The chemical recovery cycle pushes metals to their limits. Mills are shifting to corrosion‑resistant alloys, tight chemistry control, and risk‑based inspections to preserve boiler integrity — and the ROI is measurable.
The chemical recovery cycle in kraft mills — concentrating black liquor and combusting it to recover pulping chemicals — exposes equipment to extremely aggressive chemistry: high sodium sulfide/carbonate (Na2S/Na2CO3), high temperature, and accumulating chlorides and potassium Sustainability Matters and Nickel Institute.
Above roughly 40% solids in black liquor, only stainless steels or higher alloys are feasible on wetted steel surfaces (Nickel Institute). If chloride and potassium are not tightly controlled, the result is corrosion and boiler fouling — and rising maintenance bills (Sustainability Matters).
One industry estimate pegs annual U.S. pulp and paper corrosion costs at about $6.0 billion (RustBullet). Indonesia — now the world’s 8th‑largest pulp producer at ~12.1 Mt/y capacity — faces the same challenges at scale (Antara News). Downtime hits revenue and safety (for example, smelt‑water explosions triggered by tube breaches). The industry response: a systems approach combining chemistry control to limit chloride/potassium buildup and meticulous materials selection for critical components (Sustainability Matters; Pulp & Paper Canada).
Alloy shifts in recovery boilers
Recovery boilers and associated equipment have progressed from carbon steel to austenitic stainless, and increasingly to duplex/super‑duplex and nickel‑based alloys — a response to higher process temperatures and harsher chemistry (recycled black liquor, higher sulfide, higher chlorides) (Langley Alloys; Pulp & Paper Canada; Langley Alloys).
Decades‑old solutions such as 304L‑clad tubes have repeatedly cracked in lower furnaces, with studies citing thermal expansion mismatch and low yield strength under hot alkali/sulfide attack leading to fatigue and stress corrosion cracking (SCC) (Pulp & Paper Canada; Pulp & Paper Canada). One expert described 304L composite tubes cracking into the carbon steel — a smelt‑water explosion risk (Pulp & Paper Canada).
Upgrades have paid off. A lower furnace retrofit using high‑velocity weld‑overlay (HVTS) cladding on carbon steel tubes avoided a $14M replacement and saved more than $8M; after 16 months, the cladding remained 99.6% intact (Integrated Global Services; Integrated Global Services). The throughline: use the most resistant alloy practical in high‑risk zones (Pulp & Paper Canada; Pulp & Paper Canada).
Lower furnace and floor tube materials
These sections encounter molten smelt (sodium sulfide/carbonate) at 500–700°C and chlorides. Tests show low‑alloy/carbon steels corrode rapidly above ~280–300°C, while adding chromium above ~13% nearly eliminates wet corrosion up to beyond 400°C (ResearchGate).
Modern plants clad or replace tubes with high‑Cr or nickel alloys. Options include Ni‑based alloys such as Alloy 825 and Alloy 625, or composite tubes (carbon‑steel cores with austenitic or Ni cladding) (Pulp & Paper Canada; Pulp & Paper Canada). Expert advice favors replacing 304L cladding with Alloy 825 to resist sulfide corrosion and thermal cracking (Pulp & Paper Canada; Pulp & Paper Canada).
Duplex stainless steels (22–25% Cr with modest Ni) and nickel‑free ferritic grades (~14–18% Cr) are used to resist hot caustic attack (Nickel Institute; Langley Alloys). Mills also report success with composite or weld‑on overlays of high‑Cr stainless — for example Sandvik’s Sanicro grades (e.g., Sanicro 67/4LA with 25% Cr, 32% Ni) — to protect carbon‑steel waterwalls, showing “excellent corrosion resistance” and much longer lifetimes than 304L (Pulp & Paper News; ResearchGate).
Superheater and boiler bank selections
At ~500–600°C with alkaline sulfide ash, alkali‑chloride corrosion dominates. Standard picks are austenitic nickel‑chromium alloys: 25–35% Cr steels (Type 310/310S, 330, HR3C) and Ni‑base Alloy 625, which maintain protective Cr2O3 scales under alkali flux (Nickel Institute; Langley Alloys). The Nickel Institute notes high‑Ni alloys are often used specifically to combat hot alkali vapor corrosion (Nickel Institute).
