Alloy 31: The Superior All-Rounder

Alloy 31 (UNS N08031, DIN 1.4562) was introduced in 1990 by VDM Metals as Nicrofer 3127 hMo — an evolution of the earlier Nicrofer 3127 LC with higher molybdenum and a deliberate nitrogen addition. The alloy occupies a strategic position between highly alloyed austenitic stainless steels (904L, 254 SMO) and full nickel-based alloys (C-276, C-2000). Its iron-base composition with 30–32% Ni, 26–28% Cr, 6–7% Mo, 1.0–1.4% Cu, and 0.15–0.25% N delivers pitting resistance (PREN ≈ 52) approaching nickel alloys at 60–70% of the cost.

The nitrogen addition is the critical innovation. Nitrogen stabilizes the austenitic structure, allowing higher Cr and Mo contents without forming harmful intermetallic phases (sigma, chi) that plague older superaustenitics. Nitrogen also dramatically increases pitting resistance — in the PREN formula, nitrogen carries a coefficient of 16, far higher than Mo (3.3) or Cr (1.0). The result is an alloy that matches nickel-alloy corrosion performance in many environments while maintaining excellent fabricability and a pressure-vessel certification envelope of −196°C to 550°C (VdTÜV 509).

Compared to Alloy 20, Alloy 31 trades higher Cr, Mo, and nitrogen for superior high-concentration sulfuric acid and seawater pitting performance. Compared to 904L, the higher Mo + N pushes PREN from 36 to 52, enabling service in chloride environments that destroy 904L. Compared to Hastelloy C-276, Alloy 31 sacrifices some pure HCl and mixed-acid capability for a 30–40% cost reduction — making it the engineering optimum for FGD, seawater, phosphoric acid, and chlorine dioxide service.

Five Defining Advantages of Alloy 31

Chemical Composition — ASTM B625 UNS N08031

The Fe-Ni-Cr-Mo-Cu-N chemistry defines Alloy 31’s position as a nitrogen-alloyed superaustenitic bridging stainless and nickel alloys. The very low carbon (0.015% max) plus nitrogen stabilization minimizes sensitization risk; the 6–7% Mo + nitrogen combination drives pitting resistance to nickel-alloy levels:

★ Starred elements define Alloy 31’s corrosion performance — Cr, Mo, N, Ni, and Cu are the five functional pillars

Mechanical Properties — Room Temperature (Solution Annealed)

Alloy 31 is a solid-solution strengthened austenitic alloy. The 0.15–0.25% nitrogen addition provides significant solid-solution strengthening — yield strength is notably higher than 316L and 904L. Properties are stable in the solution-annealed condition. The alloy retains excellent toughness down to cryogenic temperatures, with impact energy ≥140 J/cm² at −196°C.

Impact Toughness (Charpy V-Notch)

Elastic Constants & Physical Constants (Annealed at 20°C)

Physical Properties — UNS N08031

Alloy 31’s physical properties are comparable to other high-alloy austenitic stainless steels. The density (8.05 g/cm³) reflects the iron-base composition with substantial Ni, Cr, and Mo additions. The alloy is essentially non-magnetic in all conditions — the nitrogen-stabilized austenite is highly stable and does not transform under cold work.

Heat Treatment Procedures

Alloy 31 requires solution annealing to dissolve precipitates formed during hot or cold working and to restore full corrosion resistance and ductility. The ultra-low carbon content plus nitrogen stabilization reduce sensitization risk in welded fabrications, but solution annealing after cold work is recommended for severe service:

Corrosion Resistance — The Superior All-Rounder

Alloy 31 was engineered to deliver nickel-alloy-level corrosion performance from an iron-base composition. The 26–28% Cr / 6–7% Mo / 0.15–0.25% N / 1.0–1.4% Cu combination produces a uniquely balanced corrosion profile: exceptional in high-concentration sulfuric acid, exceptional in oxidizing nitric acid, exceptional in chloride pitting and crevice corrosion, and durable in alkaline and acidic halide environments. This breadth of capability is what earns the alloy its “all-rounder” positioning.

Sulfuric Acid (H‍₂SO‍₄) ★ — DEFINING STRENGTH

Alloy 31’s signature capability — superior to Alloy 20 and 904L. The 1.0–1.4% copper addition self-polarizes the alloy surface in sulfuric acid (similar mechanism to Alloy 20 but lower Cu content), while the higher Cr (26–28%) and Mo (6–7%) extend the passive range into concentrated and hotter acid. In 50% H‍₂SO‍₄ at 80°C, Alloy 31 achieves corrosion rates below 0.05 mm/yr — matching C-276 performance at lower cost. This makes it the preferred material for high-concentration sulfuric acid service where Alloy 20 reaches its limit.

