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Alloy 20 stainless steel offers excellent corrosion resistance, including excellent resistance to oxidizing and moderate reducing corrosion, as well as excellent resistance to stress corrosion cracking and good resistance to localized corrosion. Alloy 20 has excellent resistance to stress corrosion cracking and good resistance to localized corrosion in mixed reducing media. It is suitable for use in sulfuric acid environments and industrial equipment with sulfuric acid solutions containing halogen ions and metal ions. JN Alloy 20's main products include plates, bars, pipes, strips, and flange forgings.
At 33% nickel, Alloy 20 has practical immunity to chloride stress corrosion cracking. This alloy is often chosen to solve SCC(stress corrosion cracking) problems, which may occur with 316L stainless.
| Parameter | Value |
|---|---|
| UNS Designation | N08020 |
| Werkstoffnummer | 2.4660 |
| Nickel Content | 32.0–38.0% |
| Chromium Content | 19.0–21.0% |
| Density | 8.06 g/cm³ |
| Melting Point | 1440–1450 °C (2624–2642 °F) |
| Tensile Strength (min) | 551 MPa (80 ksi) |
| Yield Strength (min) | 241 MPa (35 ksi) |
| Elongation (min) | 30% |
| Primary Service | Sulfuric acid, mixed acid, chloride SCC |
| Max Service Temp | ~ 500 °C (continuous); 980 °C (intermittent oxidizing) |
| Magnetic? | Non-magnetic (austenitic) |
| Product Form | ASTM Standard | ASME Code | Size Range |
|---|---|---|---|
| Seamless Pipe & Tube | ASTM B729 | SB-729 | OD 6–324 mm, WT 1–25 mm |
| Welded Pipe (ERW) | ASTM B474 | SB-474 | OD 21–610 mm, WT 1.5–12 mm |
| Welded Pipe (SAW) | ASTM B464 | SB-464 | OD 219–1220 mm, WT 6–30 mm |
| Plate, Sheet, Strip | ASTM B463 | SB-463 | Thickness 0.5–80 mm |
| Bars (Round, Flat, Hex) | ASTM B473 | SB-473 | Dia 6–300 mm |
| Wire & Welding Rod | ASTM B472 | SB-472 | Dia 1.0–12 mm |
| Forgings | ASTM B564 | SB-564 | Up to 2000 kg per piece |
| Welded Fittings | ASTM B366 | SB-366 | 1/2"–48", Sch 10S–80S |
| Castings (Pumps, Valves) | ASTM A351 (CN-7M) | — | As designed |
Available finishes: Cold-drawn, hot-finished, pickled, annealed, polished (#180/#240/#320/#600), electropolished. Custom cutting, beveling, and forming available on request.
Alloy 20 is primarily composed of nickel, iron, and chromium, with small amounts of carbon, manganese, silicon, molybdenum, and copper. This chemical composition makes Alloy 20 extremely corrosion-resistant in harsh environments such as strong acids, strong bases, and chlorides.
| Composition | Value |
| Ni | 32.0-38.0 |
| Cr | 19.0-21.0 |
| Cu | 3.0-4.0 |
| Mn | ≤2.0 |
| Mo | 2.0-3.0 |
| Si | ≤1.0 |
| C | ≤0.07 |
| P | ≤0.045 |
| S | ≤0.035 |
| Fe | Remain |
| Ni+Ti | 8-1.0 |
Mechanical properties | Tensile, min, ksi[MPa] | Yield, min, ksi[MPa] | Elongation, %(min) |
Alloy 20 | 80[551] | 35[241] | 30 |
Density: 8.06 g/cm³.
Melting point: 1440-1450°C (2624-2642°F).
Thermal conductivity: 10.8 W/(m·K).
Coefficient of thermal expansion: 14.0 µm/m·K.
Electrical conductivity: 1.27 × 10^6 S/m.
| Product | Standards | Show |
| Alloy 20 Pipe & Tube SMLS | ASTM B729 |  |
| Alloy 20 Welded Pipe | ASTM B474, ASTM B464 |  |
| Alloy 20 Sheet Plate | ASTM B463 |  |
| Alloy 20 Bars | ASTM B472, ASTM B473 |  |
| Alloy 20 Forgings | ASTM B564 |  |
| Alloy 20 Fittings | ASTM B366 |  |
Alloy 20 delivers best-in-class resistance to sulfuric acid across concentrations from dilute to 98%, combined with chloride SCC immunity and good resistance to phosphoric acid, nitric acid, and mixed acid streams. It is not a universal alloy—it is optimized for acid service, particularly reducing acids containing chlorides.
Sulfuric Acid (H₂SO₄) Resistance
Alloy 20 was designed for sulfuric acid. Its copper and molybdenum content provides resistance across the full concentration spectrum, with performance that far exceeds 316L and rivals more expensive nickel alloys in moderate-temperature service:
0–50% H₂SO₄: Excellent to 80 °C. Corrosion rate < 0.1 mm/year.
