Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 in Pulp and Paper
Super Duplex 2507 (UNS S32750 / EN 1.4410) is a workhorse alloy in modern bleach plants and washing lines. The combination of chlorine dioxide (ClO2) chemistry at low pH (typically 2 to 3), chloride concentration of 1000 to 3000 ppm, and process temperatures from 60 to 90 degrees Celsius is a classic pitting environment that defeats 316L within months. PREN above 41 and CPT above 50 degrees Celsius per ASTM G48 Method E let 2507 carry bleach plant duty for decades, often as the upgrade after a 316L failure event.
TorqBolt supplies 2507 plate, pipe, fittings, flanges, fasteners, and forgings to pulp and paper mills with EN 10204 Type 3.1 mill test certificates as standard, Type 3.2 third-party-witnessed available where the project specification calls for it.
Why Super Duplex 2507 in Pulp and Paper Bleach Plants
Bleach plant material selection drivers:
- Chloride pitting and crevice resistance at low pH and elevated temperature. Bleach plant ClO2 stages run at pH 2 to 3, chloride 1000 to 3000 ppm, temperature 60 to 90 degrees Celsius. 2507 PREN above 41 stays clear of the pitting threshold; 316L (PREN around 25) does not.
- Resistance to acidic ClO2 chemistry. ClO2 itself is a strong oxidiser; the residual Cl- after reaction with lignin drives the pitting. 2507 is one of the few stainless steels that runs reliably in this mix.
- Resistance to chloride stress corrosion cracking in hot bleach liquor. The duplex microstructure resists CSCC much better than austenitic 316L.
- Fatigue and erosion resistance for rotating equipment (washer drum shells, suction roll shells, screw press flights).
- Cost vs Hastelloy C-276 / C-22. 2507 is 50 to 70 percent cheaper than nickel-base alternatives for the same service life in bleach plants where temperature and chloride sit inside the 2507 envelope.
Super Duplex 2507 Pulp and Paper Design Data
| Parameter | Value | Reference |
|---|---|---|
| PREN | 41 to 43 | Cr + 3.3 times Mo + 16 times N |
| CPT (ASTM G48 Method E) | Greater than 50 degrees Celsius | ASTM G48-11 |
| Bleach plant ClO2 stage pH | 2 to 3 | Typical D0, D1, D2 stage chemistry |
| Bleach plant chloride concentration | 1000 to 3000 ppm | Mill operating data |
| Bleach plant process temperature | 60 to 90 degrees Celsius | Typical D-stage operating range |
| Maximum continuous service temperature | 250 degrees Celsius (sigma-phase risk above 300) | NORSOK M-630 |
| Hardness limit | 28 HRC max where NACE applies | NACE MR0175 / ISO 15156-3 |
| Ferrite content | 35 to 55 percent (base), 35 to 65 percent (weld) | NORSOK M-630 |
Where Super Duplex 2507 Is Specified in Pulp and Paper
- Bleach plant D0, D1, D2 stage piping and equipment. ClO2 chemistry at pH 2 to 3 and chloride 1000 to 3000 ppm is the textbook 2507 application.
- Washer drums (drum filter shells and shaft journals). 2507 resists pitting at the wash-line water-air interface where chloride concentrates by evaporation.
- Suction roll shells on bleach plant washers and dewatering presses. Drilled and machined 2507 cylindrical shells.
- Screw press flights, screen baskets, and bleach tower internals.
- Bleach plant pumps and valve trims. Centrifugal pump impellers, casings, valve gates, and seats in 2507 forgings or castings (analogous chemistry per ASTM A890 grade 5A).
- Pulp dryer rolls and exhaust ducts handling chloride-laden condensate.
- Black liquor evaporator concentrator effects at high chloride concentration.
- White-liquor and green-liquor handling in the recausticising area where chloride and oxidising species are present.
- Bolting on bleach plant flange connections and tank manways. ASTM A1082 / API 20F in matching S32750.
Super Duplex 2507 vs Competing Alloys in Pulp and Paper
| Alloy | PREN | Bleach Plant Verdict | Cost Index |
|---|---|---|---|
| 316L (austenitic) | ~25 | Pits in ClO2 stages within months; not recommended for D-stage | 1.0 |
| 317L (austenitic) | ~30 | Marginal; pits at the high-chloride end of D-stage range | 1.3 |
| 904L (austenitic) | ~35 | Common upgrade from 317L; still marginal at hot D-stage | 1.6 to 1.8 |
| 2205 duplex (S31803 / S32205) | 34 to 38 | Acceptable for cool D-stage and washer service; marginal at hot end | 1.5 to 1.8 |
| Super Duplex 2507 (S32750) | 41 to 43 | Standard pick for hot D-stage piping, washer drums, suction rolls | 2.0 to 2.5 |
| Zeron 100 (S32760) | 41 to 44 | Equivalent envelope to 2507; choice often by mill spec or supplier | 2.2 to 2.7 |
| 6Mo (254 SMO, AL6XN) | 43 to 45 | Slightly higher CPT margin; used at the most aggressive D-stage | 2.5 to 3.0 |
| Hastelloy C-276 / C-22 | 50+ | Reserved for the harshest bleach plant service or recovery boiler corrosion zones | 4.0 to 5.0 |
Super Duplex 2507 Installation Notes for Pulp and Paper
- Welding per AWS D1.6 with filler ER2594 (overmatching, preferred for bleach plant) or ER2553 (matching). Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius. Argon plus 2 to 5 percent nitrogen shielding for GTAW root.
