Alloy 31 (UNS N08031, EN 1.4562) stands as a premier nickel-chromium-molybdenum superalloy engineered to conquer the most aggressive corrosive environments. Renowned for its exceptional resistance to oxidizing and reducing acids, particularly sulfuric acid, coupled with outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC), Alloy 31 is a critical material for demanding applications across chemical processing, pollution control, and offshore industries.

Understanding the Composition: The Foundation of Alloy 31's Superiority

The outstanding performance of Alloy 31 stems from its meticulously balanced chemical composition, optimized for maximum corrosion resistance:

• Nickel (Ni): ~30-32% - Provides the fundamental austenitic matrix, ensuring excellent ductility, toughness, and inherent corrosion resistance.
• Chromium (Cr): ~26-28% - A defining feature. This high chromium content delivers exceptional resistance to oxidizing media (nitric acid, chromic acid, seawater), promotes passivation, and enhances resistance to sulfidation and high-temperature oxidation.
• Molybdenum (Mo): ~6.0-7.0% - The critical element for combating reducing acids (sulfuric, hydrochloric, phosphoric) and providing superior resistance to localized corrosion (pitting and crevice corrosion) in chloride-containing environments.
• Iron (Fe): ~Balance (~34-37%) - Provides solid solution strengthening and cost-effectiveness compared to higher-nickel alloys.
• Nitrogen (N): ~0.15-0.25% - Significantly enhances resistance to localized corrosion, stabilizes the austenitic structure, and increases mechanical strength.
• Copper (Cu): ~1.0-1.4% - Specifically added to boost resistance to sulfuric acid across a wide concentration range, especially in the highly aggressive intermediate concentrations where many other alloys fail.
• Manganese (Mn): ≤ 2.0%, Silicon (Si): ≤ 0.3%, Carbon (C): ≤ 0.015% - Controlled low levels minimize detrimental carbide formation and ensure optimal weldability and corrosion performance.
• Tungsten (W): ~0.50-1.50% - Contributes to solid solution strengthening and further enhances localized corrosion resistance.

This unique combination, particularly the high chromium, substantial molybdenum, deliberate nitrogen addition, and optimized copper content, results in a Pitting Resistance Equivalent Number (PREN = %Cr + 3.3x%Mo + 16x%N) typically exceeding 50, placing Alloy 31 among the most corrosion-resistant nickel alloys available.

Key Properties and Performance Characteristics of Alloy 31

Alloy 31 delivers a powerful combination of properties essential for severe service:

1. Exceptional Corrosion Resistance:

   • Sulfuric Acid: Outstanding resistance across virtually the entire concentration range, from dilute to highly concentrated (>98%), and at elevated temperatures. Superior to standard stainless steels and many nickel alloys like Alloy 20, 825, and 904L, particularly in the critical 40-80% concentration range.
   • Phosphoric Acid: Excellent resistance to both wet-process and thermal phosphoric acids, including resistance to fluorides and chlorides present.
   • Hydrochloric Acid: Good resistance in dilute solutions and oxidizing conditions; outperforms many stainless steels but less resistant than highly molybdenum-rich alloys like C-276 in severe reducing conditions.
   • Nitric Acid: Excellent resistance due to high chromium content.
   • Mixed Acids & Process Streams: Resistant to complex mixtures common in chemical synthesis, pickling, and metal finishing.
   • Localized Corrosion: Outstanding resistance to pitting and crevice corrosion in seawater, brackish water, and chloride-laden process fluids (PREN >50). Excellent resistance to chloride-induced stress corrosion cracking (SCC).
   • Seawater: Highly resistant, suitable for condenser tubes, piping, pumps, and valves.

2. High Mechanical Strength: Offers significantly higher strength than standard austenitic stainless steels (304L, 316L) and comparable alloys like 904L. Typical mechanical properties in annealed condition:

   • Tensile Strength (Rm): 650 - 850 MPa
   • 0.2% Yield Strength (Rp0.2): 300 - 400 MPa
   • Elongation (A5): ≥ 35%

3. Good Fabricability and Weldability: Can be readily formed and machined using techniques suitable for nickel alloys. Welding is performed using matching filler metals (e.g., Alloy 31 welding electrodes/wires) or overmatching Ni-Cr-Mo alloys (e.g., Alloy 625). Low carbon content minimizes sensitization. Post-weld annealing is generally not required for corrosion resistance but may be used for stress relief.

