Hangbo Alloy Group
Shanghai · Est. 2012
Kraft digesters, bleach washers and ClO2 stages punish carbon steel. See which nickel alloys resist white liquor, hypochlorite and acidic chloride at mill conditions.
The pulp and paper industry runs some of the most corrosive chemical environments found outside dedicated chemical plants. A modern kraft mill circulates white liquor at 150-180 oC, bleaches pulp with chlorine dioxide and hypochlorite, and washes it in acidic chloride solutions - all in equipment that must run for years between shutdowns. Carbon steel and the usual 300-series stainless grades fail quickly in these streams through pitting, chloride stress-corrosion cracking and intergranular attack. As a nickel-alloy manufacturer, Hangbo supplies the high-alloy grades that let digesters, washers and bleach-stage hardware survive these conditions; this article maps where each alloy earns its place.
A kraft mill concentrates its corrosion problems into a few recognizable zones. The digester holds white liquor - a hot, highly alkaline mixture of sodium hydroxide and sodium sulfide - where carbon steel survives only in the lower, cooler zones and suffers caustic embrittlement and sulfide attack higher up. Batch digesters see cyclic temperature and pressure swings that accelerate fatigue; continuous digesters run steady but at sustained high temperature. The recausticizing loop and green/white liquor tanks add green-liquor chloride and thiosulfate that promote localized attack.
The bleach plant is worse. The C/D stage combines chlorine dioxide in an acidic chloride medium (pH 2-4, 50-70 oC); the E stage is alkaline extraction with sodium hydroxide and hydrogen peroxide; and hypochlorite washers add oxidizing chloride at moderate temperature. Washer drums, filtrate tanks and hood structures sit in a mist of oxidizing acid-chloride that drives both pitting and chloride stress-corrosion cracking. The combined oxidizing power and chloride content is what defeats ordinary stainless; resistance is predicted far better by PREN and local corrosion metrics than by chromium alone.
Digester upper zones and relief lines. Alloy 625 (UNS N06625) handles hot sulfide plus chloride, while Hastelloy C-276 is chosen where conditions are more aggressive. The Hastelloy C-276 composition - roughly 16% molybdenum, 15% chromium - gives broad resistance across both alkaline and acidic streams.
Bleach washers, filters and filtrate. Chlorine dioxide and hypochlorite demand the highest-PREN grades: Alloy C-22, Alloy 59 and Alloy 686. The Hastelloy C-22 corrosion resistance profile is the workhorse here, with 686 and 59 stepping up where chloride and oxidizing power are highest.
Acid stages (pH below 3). Mixed-acid and low-pH filtrate favor C-276 or C-2000. The Alloy 31 grade also performs well in acidic gas-cleaning streams that resemble the mill's sulfur-oxide washers.
Warm alkaline extraction with chloride. Where temperature stays moderate and chloride is low, super-austenitic 904L or 254 SMO can be economical; once chloride and temperature climb, a nickel alloy such as 625 or C-276 is the safer choice. For the hot alkaline white-liquor side specifically, see our caustic soda alloy selection guide.
Most pulp-mill nickel-alloy failures are avoidable. Sensitization and intergranular corrosion occur when welded C-family alloys are not properly solution-annealed; our note on sensitization of Hastelloy C-276 explains how to specify the heat treatment. Crevice corrosion under washer deposits is controlled by choosing high-PREN grades and good drainage design. Microbiologically influenced corrosion shows up in stagnant filtrate chests and washer trays where biofilms create local acidic cells; our MIC article covers detection and alloy choices. Finally, welding must follow a qualified procedure - our nickel-alloy welding guide covers purge, heat input and filler selection to keep the repaired zone as resistant as the base metal.
Chlorine dioxide is a strong oxidant in an acidic chloride medium, so the highest-PREN grades win. Alloy 59 or Hastelloy C-22 typically outperform 625 in this service because of their extra molybdenum and chromium. Our PREN ranking guide shows why, and the Alloy 59 profile details the chemistry that resists ClO2 plus hypochlorite.
Only in low-temperature, low-chloride portions. The combination of sulfide, chloride and heat in the upper digester zones drives both pitting and chloride stress-corrosion cracking in 316L. Once temperatures exceed about 120 oC with chloride present, a nickel alloy such as 625 or C-276 is the safer, longer-lived choice.
White liquor is hot alkaline sulfide (caustic), which attacks by a different mechanism than the oxidizing acidic chloride of the bleach plant. Caustic handling and the right nickel grades for it are covered in our caustic soda selection guide, while the bleach side needs the high-PREN C-family alloys described above.
Yes - stagnant filtrate chests and washer trays can develop biofilms that create local acidic cells and pit even resistant alloys. Detection methods and which grades hold up best are in our MIC article; good drainage and periodic cleaning reduce the risk significantly.
As a dedicated nickel alloy manufacturer serving chemical processing sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.