- Pickling and passivation per ASTM A380 after fabrication: 20 percent nitric plus 5 percent HF, then citric or nitric passivation. Iron contamination from carbon-steel tools is the leading cause of pitting initiation in service.
- Cladding on large bleach plant tanks and reactors: 2507 roll-bond clad to carbon-steel backer. Bond-strength qualification per ASTM A263 analogue, shear test, ultrasonic disbond inspection.
- Crevice management at gasketed flanges. Spiral-wound CG gaskets with PTFE or graphite fill.
- Hydrotest with low-chloride water (less than 30 ppm chloride per ASTM A380), drain and dry within 24 hours.
- Bolting per ASTM A1082 / API 20F in matching S32750. PTFE thread lubricant on assembly.
- Inspection: ferrite count per ASTM E562, ASTM A923 Methods A and B for sigma-phase screening on critical heats. PMI per ASTM E1476 on every piece by handheld XRF or OES.
Recommended TorqBolt 2507 Products for Pulp and Paper
- 2507 pipe: ASTM A790 seamless and welded for bleach plant piping
- 2507 fittings: ASTM A815 WP-S32750 buttweld
- 2507 flanges: ASTM A182 F53 weld neck and blind
- 2507 stud bolts: ASTM A1082 / API 20F for tank manways and flange connections
- 2507 heavy hex nuts: ASTM A1082 in matching S32750
- 2507 plate, bar, forgings: ASTM A240 / A479 / A182 F53 for tanks, washer drums, suction rolls
Related Application Pages
- Chemical processing: dilute HCl, H2SO4, HF service
- FGD: wet flue gas desulfurization
- Seawater applications: sea-chest, fire-water
- Desalination: MSF, MED, RO brine
- Oil and gas: wellheads, manifolds
Super Duplex 2507 Pulp and Paper FAQ
Why does 316L fail in bleach plant ClO2 stages?
316L PREN sits around 25, well below the threshold for chloride pitting at pH 2 to 3, chloride 1000 to 3000 ppm, and 60 to 90 degrees Celsius. ClO2 itself is not the corrodant; the residual chloride from ClO2 reaction with lignin is. The chloride drives pitting, and the low pH suppresses passive-film recovery. Pinhole leaks and washer drum perforation typically appear within 12 to 24 months of commissioning.
What concentration and temperature limits apply to 2507 in bleach plant?
2507 holds up at the typical bleach plant D-stage envelope: pH 2 to 3, chloride 1000 to 3000 ppm, temperature 60 to 90 degrees Celsius. At chloride above 3000 ppm or temperature above 90 degrees Celsius, the design starts to push the 2507 CPT margin and 6Mo or Hastelloy C-276 should be evaluated. Always coupon-test borderline duty in the actual mill stream rather than rely on bench data.
Can 2507 be used for washer drum shells?
Yes. 2507 plate (ASTM A240) rolled and welded into drum shells is a standard upgrade from 316L washer drums in bleach plants. The wash-line interface concentrates chloride by evaporation and sets up a classic pitting environment that defeats 316L; 2507 PREN above 41 stays clear of the pitting threshold. Suction roll shells in 2507 have similar drivers.
Is 2507 acceptable for black liquor evaporator concentrators?
Yes, in the high-solids effects where chloride concentrates and 316L pits. The corrosion mode in concentrators is chloride pitting plus alkali-induced cracking; 2507 PREN above 41 handles the pitting and the duplex microstructure resists cracking. For the most aggressive upper effects (very high solids, high chloride, high temperature), Hastelloy C-276 / C-22 may be required on a case-by-case basis.
What welding filler should be used for 2507 in bleach plant?
ER2594 (overmatching, slightly higher PREN than the base) per AWS A5.9 is the standard pick for bleach plant ClO2 service. ER2553 (matching) is acceptable for less aggressive applications such as washer service or warm pulp washing. Heat input 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius. Argon plus 2 to 5 percent nitrogen shielding for GTAW root pass.
How does 2507 compare against 904L in bleach plant?
2507 PREN above 41 vs 904L PREN around 35 puts 2507 well clear of the typical D-stage pitting threshold, where 904L is borderline. 2507 also has roughly twice the yield strength of 904L (550 MPa vs 220 MPa), so vessel walls can be reduced. 904L can be marginally cheaper in plate form, but the wall-thickness penalty and the pitting margin tip the choice toward 2507 for new bleach plant construction.
Are 2507 fasteners acceptable for bleach plant tank manways?
Yes. Stud bolts and heavy hex nuts to ASTM A1082 / A1082M in UNS S32750 are the standard fastening for bleach plant tank manways, flange connections, and access covers. PTFE thread lubricant on assembly to prevent galling. The hardness ceiling of 28 HRC for NACE MR0175 sour-service compliance is not strictly required for bleach plant service but is good practice given the chloride environment.
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