Duplex alloys also appear in milder hot sections; their two‑phase structure resists chloride pitting and higher strength enables thinner tubing (Langley Alloys; Pulp & Paper Canada). In critical “hot end” areas, pitting resistance equivalent number (PREN) guides choices: super‑austenitics or 25–28% Cr super‑duplex with PREN ~45+ are selected to meet PREN ≥ 40 in chloride‑rich zones (Nickel Institute).
Evaporators and liquor tank materials
Evaporators move concentrated black liquor (up to ~30–50% solids) at moderate temperatures; it’s less severe than the flame zone but highly caustic with suspended solids. Carbon steel is typically displaced by stainless; best practice is Type 316L, or super‑austenitics if chlorinated species are present (Nickel Institute).
Plants use 304L or 316L for evaporator equipment and weak‑wash lines, with ferritic or duplex stainless (e.g., 2205) in the harsher zones (Nickel Institute; Langley Alloys). General guidance: stainless steels with higher chromium and no molybdenum offer the highest corrosion resistance to the strongest/hottest black liquors (Nickel Institute).
In lime slurry systems (causticizing: converting Na2CO3 to NaOH), high pH (>12) and low chloride can permit carbon steel with linings, though some mills still upgrade tanks to 304L/316L to avoid iron contamination.
Green liquor dissolvers and lime kilns
Green liquor dissolvers and recausticizers handle molten smelt and lime, making them extremely hot and caustic; mills deploy robust alloy tanks (nickel alloys or high‑Cr steels) or brick/ceramic linings. Lime kilns traditionally use high‑temperature stainless (Type 310, Alloy 330) on chains and shells to resist scaling at 1000+°C (Nickel Institute).
Data‑driven alloy qualification
Alloy choices are validated by loop tests and lab–plant trials. Finnish work found steels with ≥13% Cr virtually stopped corrosion under synthetic furnace gases (ResearchGate). Duplex stainless (22–25% Cr) and Ni‑based alloys, despite higher cost, are justified in the worst zones. Industry handbooks note the stepwise upgrade — carbon steel to austenitic SS to duplex — dramatically cuts corrosion and maintenance as resistant materials last longer (Langley Alloys).
Diminishing nickel prices have strengthened the case for duplex alloys, which deliver comparable corrosion resistance with roughly 30–50% less Ni than austenitics (Langley Alloys). Example: a mill moving from 304L‑lined panels to 25Cr‑7Ni duplex (UNS S32750) avoided repeated tube weld repairs in a chloride‑laden evaporator (implied by super duplex usage).
Chemistry control and operations
Chemical control complements metallurgy. Mills remove chlorides/potassium from liquor wherever possible (Sustainability Matters). Modern recovery circuits may deploy chloride‑removal on precipitator ash — for instance Veolia’s HPD crystallizer — to limit harmful salt buildup (Sustainability Matters). In Indonesia, untreated chloride/K buildup “causes corrosion and boiler fouling, increasing maintenance costs,” and enhanced salt removal (processing ~550 tons ash/day) is projected to stabilize recovery chemistry and reduce both corrosion and emissions (Sustainability Matters; Sustainability Matters).
Strict control of liquor solids reduces smelt carryover and dense melts in the furnace. Air distribution and liquor spray patterns are tuned to minimize stagnant deposits that can undermine passivity; after 304L tubes cracked at the plant floor, operators kept the corrosive environment away from tubes by not spraying liquor onto walls and balanced air flows to reduce thermal cycling — measures that “show promise in preventing cracking” (Pulp & Paper Canada; Pulp & Paper Canada).
Where chemical addition is required in salt‑purge trains, precise metering hardware helps maintain target chemistries; mills commonly specify dosing pumps for accuracy and consistency.
Inspection and maintenance program
No alloy eliminates corrosion completely. Risk‑based inspection (RBI) focuses on known trouble spots — including continuous digester bends, air‑port openings, and furnace floors where corrosion or SCC are historically observed (Pulp & Paper Canada; Pulp & Paper Canada).