50–90% H₂SO₄: Good to 50–60 °C. Corrosion rate < 0.5 mm/year.
90–98% H₂SO₄: Moderate at ambient temperature only. Not recommended above 40 °C.
Boiling H₂SO₄ (dilute): Good to 40% concentration at boiling point.
Chloride Stress-Corrosion Cracking (SCC) Resistance
At 33% nickel, Alloy 20 has practical immunity to chloride SCC—the failure mode that destroys 316L and 304L in warm chloride service. This makes it the standard upgrade when 316L exchangers, tanks, or piping crack in coastal or brine-handling service.
Pitting and Crevice Corrosion
Alloy 20's PREN (Pitting Resistance Equivalent Number) is approximately 25–28, comparable to 904L. It resists pitting in moderate chloride service but is not recommended for concentrated chloride brines at elevated temperatures—where C-276 or Inconel 625 would be specified.
Other Acid Resistance
Phosphoric acid: Excellent in wet-process phosphoric acid (containing impurities).
Nitric acid: Good at moderate concentrations (chromium-driven passivation).
Hydrochloric acid: Limited; only very dilute or cold. C-276 is preferred.
Organic acids (acetic, formic): Excellent.
Caustic (NaOH): Good at moderate concentrations and temperatures.
Alloy 20 is the most cost-effective metallic material for sulfuric acid plant equipment handling concentrations from dilute to 90% at temperatures up to 80 °C. It is used throughout H₂SO₄ production and handling facilities—in acid coolers, absorption towers, drying towers, storage tanks, piping, and pumps.
| Plant Equipment | H₂SO₄ Concentration | Typical Temp | Alloy 20 Suitability |
|---|---|---|---|
| Drying tower packing / internals | 93–98% | 40–80 °C | Good (ambient concentration) |
| Absorption tower internals | 98% | 80–120 °C | Marginal—consider higher alloy above 80 °C |
| Acid cooler (shell-and-tube) | 93–98% | 40–100 °C | Excellent (tube side, water-cooled) |
| Dilute acid piping | 10–50% | 20–80 °C | Excellent |
| Storage tanks | 93–98% | Ambient | Excellent |
| Pump casings & impellers | Variable | 20–80 °C | Excellent (cast version CN-7M) |
| Stripping column | Dilute + organics | 60–100 °C | Excellent |
| Acid purification unit | Variable | 20–60 °C | Excellent |
Design Notes for H₂SO₄ Service
Velocity: Keep acid velocity below 1.5 m/s in piping to avoid erosion-corrosion at concentrations above 90%. Pump discharge lines should use heavier wall schedules.
Aeration: Alloy 20 performs better in aerated sulfuric acid than in air-free service, because oxygen helps maintain the passive film. Deaerated, hot, concentrated acid is the most aggressive condition.
Contaminants: Chloride contamination in H₂SO₄ accelerates corrosion exponentially. Specify chloride limits in the acid specification and monitor regularly.
Temperature limits: For 93–98% H₂SO₄, keep Alloy 20 below 80 °C for reliable long-term service. Above 80 °C in concentrated acid, upgrade to high-silicon cast iron or Teflon-lined equipment.
Cast Alloy 20: For pump casings and valve bodies, the cast equivalent CN-7M (ASTM A351) offers the same corrosion performance with suitable mechanical properties for pressure-containing castings.
Alloy 20 is deployed across six major chemical industry segments where sulfuric acid, mixed acids, or chloride-containing process streams make stainless steel unviable. The following real-world cases illustrate typical applications, service conditions, and why Alloy 20 was selected over alternatives.
Service: HF/H₂SO₄ alkylation unit effluent cooler; 90% H₂SO₄ at 65 °C, water-cooled shell-and-tube exchanger.
Challenge: Original 316L tubes failed by pitting and SCC within 18 months; frequent retubing caused unit shutdowns costing $200K+ per day.
Solution: Retubed with Alloy 20 seamless tubes (ASTM B729, 19.05 mm OD × 2.11 mm wall). Result: 12+ years of continuous service with no tube failures. Lifecycle cost reduction of over 60% versus repeated 316L replacement.
Service: Wet-process phosphoric acid digestion reactor; 30% P₂O₅ with H₂SO₄, HF, and chloride impurities at 85 °C.
Challenge: The mixed acid + fluoride + chloride environment attacked 317L and 904L aggressively; C-276 was specified but cost-prohibitive for the large reactor volume.
Solution: Reactor vessel fabricated from 6 mm Alloy 20 plate (ASTM B463) with ERNiCr-3 welded seams.
Result: 8+ years service to date, wall thickness measurement shows < 0.05 mm/year corrosion rate. Cost roughly 40% less than the C-276 alternative.
