Nickel Alloy 20
Nickel Alloy 20 (UNS N08020, also known as Carpenter 20 or Alloy 20) is a nickel-iron-chromium alloy with copper and molybdenum additions. It was designed specifically to resist hot sulfuric acid corrosion. Niobium stabilization prevents sensitization in the as-welded condition, so it is the standard material for fabricated acid service equipment that does not require post-weld heat treatment.
1. Chemical Composition
The table below provides the full elemental composition of Nickel Alloy 20. The copper content gives it reducing acid resistance, while niobium stabilizes the alloy against intergranular corrosion after welding.
| Grade | Standard | C | Mn | P | S | Si | Ni | Nb | Mo | Cu | Nb+Ta | Fe |
| N08020 | B729 | ≤0.07 | ≤2.0 | ≤0.045 | ≤0.035 | ≤1.00 | 32.00-38.00 | 19.00-21.00 | 2.00-3.00 | 3.00-4.00 | 8*C-1.00 | BAL |
2. Mechanical Properties
The mechanical values below apply to Nickel Alloy 20 pipe and tube in the solution-annealed condition. This is the standard delivery state for acid corrosion service.
| Grade | Standard | Tensile Strength , MPa (ksi) |
Yield Strength, MPa (ksi) |
Elongation , % |
| N08020 | B729 | ≥550（80） | ≥240（35） | ≥30 |
3. Equivalent Grade
The table below maps Nickel Alloy 20 (UNS N08020) to its equivalent designations in EN/DIN, JIS, ISO, and other international material classification standards.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Alloy 20 | N08020 | — | — | — | NiCr20CuMo | 2.4660 | — |
4. Key Technical Advantages
- Targeted Sulfuric Acid Resistance via Copper Addition: The 3.0–4.0% copper content reduces the anodic dissolution rate in sulfuric acid by shifting the corrosion potential. This makes the alloy particularly effective in 5–98% H2SO4 at temperatures up to 66°C, the working range for most sulfuric acid processing equipment. It outperforms Type 316L and lower-copper nickel alloys in this medium.
- Niobium Stabilization Eliminates Post-Weld Heat Treatment: Niobium at a minimum of 8 times the carbon content ties up interstitial carbon as stable NbC precipitates before and during welding. This prevents chromium carbide formation at grain boundaries, giving full sensitization immunity in the as-welded condition. For complex acid service assemblies where heat treatment of completed weldments is impractical, this is a major fabrication advantage.
- Resistance to Both Oxidizing and Reducing Acid Contaminants: The chromium content (19–21%) provides resistance to nitric acid contamination in mixed acid streams, while the molybdenum (2–3%) adds pitting resistance in chloride-containing sulfuric acid services. Together these address the complex chemistry found in actual acid plant process streams.
- Technical Note: Nickel Alloy 20 is not recommended for pure nitric acid service. The copper content that helps in sulfuric acid accelerates corrosion in strongly oxidizing environments. For pure nitric acid, stainless steel grades with low molybdenum and copper (such as TP304L or TP310) are more appropriate.
5. Common Manufacturing Standards
ASTM B729: Standard Specification for Seamless Pipe and Tube — Nickel-Iron-Chromium-Molybdenum-Copper Alloy (UNS N08020). This is the primary standard for Nickel Alloy 20 in seamless pipe and tube form, covering chemical composition, mechanical testing, and inspection requirements for corrosion-service applications.
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes, nickel alloy grades (Material No. 2.4660)
- DIN 17459: Seamless circular tubes made of nickel alloys
JIS Standards:
- JIS H 4552: Nickel and nickel alloy seamless pipes and tubes
ASME Standards (Non-linked):
- ASME SB-729: ASME Boiler and Pressure Vessel Code equivalent of ASTM B729 for pressure-retaining components in sulfuric acid service
Summary Table — Standard Equivalents:
| ASTM | ASME | EN/DIN | JIS | GB/T |
| ASTM B729 | ASME SB-729 | EN 10216-5 / DIN 17459 | JIS H 4552 | - |
6. Primary Applications
- Sulfuric Acid Plants: Absorption towers, acid coolers, heat exchanger shells and tubes, and pump casings in contact-process and dual-absorption H2SO4 plants handling concentrated acid at temperatures from ambient to 100°C.
- Pharmaceutical Manufacturing: Reactor vessels, transfer piping, and centrifuge components in pharmaceutical synthesis processes that use sulfuric acid as a catalyst or reactant. Preventing product contamination from metallic dissolution is a primary concern.
- Pickling and Acid Cleaning Systems: Immersion tanks, fume hoods, and overflow piping in hot dilute H2SO4 pickling baths used for steel mill scale removal and metal preparation.
- Synthetic Fiber Production: Acid recovery piping, spinning bath heat exchangers, and process vessels in viscose rayon and nylon manufacturing where concentrated sulfuric acid and zinc sulfate solutions coexist at elevated temperatures.
- Explosives and Fertilizer Industry: Nitration and neutralization process piping in the production of ammonium sulfate, ammonium nitrate, and nitric acid-sulfuric acid mixed acid streams. Alloy compatibility with fluctuating acid composition is required.