Pulp bleaching throws hot caustic and strong oxidizers at equipment—an environment that pits common steels and forces mills into titanium, duplex stainless, FRP and thick linings. The right material pairing with a rigorous inspection program can shave 20–30% off corrosion costs.
In pulp bleaching, the chemistry is unforgiving. Bleaching agents such as chlorine dioxide, permanganate, ozone and peroxide, combined with process liquors like white, green and black liquor, create aggressive acids and bases—think sulfurous/sulfuric/nitric acids, NaOH, Na₂CO₃, Na₂S, CO₂, H₂S and chlorinated organics (www.cortecvci.com). Kraft white liquor—approximately NaOH + Na₂S at about 170 °C (hot caustic with sulfide)—is “considered the most aggressive of the alkaline pulping liquors” (www.scielo.br).
In chlorinous stages like ClO₂ bleaching (pH ~4–6) and in peroxide stages (NaOH/H₂O₂, pH ~10–12), carbon steels and 300‑series stainless steels rapidly pit or suffer stress corrosion cracking. One industry guide is blunt: “bleach is extremely aggressive towards metals and most industrial coating technologies” (www.advancedfrpsystems.com).
The stakes are big. One ASM source cites roughly 300 U.S. pulp mills and estimates U.S.-$6 billion per year in direct corrosion costs (www.asminternational.org). A NACE estimate for North America is $4–5 billion annually—about $30 per ton of paper—consuming 30–40% of maintenance budgets (www.pulpandpapercanada.com) (www.pulpandpapercanada.com). Studies suggest better corrosion management—materials, coatings, inspections—can save 20–30% of total corrosion cost (www.pulpandpapercanada.com).
Corrosive chemistry and cost profile
Chlorine dioxide stages (acidic, pH ~4–6) and peroxide stages (alkaline, pH ~10–12) stress both metals and coatings, while white liquor (≈NaOH + Na₂S at ∼170 °C) challenges alkaline resistance (www.scielo.br). The combined presence of halogens, oxygenated acids and hot caustics makes a single “universal” alloy unrealistic (www.advancedfrpsystems.com).
The economics are clear: $4–5 billion/year in North America (≈$30/ton), with corrosion taking 30–40% of maintenance budgets, and a potential 20–30% savings via stronger management programs (www.pulpandpapercanada.com) (www.pulpandpapercanada.com) (www.pulpandpapercanada.com) and U.S.-$6 billion/year across ~300 U.S. pulp mills (www.asminternational.org).
Specialty stainless and titanium selection
Conventional carbon steels corrode rapidly in bleaching chemicals, and 304/316 stainless works only in milder, neutral stages (e.g., pulp washing with low‑chloride water). Any significant chlorides or oxidants exceed their limits (www.advancedfrpsystems.com). The kraft process was “one of the first industries to embrace duplex stainless steels” as caustic soda/sulfide cooking fluids overran 316L (www.mdpi.com).
Modern mills often replace 316L with duplex stainless steels (DSS) such as UNS S32304 and 2205—or lean duplex—offering similar corrosion resistance with lower nickel content. Lean DSS (~22–23% Cr with N) can cost less than 316L (www.mdpi.com). DSS’ roughly 50/50 austenite–ferrite structure provides high strength (allowing thinner wall sections) and excellent pitting/SCC resistance (www.mdpi.com) (www.mdpi.com).
For pulping digesters and liquor tanks (hot NaOH/Na₂S), duplex and superduplex (25–28% Cr) vastly outperform 316 in life span (www.mdpi.com) (www.langleyalloys.com). Widespread use of S32205/2205 or S32304 in modern digesters has replaced older 316/ICP‑protected units (www.scielo.br); by contrast, when super‑alkaline 50% NaOH was once used without protection, steel failed rapidly.
For strong oxidizers and acids, titanium is the standard choice in chlorine dioxide contactors and generation systems (acidic ClO₂ suspensions), typically Grade 2 or Ti‑Pd, because it resists halogens and oxygenated acids. High‑nickel alloys (e.g., Hastelloy C‑276, Inconel 625) can also resist mixed‑acid stages but are very costly. For moderate acid bleaching (e.g., chlorine or EO stages), vessels may use 904L or high‑Mo super austenitics. In practice, brightening stages often warrant premium corrosion‑resistant alloys (Ti or Ni‑based) to avoid failures.
Data point: early adoption of 2205 DSS in pulp solved “numerous corrosion problems,” and now lean DSS (e.g., 2304) is often specified instead of 316L (www.mdpi.com) (www.mdpi.com). The higher strength of duplex means vessels can be thinner (saving weight) yet outlast cheaper steels; one source notes corrosion‑resistant alloys reduced digester wall thickness and overall cost relative to carbon steel (www.langleyalloys.com). In practice, many new mills use duplex/superduplex for pipe/tanks in chemical recovery, and austenitic plus coatings/liner in bleach.
In milder, neutral service, 304/316 stainless is still used; 316L‑grade hardware (/products/ss-cartridge-housing) follows the same limitations when significant chlorides or oxidants are present (www.advancedfrpsystems.com).
