Carbon dioxide and oxygen in condensate can chew through steel in hours. A tight playbook—deaeration, neutralizing amines, film-forming amines, and oxygen scavengers—keeps pulp and paper steam systems running.
In pulp and paper mills, large volumes of high‑purity condensate are recovered and returned to boilers. The catch: dissolved CO₂ and O₂ in condensate cause rapid corrosion, dropping pH and attacking steel or copper surfaces (www.filtox.com; www.watertechnologies.com).
One industry source puts it bluntly: “dissolved oxygen remains the most feared species, because even a few ppb (parts per billion) can pit stainless steel and erode pump impellers in hours. Carbon dioxide is equally troublesome, forming carbonic acid that lowers condensate pH and accelerates general corrosion” (www.filtox.com; corroborating guidance: www.watertechnologies.com). Untreated condensate can sit in the acidic range (~4–5), dissolving protective oxides and thinning pipe walls at low points and flanges (www.watertechnologies.com; www.filtox.com).
Failure of buried or embedded condensate piping is expensive, and corrosion debris can redeposit in boilers and turbines, pushing maintenance higher (watertechnologyreport.wordpress.com). The upside is sizable: condensate is “liquid gold” for heat and quality—“every gallon of condensate returned … saves a cubic foot of natural gas” (www.chemaqua.com)—and capturing it “saves fuel, water, and treatment chemicals” (www.filtox.com).
Condensate corrosion mechanisms
Carbon dioxide liberated from boiler water (e.g., from bicarbonate alkalinity) dissolves as carbonic acid (a weak acid that lowers pH), while oxygen drives localized pitting—both accelerate metal loss in return lines and equipment (www.watertechnologies.com). The industry repeatedly warns that even “a few parts per billion” oxygen can pit stainless or carbon steel in hours (www.filtox.com).
Neutralizing amines: pH control and distribution
Volatile neutralizing amines (volatile bases that raise pH) are a core strategy. Fed into feedwater or steam, they travel with the vapor and neutralize CO₂ at each condensation point (fr.scribd.com; www.watertechnologies.com). Common choices are morpholine, cyclohexylamine (CHDA), and diethylaminoethanol (DEAE) (www.chemaqua.com), often supplied as neutralizing amines and metered via accurate chemical dosing. The neutralization is straightforward: R–NH₂ + H₃CO₃ → R–NH₃⁺ + HCO₃⁻.
By adjusting feed, condensate pH is held mildly alkaline. Guidance targets ~pH 8.8–9.2 for mixed Cu/Fe systems (www.watertechnologies.com), or at least above pH 7.0 after each condensate stage (watertechnologyreport.wordpress.com). In practice, operators dose to neutralize estimated CO₂ with a small excess to reach ≈7.5–8.5 pH (watertechnologyreport.wordpress.com), and a well‑treated system keeps dissolved Fe and Cu far below 0.1 mg/L (ppb) (watertechnologyreport.wordpress.com).
Dosing is stoichiometric to CO₂: a 100%‑active amine needs ~2 mg/L per 1 mg/L CO₂; with 20–40% solutions, about ~5–10 mg/L of amine solution per 1 mg/L CO₂ (watertechnologyreport.wordpress.com). Example: 50 ppm CO₂ in steam requires ~250–500 ppm of 30%‑active amine (watertechnologyreport.wordpress.com).
Distribution ratios (steam/liquid partitioning) matter. Morpholine is low (~0.4), staying in early condensate; CHDA is high (~4–5), carrying downstream; DEAE is intermediate (~2) (watertechnologyreport.wordpress.com). Blends “chase” CO₂ through the system—e.g., morpholine/CHDA—so early and late condensate remain protected (www.watertechnologies.com). Selection also weighs neutralizing capacity (lower molecular weight = higher capacity), basicity (Kb), distribution ratio, and thermal stability; morpholine and CHDA resist decomposition up to ~400°C and ~2500 psig (pounds per square inch gauge) (www.watertechnologies.com; www.watertechnologies.com; www.watertechnologies.com).
Film‑forming amines: hydrophobic barrier
Where neutralizers are impractical (for example, at very high alkalinity) or oxygen ingress is a concern, film‑forming amines (FFA; long‑chain alkylamines, R ≈ C₁₂–C₂₂) volatilize with steam and deposit a molecular film on metal surfaces (www.watertechnologies.com; www.chemaqua.com). The hydrophobic tail forms a barrier that limits contact by water and dissolved gases (www.chemaqua.com; www.watertechnologies.com), often delivered as a corrosion inhibitor program for steam and condensate.
