grade 'urea grade' rather than just a similar analysis.
Urea-grade 316L limits silicon to 0.20% or below (often 0.10%) and carbon to 0.03% to resist intergranular and selective corrosion in carbamate, while standard 316L with higher silicon and impurity segregation fails rapidly in stripper service. For the wider chloride picture, see our guide to chloride stress corrosion cracking in nickel alloys.
A controlled passivator dose of roughly 0.2-0.5% oxygen keeps a protective chromium-rich oxide film on the alloy surface. Without it, carbamate strips the passive layer and corrosion accelerates quickly. This is precisely why urea-grade stainless and high-nickel alloys are specified - they hold the film under the passivated regime far better than ordinary grades.
Alloy 31 (N08031) offers higher chromium (about 27%) with added molybdenum and nitrogen, giving a wider safe operating window and better resistance to carbamate upsets. Both Alloy 31 and Alloy 28 are used where 25-22-2 reaches its practical limit, especially in condensers and pump casings.
Generally no. The high-pressure stripper and carbamate condenser demand the highest alloys (25-22-2 or Alloy 31), while lower-pressure decomposers may use urea-grade 316L. A full corrosion-resistant nickel alloy review helps match each grade to its exact duty and cost target.
As a dedicated nickel alloy manufacturer serving chemical processing 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.