At 140–200 bar and roughly 180–200 °C, the high‑pressure urea loop is a perfect storm for corrosion. Plants are winning that fight with specialized stainless steels and a small, carefully metered shot of air to keep metal surfaces passive.
High‑pressure urea synthesis (NH₃ + CO₂ → NH₄CO₂NH₂) runs near 140–200 bar and about 180–200 °C, conditions that turn ammonium carbamate (the process intermediate) into a highly aggressive medium for metal surfaces (stainless-steel-world.net) (bcinsight.crugroup.com) (stainless-steel-world.net). Early bets on titanium or 316L “urea grade” stainless often failed when oxygen was scarce at high temperature and pressure, pushing the industry toward super‑austenitic and duplex stainless steels purpose‑built for urea (bcinsight.crugroup.com).
The throughline: keep chromium‑rich steels passive with a bit of oxygen, and build critical assets from alloys that don’t panic when that oxygen dips.
Super‑austenitic steels for carbamate duty
Urea services have migrated to super‑austenitic grades such as the 25Cr‑22Ni‑2Mo‑N family (“25‑22‑2”), designed for ammonium carbamate exposure with very low ferrite and a high pitting resistance equivalent number (PREN ≈33) that also helps against chlorides (energy-fasteners.co.uk) (matweb.com). Alleima’s 2RE69 (a 25.22.2 variant at ≈25 Cr, 22 Ni, 2 Mo, ~0.1 N) has logged more than 27 years in stripper tubes at IFFCO Kalol (India), a longevity well beyond typical 5–10 year runs of 316L (alleima.com).
Laboratory Huey tests (a standard nitric acid corrosion benchmark) illustrate the margin: 316L‑UG (≈18 Cr‑12 Ni) around ~0.6 mm/yr versus ~0.3 mm/yr for 25‑22‑2 (bcinsight.crugroup.com) (bcinsight.crugroup.com). Electrochemical work at typical urea conditions (~195 °C, 20 MPa) found that high‑Ni content is detrimental under low‑oxygen, increasing active‑region dissolution and making low‑Ni, high‑Cr steels preferable—or, failing that, ensuring ample passivation oxygen (meridian.allenpress.com) (meridian.allenpress.com).
Duplex stainless steels in high‑pressure strippers
Duplex stainless steels (two‑phase ferrite + austenite microstructure) balance corrosion resistance and strength. Sandvik/Stamicarbon’s Safurex (UNS S32906, ~23 Cr‑7 Ni‑3 Mo) and the newer Safurex Star (30 Cr‑7 Ni‑3 Mo) were developed specifically for high‑pressure urea strippers (stainless-steel-world.net) (stainless-steel-world.net).
Field experience reports essentially zero corrosion in practice—often less than 0.01 mm/yr—even at stripper bottoms (about 205–210 °C), where these alloys remain largely passive without added oxygen (stainless-steel-world.net) (bcinsight.crugroup.com). The duplex structure also brings low thermal expansion and strengths approaching carbon steel (stainless-steel-world.net). Sumitomo/Toyo’s DP28W™, a W‑bearing super‑duplex, targets the same mix: urea/carbamate corrosion resistance, chloride stress corrosion cracking resistance, and high strength (stainless-steel-world.net).
Design and fabrication are as consequential as composition: beta‑phase control to avoid Cr₂N or sigma phase and careful welding procedures are critical, as detailed in development work (including dilatometry) reported alongside alloy families up to UNS S33007 (stainless-steel-world.net) (stainless-steel-world.net).
Claddings, linings, and bimetallic designs
Beyond bulk alloy selection, plants frequently specify carbon‑steel shells clad or lined with stainless, nickel alloys such as 625, or even glass/ceramic for certain lower sections (bcinsight.crugroup.com). Bimetallic solutions like OmegaBond™ combine Zirconium and Titanium layers; zirconium (33 Cr) is nearly inert, and titanium resists crevice attack, but the configuration is expensive (bcinsight.crugroup.com).
Licensor claims for OmegaBond strippers include bottom temperatures around 210 °C, a notch above the ~205 °C often cited (bcinsight.crugroup.com). Pure Zr or Ti (with niobium additions) can be used in the most severe zones on the CO₂ side of the stripper, though erosion and weld issues have been documented (bcinsight.crugroup.com).
Corrosion mechanisms and rates
Ammonium carbamate condensation and urea decomposition products—biuret, cyanurates, and more—drive corrosion. Uniform attack can occur under deposits; cyanuric acid, in particular, can exploit chromium‑depleted grain boundaries (mdpi.com) (mdpi.com) (mdpi.com) (mdpi.com).
Chromium content is critical because the protective Cr₂O₃ film that forms in the presence of oxygen limits metal dissolution; by contrast, nickel tends to raise active‑region dissolution when oxygen is scarce. Studies at ~195 °C and ~20 MPa found 25–26% Cr steels showed low corrosion and higher passive currents, whereas high‑Ni alloys suffered worse active attack—one author stressing that modern lower‑oxygen processes require low‑Ni stainless (meridian.allenpress.com) (meridian.allenpress.com).
Quantitatively, Sandvik reports Safurex corrosion below 0.01 mm/yr (effectively negligible), though localized rates around 0.07–0.09 mm/yr have been seen in the hottest stripper zones (stainless-steel-world.net). Trials and experience put 25‑22‑2 near ~0.3 mm/yr (Huey), and plain 316L at ~0.6 mm/yr (bcinsight.crugroup.com) (bcinsight.crugroup.com). Titanium shows around ~0.06 mm/yr (Huey) and zirconium ~0.005 mm/yr (bcinsight.crugroup.com).
