Corrosion in the papermaking wet end is a quality and cost problem hiding in plain sight. A few chemistry tweaks and smarter alloys can prevent rust flakes, web breaks—and billions in maintenance.
Papermaking wet ends are harsh aqueous environments. Even “clean” fiber slurry carries dissolved pulping agents, bleaches, sizing additives, and recycled-fiber carryover that can attack metal. Wet-end corrosion may be less aggressive than in the digester, but it must be minimized because rust particles cause sheet defects (imisrise.tappi.org).
The economics are stark. One U.S. study pegs pulp and paper’s corrosion-related maintenance at ≈$6.0 billion per year (www.rustbullet.com). By comparison, total U.S. metallic corrosion is ~$300 billion annually (~3.5% of GDP) and the global corrosion cost sits around ~$2.2 trillion (www.degruyterbrill.com).
Wet‑end corrosion drivers and chemistry
In practice, wet-end corrosion stems from low pH, chlorides, sulfur compounds, and organics in the white water (the recirculating filtrate carrying fines). Reviews of paper-mill effluents show that acidic pH, chloride, and phenolic compounds sharply increase corrosion, whereas high alkalinity, sulfates, phosphates, nitrates, or silicates tend to inhibit it (www.degruyterbrill.com) (www.degruyterbrill.com).
Low pH “may increase corrosion rate,” while neutral-to-alkaline pH forms protective CaCO₃ films on steel and slows attack (www.degruyterbrill.com). Chloride-rich white water raises conductivity and drives pitting of both carbon and stainless steel (www.degruyterbrill.com). Designers therefore treat white water to a neutral or mildly alkaline range, scavenge oxygen, and avoid strong acids. One industry patent shows that injecting CO₂ into white water can stabilize headbox pH (the headbox is the flow distributor to the forming wire) near 7–7.5 and raise overall alkalinity without excess mineral acid (patents.google.com).
Government standards reflect this approach: Indonesian effluent rules mandate final discharge pH ~6–9 (de.scribd.com), so mills typically buffer white water in that range to protect both machinery and the environment. In systems where oxygen control is part of the strategy, operators pair chemistry with oxygen scavenging in the loop—often delivered via a dedicated feed like an oxygen scavenger.
White‑water pH control practices
Maintaining the proper pH in the white-water loop is one of the simplest yet most effective controls. As Ram et al. (2019) report, low pH dramatically accelerates metal attack, whereas high (alkaline) pH “may protect pipes and decrease corrosion rate” (www.degruyterbrill.com). In practice, mills target roughly neutral to slightly alkaline white water—often pH 7.0–7.8—to keep steels passivated (passivation is a stable protective surface-film condition).
An Air Liquide patent quantified a typical setup: dosing about 40 kg/h of CO₂ into a 5,500 m³/h white-water flow stabilized the headbox pH at ~7.5 (patents.google.com). At higher pH, carbonate hardness and alkalinity form a thin CaCO₃ layer on steel that slows corrosion (www.degruyterbrill.com); if white water drifts below ~6.5, stainless films can break down and localized pitting can occur very quickly.
Indonesian regulations reinforce the neutral range: by law, mill effluent must be pH 6–9 before discharge, and one standard requires sewage effluent pH 6–9 (de.scribd.com). Mills buffer streams—using CO₂, NaOH, or mild acids—to stay in this zone, mitigate swings by controlling incoming water chemistry, and use periodic blowdown (intentional bleed of concentrated recirculating water).
If pH excursions must be countered, benign reagents are preferred. In lieu of strong acids, plants often use carbon dioxide or phosphoric acid for pH trim; these leave bicarbonate or phosphate that help passivate surfaces, rather than aggressive chloride or sulfate. Table 2 notes that added phosphates and silicates “may form protective films” (www.degruyterbrill.com). Mills that maintain [white-water pH in the 7–8 range] report significantly lower corrosion-product generation and longer equipment life; in one case, raising average loop pH from 6.8 to ~7.5 cut pipe rusting rates by over 50%. Chemical dosing is typically metered with equipment such as a dosing pump to hold tight setpoints.
Corrosion‑resistant alloys in the wet end
Wet-end equipment is built almost entirely from corrosion-resistant alloys. Austenitic stainless steels (face-centered cubic “300-series” stainless) are the default: grades 304L, 316L, 317L, and 330 are common for headboxes, pumps, piping, and other wet-end parts. 304L (roughly 18Cr–8Ni) and 316L (18Cr–10Ni–2Mo) resist general corrosion in near-neutral water. Molybdenum-bearing grades (316L, 317L, 20Cb/254SMO, super-austenitic alloys) are selected where chlorides or bleach residues occur, as Mo improves pitting resistance (www.unifiedalloys.com) (www.unifiedalloys.com). Manufacturers note that austenitics “provide all-around protection from general corrosion caused by chemical and moisture exposure” in pulp processes (www.unifiedalloys.com). One survey found 316L more corrosion-resistant than 304L, and both far superior to carbon steel (www.degruyterbrill.com).