FRP vessels and polymer linings
Where metals fail or cost too much, non‑metals dominate. FRP (fiberglass‑reinforced plastic) is common for bleach washers, dilution tanks, long piping runs and containment. FRP resins—often vinyl ester or novolac—resist caustic and chlorine, so FRP vessels “generally last longer” than steel in bleach service (www.advancedfrpsystems.com). Vendors note that fiberglass bleach tanks can provide decades of service if designed properly; one source analogizes to fiberglass boats lasting 50–60 years (www.hbrunlinhb.com). This FRP category includes fiberglass housings (/products/fiberglass-filter). Lightweight composite housings sit within the same family (/products/pvc-frp-cartridge-housing).
FRP is not invulnerable: it can undergo stress‑corrosion or erosion‑corrosion, especially if poorly cured or exposed to high‑peroxide conditions (www.advancedfrpsystems.com). Many operators monitor FRP tanks for bellying or creep cracks.
Rubber/polymer linings are common on carbon‑steel vessels in moderately aggressive service. Thick elastomer layers (neoprene, chlorobutyl, fluoroelastomer) or PTFE linings are used in ClO₂ scrubber towers and hypochlorite tanks; Japan’s pulp industry has long used thick elastomer layers inside acid/bleach contactors. Typical design life for a well‑maintained rubber lining in bleach liquor is on the order of 10–15 years before re‑lining (data are scarce).
Heavy‑duty epoxy and polymer coating systems are also applied. For example, Blome Inc. reports using high‑net‑polymer epoxy/vinyl ester mortars packed with glass flake in ClO₂ towers and washers (blome.com). Their product line includes novolac epoxy and vinyl‑ester linings rated for ClO₂/NaOCl exposure (blome.com). Multi‑layer systems (primer + glass‑filled topcoat) are common, can extend metal life, and require rigorous application quality control; failures (pinholes, delamination) can occur if not inspected.
Deployment trends reflect the shift: since the 1950s, bleach plants globally have increasingly used FRP and special linings. Today it is routine to see bleach towers of FRP and steel tanks 20–25 mm thicker but rubber‑lined. Design codes (e.g., RTP‑1 for FRP) reflect decades of experience. Modern mills often balance cheaper alloys plus lining versus expensive solid alloys; one vendor notes fiberglass “is a very good material for pipes and tanks in corrosive environments, and often it is more cost‑effective” than coating steel (www.hbrunlinhb.com). In summary, FRP or lined systems are favored for most bleach storage/washer equipment, while critical high‑pressure or high‑temperature vessels use metal corrosion‑resistant alloys.
Inspection and integrity management
Even with good materials, periodic inspection is essential. Robust preventive maintenance combines visual, ultrasonic and other NDT to catch corrosion before leaks. Mills typically schedule thickness measurements on high‑risk items (digesters, bleach contactors, scrubbers) every 1–5 years depending on age and severity. Contractors use ultrasonic UT scanning of welds/fittings and pulsed eddy‑current probes on FRP/liners. One NDT provider notes “digesters, evaporators, and bleaching systems are exposed to corrosive environments, leading to corrosion and cracking challenges,” and that corrosion detection inspections “help minimize production shutdowns and equipment repair costs” (nucleom.ca).
Any indication of wall loss prompts repair or replacement. In hard‑to‑access areas (e.g., caustic towers), cameras or guided wave ultrasonic tools may be used. Owners also deploy on‑line corrosion probes in liquors—tracking pH (a measure of acidity/alkalinity), ORP (oxidation‑reduction potential), and conductivity—to detect anomalies. Critical controls (block valves, ducts) are part of regular shutdown inspections. For rubber/linings, routine inspections (visual/adhesion checks) are done on a 1–3 year basis; coatings are scraped and recoated per manufacturer schedules (often 5–10 years).
The payoff is material. Plants report corrosion‑related repairs consuming 30–50% of annual maintenance costs (www.pulpandpapercanada.com); halving that through monitoring would free millions. Overall analyses echo the earlier finding: mills spend 30–40% of maintenance budgets on corrosion work, yet 20–30% can be saved by improved management (www.pulpandpapercanada.com) (www.pulpandpapercanada.com).
Regulatory and life‑cycle context
Material and maintenance choices align with business and regulatory goals. Premium alloys or linings raise capex but reduce life‑cycle cost via longer outage intervals; industry experience and ISO 15686 life‑cycle concepts support that corrosion‑proof materials often yield lower total cost of ownership for critical equipment. Indonesian regulators now push “green” pulp mills—2015 Ministerial Green Industry Standard (legalcentric.com) and 2024 sustainable industry initiatives (www.industry.co.id)—requiring closed‑loop, zero‑leak processes. Leakages of bleach chemicals or boilers can incur penalties or shutdown. Engineering teams increasingly view improved materials and NDT programs as compliance measures as well as cost savers in Indonesia.
Bottom line for bleach plants
High‑chloride or caustic stages typically use duplex/superduplex stainless steels (or nickel alloys) for tanks and heat exchangers; acid/oxidative stages use titanium or heavy‑duty linings. FRP and rubber linings serve where metal cannot go. Crucially, these selections must be paired with vigorous inspection and maintenance—UT scans, coupons, thickness monitoring—to ensure integrity. Together, the data show such investments significantly cut downtime and repair costs while supporting regulatory compliance (www.pulpandpapercanada.com) (www.asminternational.org) (www.scielo.br) (www.mdpi.com) (www.advancedfrpsystems.com) (blome.com) (www.industry.co.id) (www.scielo.br).