Phosphoric Acid (H‍₃PO‍₄) — Wet-Process Service

Excellent resistance to wet-process phosphoric acid. The high Cr and Mo content handles the chloride, fluoride, and sulfate contaminants typical of wet-process acid, while the alloy resists both general corrosion and erosion-corrosion in evaporators and heat exchangers. Used extensively in phosphoric acid concentrators and superphosphoric acid service.

Nitric Acid (HNO‍₃) — Boiling 67% Service

Excellent resistance, including to boiling 67% nitric acid (Huey test conditions). The 26–28% chromium content forms a stable Cr‍₂O‍₃ passive film that resists oxidizing acid attack. Corrosion rate in boiling 67% HNO‍₃ is approximately 0.03 mm/yr — superior to Alloy 20 (0.15), 904L (0.10), and even C-276 (0.30, due to Mo-bearing transpassive dissolution). This makes Alloy 31 a strong candidate for mixed nitric/sulfuric acid environments.

Chloride Pitting & Crevice Corrosion ★

Outstanding resistance. PREN ≈ 52 places Alloy 31 between 254 SMO and C-276. Critical pitting temperature (CPT) in ASTM G48 exceeds 90°C — far above 316L (~15°C), Alloy 20 (~35°C), and 904L (~50°C). This is the key capability that qualifies Alloy 31 for seawater coolers, FGD absorbers, and other chloride-rich service where pitting is the governing failure mode. The nitrogen contribution is decisive — removing nitrogen from the formula drops PREN by ~3 points.

Alkaline & Acidic Halides, Chlorine Dioxide

Excellent resistance in alkaline halide and acidic halide environments, and outstanding performance in chlorine dioxide (ClO‍₂) bleaching service — the dominant failure environment in pulp and paper bleach plants. The combination of high Cr, Mo, and N resists the aggressive oxidizing chloride conditions that destroy 316L and 904L bleach plant equipment.

Chloride Stress-Corrosion Cracking (SCC)

The 30–32% nickel content provides immunity to chloride SCC — the failure mode that limits 316L service life above 60°C in chloride environments. Combined with the high PREN, this makes Alloy 31 suitable for seawater and brackish water service at temperatures where 316L, 317L, and even 904L fail by SCC or pitting.

Intergranular Corrosion (IGC)

Passes ASTM G28 Method A in the solution-annealed condition. The ultra-low carbon (0.015% max) reduces sensitization risk, while nitrogen stabilization further limits chromium carbide precipitation. Welded fabrications retain good IGC resistance when proper low-heat-input procedures are followed. For severe service, post-weld solution anneal + pickling restores full IGC resistance.

Corrosion Rate Comparison — Alloy 31 vs Alloy 20 vs 904L vs C-276

Green = excellent; Red = unacceptable. Values are typical — actual performance depends on specific process chemistry, aeration, velocity, and contaminants. Note C-276’s poorer performance in boiling HNO‍₃ (Mo transpassive dissolution) vs Alloy 31’s Cr-driven resistance.

Grade Selection Guide — Alloy 31 Positioning

Alloy 31 is positioned as “The Superior All-Rounder” — the optimal choice when service conditions exceed the capability of Alloy 20 and 904L but the cost of C-276 is not justified. It delivers near-nickel-alloy pitting and sulfuric acid resistance at 60–70% of C-276 cost:

Key Applications

Alloy 31 is the workhorse material for FGD systems, seawater-cooled heat exchangers, pulp-and-paper bleach plants, and high-concentration sulfuric acid service where 316L, 317L, and 904L fail prematurely and C-276 cost is difficult to justify. Its balanced chemistry and broad corrosion capability make it the default upgrade specification when service conditions push beyond standard austenitic stainless steels:

Applicable Standards & Specifications

Available Product Forms from HT PIPE

We supply Alloy 31 in solution-annealed + water-quenched condition with EN 10204 3.1 certification on every shipment. ASTM G28 Method A intergranular corrosion testing per heat is standard for severe-service applications. Welding filler metal FM 59 (2.4607 / UNS N06059, ERNiCrMo-13) is available for fabrications. Contact us for project quotations and current stock.

HT PIPE Project Experience — Alloy 31

For additional project references matching your specific process chemistry — FGD, seawater cooling, phosphoric acid, HPAL, pulp and paper — contact our technical sales team. We can provide anonymized case studies and material selection recommendations for your corrosion environment.

Frequently Asked Questions — Alloy 31