SS 904L Fasteners
Manufacturer
India · Mumbai
SS 904L Explained — Composition, PRE, and the Cu Advantage
SS 904L (UNS N08904, EN 1.4539) is a high-alloy austenitic stainless steel developed specifically for the sulphuric acid industry. It sits between standard 316L (Cr 16–18%, Mo 2–3%, PRE≈24) and the nickel alloys in the corrosion resistance hierarchy — providing substantially better resistance to pitting, crevice corrosion, stress corrosion cracking, and reducing acids than 316L, at a cost well below Hastelloy C276 or Incoloy 825.
The four key alloy additions that make 904L superior to 316L: (1) Higher Ni (23–28%) — increases SCC resistance and general corrosion resistance in reducing acids; (2) Higher Mo (4–5%) — raises PRE from ≈24 (316L) to ≈36 (904L), dramatically improving pitting and crevice corrosion resistance in chloride-containing media; (3) Copper (1–2%) — improves resistance to dilute and moderate-concentration sulphuric acid, the defining application for 904L; (4) Higher Cr (19–23%) — improves general oxidising acid resistance vs 316L’s 16–18% Cr.
The copper content is 904L’s most distinctive feature. Copper in the 1–2% range provides specific resistance to sulphuric acid in the concentration range 5–40% at temperatures up to approximately 60°C — the conditions found in fertiliser production (sulphuric acid attack on phosphate rock), acid mine drainage treatment, and dilute H₂SO₄ chemical processing. For stronger H₂SO₄ resistance beyond 40% or above 60°C, Incoloy 825 or Hastelloy C276 are specified instead of 904L.
| Element | Min % | Max % |
|---|---|---|
| Iron (Fe) | Balance | — |
| Nickel (Ni) | 23.0 | 28.0 |
| Chromium (Cr) | 19.0 | 23.0 |
| Molybdenum (Mo) | 4.0 | 5.0 |
| Copper (Cu) | 1.0 | 2.0 |
| Manganese (Mn) | — | 2.0 |
| Silicon (Si) | — | 1.0 |
| Carbon (C) | — | 0.020 |
904L PRE≈36 — Where It Sits in the SS Hierarchy
PRE (Pitting Resistance Equivalent) quantifies a stainless steel’s resistance to chloride pitting. 904L’s PRE≈36 is 50% higher than 316L’s PRE≈24 — a substantial corrosion engineering step up for applications at the 316L failure boundary. The copper addition provides a separate, unique advantage in H₂SO₄ not captured by PRE.
PRE = %Cr + 3.3×%Mo + 16×%N. Higher = better pitting resistance.
The 1–2% copper in 904L creates a Cu-rich passive film layer that resists sulphuric acid corrosion in the 5–40% concentration, 20–60°C range — the working conditions of fertiliser production, acid mine drainage neutralisation, and dilute H₂SO₄ chemical plant. This benefit is separate from the Mo-driven PRE improvement. For hot concentrated H₂SO₄ (>60% or >80°C): specify Incoloy 825 or Hastelloy C276.
SS 904L Fastener Types & Standards
All forms machined from ASTM B649 certified UNS N08904 annealed bar stock. PMI-verified before production. Matched sets from same heat for single-MTC assembly documentation.
904L Stud Bolts
Fully threaded stud bolts for ASME B16.5 pressure flanges in H₂SO₄ plant, fertiliser production, and seawater service where 316L fails. Class 150–600, M12–M100, lengths to 3,000mm. Matched heavy hex nuts from same certified heat. PRE≈36 stud bolt sets with single MTC for complete assembly documentation.
Hex Bolts
Full thread and partial thread hex bolts in 904L. M6–M100. Fertiliser plant H₂SO₄ equipment cover bolts, acid mine drainage treatment equipment, seawater heat exchanger body bolts, and all chemical plant structural connections where 316L pitting failure is documented and 904L’s PRE≈36 provides the needed margin.
Hex Nuts
Standard hex and heavy hex nuts in 904L. M6–M100. Matched from same ASTM B649 certified heat as stud bolts. For 904L stud bolt assemblies in H₂SO₄ plant, phosphoric acid, and seawater service — same-grade nuts eliminate galvanic corrosion at the nut-stud thread engagement crevice where mixed-alloy assemblies corrode preferentially.
Washers
Plain flat and spring helical washers in 904L. M5–M100. Matched-grade washers for complete 904L stud bolt assembly documentation in H₂SO₄ plant, fertiliser production, and phosphoric acid equipment — one MTC covers stud bolt, nut, and washer from the same certified heat of N08904 bar stock.
Threaded Rods
Full thread and end-thread in 904L. M6–M100, lengths to 3,000mm. H₂SO₄ plant hanger systems, acid mine drainage treatment structural rods, seawater desalination tie-rods, and phosphoric acid fertiliser plant equipment support rods where 316L threaded rod pitting resistance is marginal.