Service: Continuous strip pickling tank; 15% H₂SO₄ + 5% HCl at 70 °C, with iron dissolution increasing over batch life.
Challenge: Acid concentration and temperature swings, plus accumulated Fe³⁺ ions (oxidizing), caused rapid failure of 316L tank liners and heating coils.
Solution: Alloy 20 heating coils (ASTM B729 seamless tube) and tank lining (3 mm clad plate).
Result: Coil life extended from 6 months (316L) to 5+ years (Alloy 20). Production uptime improved by 12%.
Service: Active pharmaceutical ingredient (API) reactor; batch process with catalyst in 40% H₂SO₄ at 60 °C, followed by caustic neutralization.
Challenge: Acid-to-caustic cycling caused stress-corrosion cracking in 316L vessels within 2 years; product contamination from iron ion leaching was a regulatory concern.
Solution: Alloy 20 reactor vessel and agitator (ASTM B463 plate + B473 bar), electropolished to Ra < 0.4 µm for GMP compliance.
Result: 7+ years without cracking or product contamination. Passed FDA inspection with no material-related observations.
Service: Saturated brine preheater; NaCl-saturated water at 90 °C feeding electrolysis cells.
Challenge: 316L heater tubes failed by chloride pitting and SCC at the tube-to-tubesheet joints within 3 years.
Solution: Alloy 20 U-tube bundle (ASTM B729) with Alloy 20 tubesheet.
Result: 10+ years service. The 33% nickel content provides SCC immunity that 316L cannot achieve at this temperature in saturated chloride.
Service: Viscose rayon spinning bath; 10% H₂SO₄ + 18% Na₂SO₄ + 1% ZnSO₄ at 50 °C.
Challenge: The mixed sulfuric acid / sulfate / zinc environment is highly corrosive to stainless steel; equipment life was 1–2 years with 316L.
Solution: Alloy 20 spinning bath troughs, pump casings (cast CN-7M), and piping.
Result: 15+ year service life. Alloy 20 is the industry standard for viscose rayon equipment worldwide.
| Standard | Designation |
|---|---|
| UNS (USA) | N08020 |
| DIN / Werkstoff Nr. (Germany) | 2.4660 / NiCr20CuMo (DIN 17740) |
| AFNOR (France) | Z2NCUD31-20AZ |
| EN (Europe) | NiCr20CuMo |
| BS (UK) | — |
| JIS (Japan) | NCF 8020 |
| GB (China) | NS 143 |
| Commercial Name | Carpenter 20 / 20Cb-3 |
| Cast Equivalent | CN-7M (ASTM A351) |
Alloy 20 is readily welded and formed using standard austenitic-stainless techniques, but requires controlled heat input and matching or over-matching filler metal to preserve corrosion resistance in the weld zone. No post-weld heat treatment is required.
| Process | Application | Filler Metal |
|---|---|---|
| GTAW (TIG) | Tube-to-tubesheet, thin-wall pipe | ERNiCr-3 or ER320LR |
| GMAW (MIG) | Medium-thickness plate, shop fabrication | ERNiCr-3 or ER320LR |
| SMAW (Stick) | Field welding, repair | ENiCr-3 or E320LR |
| SAW | Thick plate, heavy structural | ERNiCr-3 with neutral flux |
Key Welding Parameters
Heat input: 0.5–1.5 kJ/mm. Excessive heat input causes microfissuring and reduces corrosion resistance in the HAZ.
Interpass temperature: Keep below 150 °C. Clean between passes with stainless wire brush.
Shielding gas: Argon (GTAW/GMAW); Ar + 2–5% H₂ for improved penetration on clean material.
Backing gas: Argon purge on ID of pipe welds to prevent sugaring (oxidation) on the root pass.
Filler selection: ERNiCr-3 (Alloy 82) is the general-purpose choice. ER320LR (low-residual Alloy 20) is used when matching chemistry is required for weld metal corrosion resistance.
No PWHT required: Columbium stabilization prevents sensitization, so the as-welded condition maintains full corrosion resistance.
Forming and Machining
Cold forming: Alloy 20 work-hardens rapidly (faster than 316L). Use slow feed rates, generous bend radii (≥ 2× thickness), and intermediate annealing for deep draws.
Hot forming: 1150–1230 °C; reheat if temperature drops below 980 °C.
Machining: Use sharp carbide tooling, low surface speeds, positive feed, and rigid setups. The alloy's gummy, work-hardening behavior requires techniques similar to 316L but with more conservative parameters.
| Treatment | Temperature | Cooling | Purpose |
|---|---|---|---|
| Solution Anneal | 980–1010 °C (1800–1850 °F) | Water quench | Restore full corrosion resistance after cold work |
| Stress Relief | Not recommended | — | Columbium stabilization eliminates the need |
| Hot Working Range | 1150–1230 °C | Air or water | Forging, rolling, extrusion |
Important: Solution annealing must be followed by rapid water quenching. Slow cooling through the 540–760 °C range can precipitate carbides and sigma phase, reducing corrosion resistance. All Alloy 20 products supplied by JN Alloy are delivered in the solution-annealed condition.
