Pulp bleaching swings from pH ~14 to ~1 and pushes chlorine dioxide vapor around ~1 ppm — conditions that can wreck ordinary alloys and cost mills billions. A materials-first strategy, backed by rigorous inspection, is proving cheaper over the life of the plant.
Hot caustic digester liquors can reach pH ~14, and oxidizing bleach stages can drop to pH ~1, according to ASM International. Add agents like Cl₂, ClO₂ and chlorate salts, and most common alloys are under attack. Typical bleach towers and washers see chlorine dioxide levels ~1 ppm (vapor) (Bry-Air), which helps explain why carbon steels and basic 304/316 stainless steel often fail rapidly.
Inspections have found severe caustic stress‑cracking in unprotected digesters and corrugated boilers decades ago, a reminder that the cost of corrosion is not theoretical. In the U.S., a ~$165 B industry faces roughly $6 B/year in corrosion-related costs (ASM International). Planning materials and maintenance accordingly avoids spills, downtime and fines — and is increasingly central to sustainability.
Bleach‑stage chemistry and attack mechanisms
Bleaching chemistry is an aggressive mix. In addition to extreme pH swings (pH ~14 to ~1), oxidizing species such as chlorine dioxide and chlorate target passive films on standard stainless grades (ASM International). At ~1 ppm ClO₂ vapor, even external surfaces can corrode (Bry-Air).
The result is a high incidence of pitting, crevice corrosion, and SCC (stress corrosion cracking — cracking driven by tensile stress and corrosive environments) in conventional 304/316 equipment. The industry’s corrosion bill — roughly $6 B/year on a ~$165 B base — underscores the stakes (ASM International).
Titanium applications and economics
Titanium’s “outstanding corrosion resistance… in chloride environments” has been documented for decades, with early work already predicting more Ti use as mills close water cycles (ASME Digital Collection). In practice, many bleach towers (including D0 towers), sock filters, washers and digester vents are built in Titanium grade 2 or grade 7.
While Ti’s purchase price can be 10–20× carbon steel, total life‑cycle cost is often lower because of near‑“permanent” corrosion resistance (Strongwell; ASME Digital Collection). North American mills have replaced aging FRP/metal lines with titanium spools for “better reliability and service longevity” (Apex; Apex). Design guidelines warn that conventional SS (316L/317L) cannot survive rising chlorides in closed‑circuit bleach filtrates, so upgrading to Ti (or Ni alloys) is needed (ASME Digital Collection).
Duplex stainless steels in service
Duplex grades — e.g., UNS S31803 (2205), S32205 (2304), and SUPERDUPLEX S32750 — combine ~50% ferrite and ~50% austenite, delivering very high strength and corrosion resistance. Compared to 304/316, duplex alloys tolerate higher chlorides and are far more SCC‑resistant in hot, caustic liquor mixings (Pulp & Paper Canada).
Field experience is striking. One corrosion engineer says “new duplex digesters are no more costly than new digesters built using carbon steel,” and in service duplex equipment “just don’t experience appreciable corrosion or cracking” (Pulp & Paper Canada; Pulp & Paper Canada). Meanwhile, modern 316L often sits at only ~2.0% Mo (the low end of spec) versus older material at ~2.8% Mo (Pulp & Paper Canada). A duplex like 22%Cr–5%Ni contains ~3.5% Mo, boosting pitting/crevice resistance. As a result, most new batch or continuous digesters, flash vessels, level tanks, and bleach washer tanks/installations now use 2205/2304, and equipment constructed in duplex “needs far less inspection and maintenance” due to SCC resistance even under wet insulation (Pulp & Paper Canada).
Table 1 from industry sources (MDPI) illustrates typical duplex SS applications in a Kraft mill: digesters (high P) in 2205; washer and flash tanks (mid P) in 2304/2205; liquor piping (n/a) in 2205/2101; O₂‑delignification separator in 2205; brownstock washers (low P) in 2101/2205; and BL storage often in standard 2202 (25Cr), etc. (“P” = pressure; n/a = none.)
Other high‑alloy choices for hot spots
For the rare cases where even duplex fails — vent condensers or certain acid mixers — mills turn to Ni alloys (e.g., Alloy 20, 825, Hastelloy C276) or zirconium. That said, titanium and duplex cover most bleach‑plant needs while balancing cost.
Non‑metallic options: FRP and elastomer linings
FRP (fiberglass‑reinforced plastic — glass fiber in a corrosion‑resistant polymer such as vinyl ester) and rubber linings are workhorses in bleach service. FRP tanks, vessels and piping became ubiquitous from the 1950s onward because of excellent corrosion resistance, high strength‑to‑weight and design flexibility (OnePetro). FRP does not corrode in hot caustic or low‑pH bleach chemicals (except in worst cases), yielding very long life. One pulp mill’s buried FRP effluent line ran 30 years with virtually no degradation and was removed only for capacity expansion (OnePetro). Modern FRP service lives of 25–40+ years in bleach duty are reported, and FRP is often cost‑competitive with exotic alloys over time (OnePetro).