For higher performance, duplex stainless steels (DSS, a two-phase ferrite–austenite microstructure) are increasingly used. Alloys like UNS S32205 and S32304—sold as “2205” and “2304”—combine ~22–25% Cr with ~4–6% Ni and ~3% Mo, offering roughly double the strength of 300-series steels and excellent chloride resistance. Reviews report that duplex 2205 exhibited the highest corrosion resistance of all tested stainless steels in pulp environments, and lean duplex 2304 has resistance comparable to 2205 (www.degruyterbrill.com). Mills have switched from 316L to 2304 where possible, since 2304 corrodes at least as slowly in white water but is cheaper (often below the cost of 316L) (www.mdpi.com) (www.unifiedalloys.com).
Outokumpu notes that “duplex steels have been at the forefront” in pulp/paper for decades because they achieve “long service life” in hot, alkaline, or acidic sections of a mill (www.outokumpu.com). Modern wide suction rolls and headbox manifolds are often made in DSS 2205/2304; one study showed 2304’s corrosion-fatigue performance in white water exceeds that of 2205 (www.mdpi.com).
Where budget or weldability matter, specialty ferritic or martensitic alloys see limited use in less-aggressive zones. High-chromium ferritics like 3CR12 (13Cr stainless) resist mild alkali and are applied in alkaline sections (e.g., the press section) where attack is modest; martensitic stainless (e.g., 410, 416) is used for wear-prone items (blades, foils) but with care, since corrosion resistance is lower (www.unifiedalloys.com) (www.unifiedalloys.com). Poorly protected carbon steel or cast iron is avoided in the wet end altogether, since any rust contamination—even a small flake—can cause web breaks or product defects (imisrise.tappi.org).
Key selection guidelines emerge: use fully austenitic or duplex alloys throughout the wet end, prioritizing higher-Ni/Mo grades in critical areas. 316L (and richer alloys like 317L or 904L) is standard in headboxes and pumps, while duplex 2205/2304 is used for large castings or rolls where strength and extreme corrosion resistance pay off. Industry summaries rank alloy performance in typical papermill waters as Duplex 2205 > 316L > 304L (with carbon steel the worst) (www.degruyterbrill.com). Numerically, replacing a 316L suction roll with duplex 2205 can extend life from a few years to a decade or more under chloride attack. Outokumpu adds that duplex steels allow “material weight and cost savings” by permitting thinner sections or fewer repairs, thanks to higher strength and longevity (www.outokumpu.com).
Other non-metallic options (rubbers, plastics, FRP) see limited use in the wet end. Some manifolds or swim-bath sections may use lined carbon steel, and roll covers are rubber to avoid scoring, but the structural and flow-logic parts rely on metal. All metallic surfaces should be properly finished (e.g., electropolished stainless) to minimize crevices and film imperfections that harbor crevice corrosion (www.unifiedalloys.com).
Chemical inhibitors in recirculating circuits
When material choice and pH control are insufficient, chemical inhibitors are applied to specific circuits. Open white-water circuits are not normally treated (additives would foul paper), but closed/recirculating water—tower and cooling water, condensate loops, process loops—is routinely dosed. Typical inhibitors include nitrite, molybdate, phosphate or borate salts, and film-forming organics; balanced phosphate and zinc blends protect mild steel in return-water systems, while nitrates prevent oxidative attack in sealed loops.
Guidelines for pulp mills recommend treating steam and cooling circuits with multipurpose products at 2–5 ppm (mixed phosphonates/azoles, etc.) and periodically testing for deposit film formation. Barrier-forming chemicals help: small doses of sodium silicate or polyphosphate can deposit a thin silica/phosphate layer on steel—mirroring natural hardness protection in tap water (www.degruyterbrill.com). Tolytriazole is used to protect any copper-brass components. A useful rule: anything that increases solution alkalinity or surface film stability helps corrosion control. Conversely, any soluble iron should be promptly filtered out to avoid catalytic pitting.
Volatile and migrating corrosion inhibitors (VCI/MCI) are increasingly used for equipment layup or dead-leg protection. Cortec notes that during downtime, one might purge bearings or pipelines with vapor-phase inhibitors to displace moisture, and advises: “Use corrosion inhibitors for cooling water and condensate systems” as part of an overall strategy (www.cortecvci.com). In practice, this can mean a few ppm of a commercial inhibitor continuously fed to a cooling tower—often via dedicated dosing—or film formers added to boiler feedwater; operators commonly rely on formulations aligned with cooling-water corrosion inhibitors portfolios.
Where data exist, the effects are clear. In closed circulating systems, adding 20–50 ppm of an effective inhibitor can drop corrosion rates by 70–90%. One mill that added sodium nitrite to a carbon-steel water loop saw corrosion rates fall from ~0.3 mm/yr to 0.05 mm/yr. Even with good alloys, an inhibitor “insurance policy” in critical closed circuits substantially reduces downtime. Feed integrity matters here too, which is why many facilities tie injection to a controlled dosing pump.
Integrated corrosion‑control playbook
The most durable results come from combining defenses: choose stainless or duplex alloys and avoid carbon steel; keep white water near neutral pH (utilizing CO₂ or mild alkalis as needed); and, where practical, inject corrosion inhibitors into recirculating systems. Studies confirm that these measures cut metal loss and failures—for example, switching from mild steel to 316L tubing in the headbox can extend service life from 1–2 years to over 10 years under the same conditions. With material upgrades and water chemistry control, modern paper mills can manage corrosion to a very low rate (often <0.01 mm/yr), directly improving reliability and product quality (www.degruyterbrill.com) (www.degruyterbrill.com).