Alloy 20 is specified across industries where sulfuric acid, mixed acids, or chloride SCC drive material selection:
| Industry | Typical Equipment | Why Alloy 20 |
|---|---|---|
| Sulfuric acid production | Acid coolers, towers, tanks, piping | Purpose-designed for H₂SO₄ |
| Petrochemical refining | Alkylation units, overhead condensers | H₂SO₄ + chloride resistance |
| Fertilizer manufacturing | Phosphoric acid reactors, digestion vessels | Mixed acid + impurity resistance |
| Pharmaceuticals | API reactors, crystallizers, piping | Acid cycling, product purity |
| Steel pickling | Pickling tanks, heating coils, fume scrubbers | H₂SO₄ + HCl + Fe³⁺ resistance |
| Rayon & synthetic fiber | Spinning baths, pump casings | Industry standard for viscose |
| Chemical processing | Heat exchangers, reactor vessels, columns | Versatile acid resistance |
| Food & dye production | Process tanks, piping | Organic acid resistance |
| Plastics & rubber | Synthetic rubber equipment | Catalyst acid resistance |
| Marine / coastal | Heat exchangers in chloride service | SCC immunity from 33% Ni |
Alloy 20 is primarily used in sulfuric acid systems, chemical processing equipment, pharmaceutical manufacturing, petrochemical plants, and any environment involving strong acids combined with chlorides. Common applications include heat exchangers, reactor vessels, pickling tanks, piping systems, pumps, and valves exposed to H₂SO₄ concentrations up to 98% at moderate temperatures.
Alloy 20 contains 32–38% nickel (vs 10–14% in 316L), 3–4% copper, and 2–3% molybdenum, giving it far superior resistance to sulfuric acid and chloride stress-corrosion cracking. 316L will crack and pit in warm chloride service above 60 °C, while Alloy 20 is practically immune to chloride SCC due to its high nickel content. However, Alloy 20 costs roughly 2.5 to 3 times more than 316L.
Alloy 20 is technically a nickel-iron-chromium alloy that bridges the gap between stainless steel and nickel superalloys. With 32–38% nickel and roughly 40% iron, it is sometimes classified as a stainless steel (it appears in ASTM standards alongside austenitic grades) but behaves like a nickel alloy in corrosion performance. It is most accurately described as a nickel-based corrosion-resistant alloy.
Alloy 20 performs excellently in sulfuric acid at concentrations up to 50% at temperatures up to 80 °C. At higher concentrations (50–90%), the safe temperature drops to 40–60 °C. Above 90% H₂SO₄, Alloy 20 is not recommended at elevated temperatures. Always consult an isocorrosion chart for the specific concentration and temperature of your service.
Yes. Alloy 20 is readily welded using GTAW (TIG), GMAW (MIG), and SMAW processes with ERNiCr-3 or ER320LR filler metal. The alloy is stabilized with columbium (niobium), which prevents sensitization during welding. No post-weld heat treatment is required for corrosion performance. Heat input should be controlled to 0.5–1.5 kJ/mm with interpass temperature below 150 °C.
Alloy 20 products are governed by: ASTM B729 (seamless pipe and tube), ASTM B474 (welded pipe, ERW), ASTM B464 (welded pipe, SAW), ASTM B463 (plate, sheet, strip), ASTM B472 (wire and welding rod), ASTM B473 (bars and forgings), ASTM B564 (forgings), and ASTM B366 (welded fittings). ASME code-stamped equivalents use the SB- prefix.
Alloy 20 does not rust in the conventional sense because its high chromium (19–21%) and nickel (32–38%) content forms a stable passive oxide film. However, in extremely aggressive reducing acid environments at high temperatures, or under crevice conditions with chloride deposits, localized attack is possible. In its designed sulfuric acid service range, Alloy 20 maintains a bright, corrosion-free surface for decades.
Alloy 20 is designated as UNS N08020, W.Nr. 2.4660, AFNOR Z2NCUD31-20AZ, EN NiCr20CuMo, and is also known commercially as Carpenter 20 or 20Cb-3. These designations refer to the same alloy chemistry and are fully interchangeable in specification. The cast equivalent is CN-7M (ASTM A351).
20Cb-3 is the improved version of the original Alloy 20 (20Cb). The "3" designates the addition of 3% copper for enhanced reducing-acid resistance, and the switch from titanium to columbium (Cb) stabilization for better weldability. All modern Alloy 20 produced to UNS N08020 is effectively the 20Cb-3 composition. The terms are used interchangeably today.
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