There are limits. High‑concentration ClO₂ or hot concentrated acids can attack FRP resin. UTComp notes ClO₂ “can attack and degrade FRP” and cause thickness loss (UTComp). Some mills found cracks and thinning in FRP bleach washers after ~15–20 years, driving repairs (UTComp; UTComp). Monitoring mitigates risk: external ultrasonic systems can map FRP wall thickness. In one case, 28‑year‑old FRP seal tanks with ClO₂ filtrate showed no immediate need for relining and an estimated ~29 more years of life (UTComp). Where oxidizers are present, inspection frequency typically increases, and cracked panels may be repaired or replaced (UTComp).
Rubber linings (neoprene, EPDM, Hypalon) and polymer linings on steel or FRP substrates are common in washers, towers and lye tanks. These elastomers provide toughness and chemical resistance in mildly oxidative environments and often last 5–10 years before relining, depending on chemistry. Technical lining systems also use vinyl ester or ceramic‑filled polymer concretes in bleach towers (Blome). Elastomers age — chlorides can embrittle some formulations — so lined vessels are inspected for gap corrosion or lining failure. Notably, composites often outlast metals “through multiple life cycles” of their alloy counterparts (Strongwell).
Corrosion control beyond base materials
Supplementary protections are widely applied. Cathodic protection or anodic straps are sometimes used in digesters, though not usually in bleed piping due to mixed waters; any such systems require periodic checks.
Special coatings add insurance. Mills use sprayed novolac epoxy coatings (e.g., TL‑47, TL‑91 types) or high‑build urethane membranes to protect carbon steel internals from NaOH/chlorine; proper QA on thickness and holiday checks is critical because coating failures localize attack (Blome).
Chemical inhibitors are limited in bleaching itself but may be added in water‑scrubbers or heat exchangers to buffer pH or tie up oxidants (phosphates, amines, silicates). Where programs are used, mills turn to tools such as a corrosion inhibitor for general protection or a neutralizing amine for pH control, with precise addition supported by an accurate chemical dosing pump. Process control helps too — e.g., recirculating filtration until acid concentrations drop (as one mill did to preserve FRP piping (OnePetro)), or balancing washer temperatures and flows to minimize erosion‑corrosion. Solutions depend on wood type, liquor composition and flows, so each mill’s history matters.
Inspection and maintenance program essentials
Even the best materials demand surveillance. Regular thickness monitoring via UT (ultrasonic testing) on vessels and piping in bleach areas is standard; high‑risk items like bleach towers, digesters and washer drums warrant annual or biennial checks. Industry practice maps internal thickness in known thinning zones of digesters yearly because cracks can grow ~10 mm/year (Pulp & Paper Canada). Measured wall‑thickness data drive capital calls — for example, whether to install a $1M stainless‑steel overlay versus cheaper options (Pulp & Paper Canada).
NDT (non‑destructive testing) goes beyond UT. UTComp’s UltraAnalytix detects FRP delamination or thinning without shutdown (UTComp; UTComp). Mills also use dye‑penetrant, thermal imaging, or guided‑wave ultrasound. Crucially, plans “tailor” to high‑risk areas — welds, nozzle corners, bottom plates — rather than a shotgun approach (Pulp & Paper Canada).
Visual and safety inspections at turnarounds check for pitting, cracking and lining failure. High‑voltage holiday tests on linings validate integrity, and leak detectors (sumps or sensors) provide early warning between outages. PMV (positive material verification) at construction matters too: one $30K analyzer flagged a wrong alloy substitution (carbon steel instead of the specified 310), preventing a major leak (Pulp & Paper Canada).
Maintenance tracking in a CMMS (computerized maintenance management system) helps quantify improvement. Define KPIs such as corrosion rate, outages prevented, and repair‑versus‑replacement cost. Corrosion management experts report strong ROI — NACE often cites 5–15× — though site‑specific analysis is essential. In practice, data has extended asset lives: an FRP seal tank slated for re‑lining at 25 years stayed in service and gained ~3 more years after UT data showed it was sound (UTComp). Conversely, neglect can be catastrophic: Wensley recounts a boiler explosion and $100M loss after a cheap process gas analyzer discouraged future inspection and a carbon‑steel tube failed (Pulp & Paper Canada).
Regulatory context and outcomes
Regulators are raising the bar. Indonesia’s environmental program is pushing pulp mills toward “PROPER Hijau” (highest environmental performance), which in practice calls for best‑in‑class process integrity alongside emissions control (KemenLH). The path is clear in bleach plants: choose titanium or duplex stainless where it counts (Pulp & Paper Canada; Apex), deploy FRP or rubber linings where they shine, and backstop it all with rigorous, targeted NDT and maintenance. The payoff shows up in lower downtime, lower lifetime costs, and reduced risk of spills or fines — and, ultimately, in maximized equipment life and reliability in the harsh bleach‑plant service.
Sources: ASME Digital Collection; Pulp & Paper Canada; OnePetro; Apex; ASM International; UTComp; MDPI; Strongwell; Bry‑Air; Blome; UTComp.