Surfactant action can lift old corrosion products and oxides, after which a new protective film forms (www.watertechnologies.com). Films are mono‑ or multi‑molecular, so once established, only very small residuals (fractions of ppm) are needed (fr.scribd.com; www.watertechnologies.com). FFAs are often applied with neutralizers to cover both pH and surface protection, including “next‑generation” products for high‑alkalinity cases (www.chemaqua.com).
Application is deliberate: add gradually—particularly on re‑treat loops during start‑up—because heavy initial dosing can dislodge scale and clog traps (www.chemaqua.com). A typical approach is to start at ~⅓ of final feed and ramp. Feed is often tied to steam flow (e.g., 0.3×–1× ppm per kg/kg steam) to keep ~0.1–0.5 ppm residual in condensate (watertechnologyreport.wordpress.com). Small amounts of emulsifiers or neutralizers may be co‑fed to improve wetting in complex systems (www.watertechnologies.com). Monitoring uses corrosion coupons or “wettedness tests” (water beading on a protected coupon) (www.chemaqua.com).
Deaeration performance and oxygen scavenging
A robust deaerator (a device that strips dissolved gases) is the first line of defense. By Henry’s law, spraying make‑up into steam strips ~97–98% of dissolved oxygen (www.watertechnologies.com). Pressurized units heat to near saturation and vent almost all free O₂; vendors typically guarantee ≤0.005 cm³/L (~7 ppb) O₂. Vacuum‑style units achieve ~330–650 ppb (www.watertechnologies.com). One supplier notes thermal deaerators routinely reduce O₂ below 0.007 mg/L (7 ppb) (watermech.com), so single‑digit ppb oxygen is expected after a good deaerator.
Even low levels are harmful, so plants add oxygen scavengers alongside. Sodium sulfite and hydrazine are common: practical sulfite feed is ~10 ppm per 1 ppm O₂, while hydrazine as a 35% solution is ~3 ppm feed per ppm O₂ (watertechnologyreport.wordpress.com; watertechnologyreport.wordpress.com). Handbooks emphasize that “nearly complete oxygen removal is required” in industrial systems (www.watertechnologies.com), and tray or spray‑type units are designed to strip O₂ to 5–7 ppb while purging most CO₂ (handwiki.org).
CO₂ from alkalinity and condensate targets
Deaeration removes free CO₂ and O₂, but bicarbonate‑bound CO₂ rides through and reforms carbonic acid in condensate (www.watertechnologies.com). Boiler chemistry liberates significant CO₂: ~0.79 mg/L CO₂ per 1 mg/L NaHCO₃ and 0.35 mg/L per 1 mg/L Na₂CO₃ (www.watertechnologies.com; watertechnologyreport.wordpress.com). For instance, 100 mg/L sodium bicarbonate alkalinity produces about 79 mg/L CO₂ in the steam (www.watertechnologies.com; watertechnologyreport.wordpress.com). This is the primary load targeted by neutralizing amines. If venting or temperature control slips, deaerators can reintroduce both O₂ and CO₂ into feed, which must then be countered by amines or corrosion inhibitors (www.watertechnologies.com).
Best practice is multi‑barrier: use corrosion‑resistant pipeline materials or coatings; ensure feedwater deaeration removes ≥98% oxygen and degasses just before the boiler (www.watertechnologies.com; www.watertechnologies.com) with targets of <0.01 mg/L O₂ and <10 ppb CO₂; dose oxygen scavengers to capture the last traces (www.watertechnologies.com; watertechnologyreport.wordpress.com); maintain condensate pH ≈8–9 with amines, where programs commonly dose 50–500 ppm solution (depending on CO₂ load) to achieve pH 7.5–9 (watertechnologyreport.wordpress.com; www.watertechnologies.com); and apply film‑forming amines so a hydrophobic monolayer covers the steam/condensate path, with visual “beading” on coupons and negligible corrosion rates on racks (www.chemaqua.com; www.watertechnologies.com).
Program outcomes and cost impacts
Well‑executed programs show measurable protection: condensate iron/copper typically <0.1 mg/L and corrosion coupons near zero; untreated systems can see several tenths of mm/yr metal loss (watertechnologyreport.wordpress.com). Treatment costs are offset by avoiding pipe replacement and boiler tube failures, while reusing condensate with proper chemistry control can cut feedwater costs and reduce fuel use. One trade source estimates returning 90% of boiler condensate can save on the order of 10–15% of a mill’s fuel bill (www.chemaqua.com; www.filtox.com).
The through line is clear: a robust deaerator removing >98% O₂, backed by neutralizing amines and film‑forming amines, yields very low oxygen and acid stress in condensate and extends asset life and efficiency (www.watertechnologies.com; www.chemaqua.com).