Socket Cap Screws
Hex socket cylindrical head in 904L for confined-access bolting in H₂SO₄ plant, phosphoric acid equipment, and seawater service. M4–M30. Heat exchanger baffle screws, acid pump internals, instrument flange connections, and reaction vessel internal fittings where 316L socket cap screw corrosion is the failure mode.
Matched Assembly Sets
904L stud bolts + heavy hex nuts + washers from the same ASTM B649 certified N08904 heat. Single MTC, PMI XRF, and COC for the complete flange assembly — simplified TPI hold plan for fertiliser, sulphuric acid, and seawater EPC projects where 904L is the specified assembly alloy.
Custom 904L
Non-standard AF dimensions, inch UNC/UNF thread, fine pitch, flanged hex (DIN 6921), coupling nuts (DIN 6334), and drawing-specific configurations in 904L. Contact sales@akbarfasteners.in with your drawing, size, and documentation requirement.
904L vs 316L vs Incoloy 825 — Grade Selection Matrix
904L fills the gap between 316L (cost-first stainless) and Incoloy 825/Hastelloy C276 (exotic alloys). This matrix identifies when 904L is the correct, cost-effective intermediate choice.
| Environment / Criterion | SS 904L This Page | SS 316L | Incoloy 825 | Hastelloy C276 |
|---|---|---|---|---|
| CORROSION RESISTANCE | ||||
| PRE (Pitting Index) | ≈36 | ≈24 | ≈26 | ≈66 |
| H₂SO₄ 5–40%, up to 60°C | ✔ Good (Cu helps) | ✗ Fails >30% | ✔ Good | ✔ Excellent |
| H₂SO₄ >40% or >80°C | Marginal | ✗ Fails | ✔ Good — use 825 | ✔ Excellent |
| H₃PO₄ (phosphoric acid) | ✔ Good | Limited | ✔ Good | ✔ Excellent |
| Seawater (ambient) | ✔ Good (PRE≈36) | ✗ Pitting risk | ✔ Good | ✔ Excellent |
| Chloride SCC resistance | Excellent (High Ni) | Moderate (SCC risk) | Excellent | Excellent |
| COST POSITION | ||||
| Relative cost vs 316L | 3–4× 316L | Baseline 1× | 5–7× 316L | 8–12× 316L |
| Best value position | When 316L fails, C276 not justified | Standard corrosion | When 904L fails | Aggressive mixed acid |
If 904L insufficient: Incoloy 825 · Hastelloy C276
904L Application Areas
Dilute to moderate H₂SO₄, H₃PO₄, seawater, and chloride service where 316L pitting resistance (PRE≈24) is insufficient and exotic alloy cost is not justified.
Sulphuric Acid Plant
dilute acid process bolts
Fertiliser Production
phosphoric acid plant bolts
Acid Mine Drainage
low-pH process equipment
Seawater Service
desalination plant bolts
Oil & Gas
high-Cl marine flanges
Pharmaceutical
mild acid process flanges
Chemical Processing
high-Ni SCC-resistant bolts
Water Treatment
PRE≈36 pump body bolts
SS 904L Standards Coverage
Same ISO 9001:2015 quality system as our complete stainless steel fastener range. PMI XRF before machining — critical for 904L to confirm the elevated Mo (4–5%) and Cu (1–2%) that distinguish it from 316L.
Complete Documentation Package
Heat & Lot Traceable
Conformance
Mo+Cu Confirmed
Tensile · Hardness
TÜV · BV · Lloyds
Related Fastener Alloys
904L when 316L fails but exotic alloy is over-specified. Incoloy 825 when 904L is insufficient. Hastelloy C276 for aggressive mixed acid. Duplex 2205 for strength + chloride.
SS 316L Fasteners
The step below 904L — correct for standard chemical, marine, and food processing applications where PRE≈24 is sufficient. When 316L passes the service environment corrosion check, 904L’s higher cost is not justified.
View SS Range →Incoloy 825 Fasteners
The step above 904L — for hot concentrated H₂SO₄ above 40% or 80°C where 904L is marginal. Incoloy 825’s higher Ni (38–46%), Mo (2.5–3.5%), and Cu provides better H₂SO₄ resistance at elevated temperature and concentration.
View Incoloy Range →Hastelloy C276 Fasteners
For mixed acid media or aggressive H₂SO₄ beyond Incoloy 825 — C276’s Ni-Cr-Mo-W alloy (PRE≈66) provides the broadest corrosion resistance in the chemical industry at significantly higher cost than 904L or 825.
View C276 Range →Duplex 2205 / 2507
An alternative to 904L for chloride and seawater service — Duplex 2205 (PRE≈35) provides similar PRE to 904L with higher strength (≥620 MPa vs 904L’s ≥490 MPa) and lower cost. Best when strength is also a requirement alongside corrosion resistance.
View SS Range →Need SS 904L
Fasteners?
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