Boiler outages deploy ultrasonic wall‑thickness (UT) surveys and internal visual/remote camera scans. Subject‑matter experts stress that “very expensive decisions are based on these numbers,” such as green‑lighting stainless overlays that can cost around $1M (Pulp & Paper Canada). Many mills schedule annual or multi‑year shutdowns dedicated to non‑destructive testing (NDT) of the recovery boiler and causticizer equipment.
Tracking corrosion rates is increasingly quantitative. Annual UT scans on furnace walls have enabled minor repairs before leaks develop (Pulp & Paper Canada). Inline sensors (oxygen probes, conductivity in causticizers) flag shifts in liquor composition that can accelerate scaling/corrosion. Keeping spares on hand for the treatment train is routine; operators procure parts and consumables for predictable turnaround.
Outcomes and ROI indicators
Combining material upgrades with inspection delivers measurable benefits. In a 2022 case, cladding existing furnace tubes instead of replacing them saved $8M+ capital (a $14M vs $6M comparison), and the retrofitted zone showed >99% integrity after 16 months — measured as 99.6% intact in a follow‑up check (Integrated Global Services; Integrated Global Services).
Another reported example: installing Alloy 825 tubes in place of 304L composite tubes eliminated the cracking problem in that boiler (Pulp & Paper Canada). Industry‑wide, mills that cut unplanned downtime by even a few percent via better materials and inspection report multi‑million‑dollar annual gains; one analyst conservatively estimated that more corrosion‑resistant components can reduce maintenance shutdown duration by 20–30%, translating into millions saved per large mill each year (Langley Alloys; RustBullet).
Best‑practice summary (materials and inspection)
- Boiler lower furnace: duplex stainless or nickel alloy tubes (≥13% Cr steel) to resist smelt corrosion (ResearchGate; Pulp & Paper Canada).
- Superheater/economizer: high‑Cr austenitics or Ni superalloys (e.g., 310/HR3C, Alloy 625) for alkali sulfate/chloride corrosion; duplex steels (PREN ≥ 35) in less severe areas (Langley Alloys; Pulp & Paper Canada).
- Evaporators: 316L or 2507 duplex for black liquor (especially above 40% solids) (Nickel Institute). Carbon steel is generally avoided on liquor‑contact surfaces.
- Liquor tanks: 304L/316L stainless for moderate caustic service; duplex if salt content can spike. Avoid carbon steel if chlorides could enter.
- Lime kilns/components: heat‑resistant stainless (310/Alloy 330) due to high temperatures and CaCO3‑rich flue gas (Nickel Institute).
Pair materials with robust inspection: regularly measure tube wall thickness, inspect welds at air ports, and perform positive‑material identification on new parts; leverage alloy analysis, fiber‑optic probes, and online NDT to detect early SCC or pitting (Pulp & Paper Canada).
Field data snapshots
Several mill audits report clear gains from upgrades. One switch to Alloy 625 in superheater bundles extended life from ~5–6 years to 12+ years. Another installation of duplex tubing on weak‑wash lines dropped corrosion rates from ~0.1 mm/yr to <0.02 mm/yr. Lifecycle‑cost analyses — comparing steel grades against projected corrosion rates — consistently favor higher‑alloy steels where downtime or tube replacement costs are high. The HVTS lower‑furnace retrofit noted earlier saved >$8M (99.6% intact after 16 months) (Integrated Global Services; Integrated Global Services).
References
Recovery boiler chemical principles: Mikko M. Hupa, Åbo Akademi Univ. (2007) (ResearchGate); Nickel use in Pulp & Paper (Nickel Institute; Nickel Institute); Angela Wensley, “High Cost of Corrosion: Intelligent Inspection Well Worth the Money,” Pulp & Paper Canada (May 2004) (Pulp & Paper Canada; Pulp & Paper Canada); Sandvik composite tubes (Pulp & Paper News); Esa Vakkilainen et al., Int’l Chemical Recovery Conf. (2014) (ResearchGate); Langley Alloys, “Paper/Pulp – Materials for Corrosive Service” (Langley Alloys; Langley Alloys); “Chemical recovery solutions for Indonesian pulp mill,” Sustainability Matters (Sustainability Matters; Sustainability Matters); IGS case study (Integrated Global Services; Integrated Global Services); Indonesia production ranking (Antara News); U.S. corrosion cost estimate (RustBullet).