In practical terms, a 10 mm wall of 25‑22‑2 corroding at ~0.3 mm/yr could last roughly ~30 years, versus ~16 years for 316L at ~0.6 mm/yr—aligning with the long 2RE69 service life and the fact that first‑generation 316L strippers in worst cases “lasted only a few months” (alleima.com) (bcinsight.crugroup.com) (bcinsight.crugroup.com).
Oxygen passivation in the high‑pressure loop
A small, steady dose of oxygen—typically fed as air—is the cornerstone of corrosion control. The goal is ~0.01–0.02 wt% O₂ dissolved in the liquid carbamate/urea stream, described as a “normal” passivation level (bcinsight.crugroup.com). In practice, licensors call for roughly 0.2–0.6 vol% O₂ in the CO₂ stream—Saipem plants often run ~0.35 vol% air (O₂) in CO₂, and CF Industries has reported ~0.6 vol% O₂ via CO₂ compressor blowers (bcinsight.crugroup.com) (bcinsight.crugroup.com). Patent literature specifies 200–2000 ppm O₂ (0.02–0.2 vol%) in the CO₂ feed to the bottom of the CO₂ stripper (patents.google.com), with supporting data on NH₄CO₂NH₂ passivation (patents.google.com).
That oxygen reacts to form a thin, protective Cr₂O₃ film on wetted stainless surfaces. Adequate exposure matters: at least ~2 hours of aerobic conditioning is recommended before ramping to full operation. Simply “blocking in” the loop with no fresh O₂ depletes the inventory, and after about ~72 hours corrosion can accelerate; if a start‑up is interrupted before the two hours of passivation, draining and repassivating is advised (bcinsight.crugroup.com) (bcinsight.crugroup.com). In one zero‑O₂ start‑up, multiple pinholes were observed, contrasted with normal O₂ starts that preserved the liner (bcinsight.crugroup.com).
The oxygen demand is modest: 0.35 vol% O₂ in a 2,500 lb/hr CO₂ stream equates to only a few hundred standard liters per minute of air, and oxidation risk at these levels is low. The real hazard is lack of oxygen—Cr‑oxide “burn‑off” in high‑temperature steam can strip passive films—so procedures call for co‑current O₂ addition during warm‑up (often separately metered from CO₂) and continuous monitoring, typically via O₂ analyzers on the CO₂ compressor. With O₂, steels like 25‑22‑2 or 316L‑UG hold their oxide films up to ≥205 °C; without O₂, even high‑Cr alloys can corrode actively (bcinsight.crugroup.com) (bcinsight.crugroup.com).
Start‑up practices and lifecycle trade‑offs
Complementary controls help the metallurgy and oxygen do their work. Heat‑up rates are kept slow—about 30–50 °F per hour—to limit thermal stress and enable even passivation (bcinsight.crugroup.com) (bcinsight.crugroup.com). Start‑up sequences use steam first for initial heating (steam has negligible O₂), then introduce CO₂ with air as soon as possible, per licensor procedures (bcinsight.crugroup.com) (bcinsight.crugroup.com).
Surface preparation matters: welds and cut edges—tube ends, headers—are ground and acid‑cleaned to remove heat tint and embedded oxides that can consume oxygen and block passivation (bcinsight.crugroup.com) (bcinsight.crugroup.com). Routine inspections (e.g., eddy‑current thickness checks) guide maintenance; many plants re‑line reactors/strippers every 10–20 years even when using advanced metals.
The economics hinge on the alloy‑oxygen pairing. Lower‑cost 316L‑UG may suffice if >0.5% O₂ is consistently ensured, but any oxygen loss risks rapid attack. Duplex Safurex steels are nearly immune even if passivation lapses, at higher capital cost (stainless-steel-world.net). Upgrading to 25‑22‑2 often doubles or triples service life, as the 2RE69 field record suggests (alleima.com) (bcinsight.crugroup.com). Injecting a few hundred ppm of air transforms corrosion rates from ~0.3–0.6 mm/yr (unstable regimes) to <0.01 mm/yr (stable passivation), shifting liner lifetimes from a few years to multiple decades (alleima.com) (bcinsight.crugroup.com) (stainless-steel-world.net).
Source notes and data trail
The operating envelope (140–200 bar, ≈180–200 °C) and alloy performance are drawn from Sandvik/Stamicarbon technical papers (stainless-steel-world.net) (stainless-steel-world.net); super‑austenitic 25‑22‑2 data and composition details appear in Alleima/Industeel datasheets (energy-fasteners.co.uk) (matweb.com), with a >27‑year 2RE69 case study at IFFCO Kalol (alleima.com). Practical perspectives and passivation air levels are documented in industry forums and reports (bcinsight.crugroup.com) (bcinsight.crugroup.com); electrochemical behavior under low‑oxygen urea conditions is detailed by Mancia & Tamba (NACE 1987) (meridian.allenpress.com) (meridian.allenpress.com); corrosion from urea decomposition products is reviewed by Galakhova et al. (mdpi.com) (mdpi.com); and passivation dosing ranges appear in U.S. patents (patents.google.com) (patents.google.com). Alloy development notes—including temperature profile experiments and dilatometry up to UNS S33007—are captured in Stainless Steel World features (stainless-steel-world.net) (stainless-steel-world.net).