4. Thermal Stability: Maintains good mechanical properties and corrosion resistance at elevated temperatures, suitable for applications up to approximately 400°C (750°F). Resists oxidation and sulfidation scaling.

Primary Applications of Alloy 31 (N08031)

Due to its unparalleled corrosion resistance profile, Alloy 31 is the material of choice for critical components in:

1. Chemical & Petrochemical Processing:

   • Sulfuric acid production, concentration, handling, and storage (pickling plants, acid coolers, piping, pumps, valves, tanks).
   • Phosphoric acid production (reactors, heat exchangers, evaporators, piping).
   • Handling halogen acids and salts (HCl, HF).
   • Reactors, columns, piping, and heat exchangers in aggressive organic and inorganic chemical synthesis.

2. Pollution Control & Flue Gas Desulfurization (FGD):

   • Scrubbers, absorber towers, ducting, dampers, fans, reheaters, and mist eliminators handling hot, corrosive, chloride-containing flue gases and sulfuric acid condensates.

3. Offshore Oil & Gas:

   • Seawater piping systems, pumps, valves, heat exchangers.
   • Process equipment handling corrosive production fluids containing H2S, CO2, and chlorides.
   • Umbilicals, subsea components.

4. Pulp & Paper Industry: Digesters, bleach plants, piping handling corrosive liquors and chlorides.

5. Pharmaceutical Industry: Reactors, vessels, piping for aggressive chemical synthesis.

6. Acid Mining & Hydrometallurgy: Leaching tanks, solvent extraction equipment, electrowinning cells, piping.

7. Seawater Desalination: Heat exchanger tubes, evaporator shells, piping.

Standards and Common Designations

• UNS: N08031
• EN / DIN: 1.4562 (X1NiCrMoCu32-28-7)
• ASTM:
  • B366 (Wrought Fittings)
  • B462 (Forged or Rolled Pipe Flanges, Fittings, Valves)
  • B463 / B463M (Sheet, Plate, Strip)
  • B464 / B464M (Welded Pipe)
  • B729 / B729M (Seamless UNS N08020, N08024, and N08026 Pipe)
  • B751 / B751M (Seamless UNS N08020, N08024, and N08026 Tubes)
  • B775 / B775M (General Requirements for Nickel and Nickel Alloy Welded Pipe)
• Other Names: Nicrofer® 3127 hMo (ThyssenKrupp trademark), Cronifer® 1925 hMo (VDM Metals trademark), Nickel Alloy 31, Alloy N08031.

Why Choose Alloy 31?

Select Alloy 31 when:

• Severe sulfuric acid corrosion is the primary challenge, especially across a wide concentration and temperature range.
• Resistance to pitting, crevice corrosion, and SCC in high-chloride environments (seawater, brines) is critical (PREN >50).
• High resistance to complex mixtures of oxidizing and reducing acids is required.
• Higher mechanical strength than standard stainless steels or alloys like 904L is needed.
• Cost-effectiveness compared to higher-nickel, higher-molybdenum alloys (like C-276, C-22) is a factor, while still offering exceptional performance.

Conclusion

Alloy 31 (UNS N08031) represents a pinnacle of corrosion-resistant alloy design, particularly excelling in sulfuric acid service and chloride-rich environments. Its high chromium content, substantial molybdenum, deliberate nitrogen addition, and strategic copper content synergistically create a material with unmatched versatility and durability against the most aggressive industrial corrosives. For engineers and designers facing severe corrosion challenges in chemical processing, FGD, offshore, and related industries, Alloy 31 offers a robust, reliable, and often cost-effective solution, ensuring long-term equipment integrity and operational safety. Understanding its unique composition and broad capabilities is key to leveraging its full potential in critical applications.