Carpenter 20 / 20Cb-3 — Niobium-Stabilized Austenitic Alloy for Sulfuric Acid and Chloride Environments  |  ASTM B473  |  Special Alloys

Alloy 20: The Sulfuric Acid Benchmark

Alloy 20 (UNS N08020) was developed in the 1930s and refined into the modern 20Cb-3 variant by Carpenter Technology specifically to handle sulfuric acid environments that destroy standard austenitic stainless steels. The alloy occupies a unique position between stainless steel and nickel alloys — it carries enough nickel (32–38%) to resist chloride stress-corrosion cracking, enough chromium (19–21%) and copper (3–4%) to handle sulfuric acid across a wide concentration range, and enough molybdenum (2–3%) to resist pitting and crevice corrosion.

The niobium (columbium) addition is the critical stabilizing element. By tying up carbon as niobium carbide, Nb prevents chromium carbide precipitation at grain boundaries during welding and heat exposure. This eliminates sensitization — the mechanism that causes intergranular corrosion in non-stabilized stainless grades like 304 and 316. As a result, Alloy 20 can be used in as-welded condition for most chemical process applications without post-weld annealing.

While newer nickel alloys like Hastelloy C-276 and C-2000 offer superior performance in the most aggressive mixed-acid environments, Alloy 20 remains the cost-effective benchmark for moderate sulfuric acid service where its lower nickel content delivers adequate performance at 40–60% of the cost of premium Ni-Cr-Mo alloys.

Five Defining Advantages of Alloy 20

Chemical Composition — ASTM B473 UNS N08020

The balanced Ni-Fe-Cr-Cu-Mo-Nb chemistry defines Alloy 20’s position as a cost-effective sulfuric acid specialist. Each element serves a specific corrosion-resistance function:

★ Starred elements define Alloy 20’s corrosion performance — Ni, Cr, Cu, Mo, and Nb are the five functional pillars

Mechanical Properties — Room Temperature (Annealed)

Alloy 20 is a solid-solution strengthened austenitic alloy. Properties are stable in the annealed condition. The work-hardening rate is comparable to 316L stainless steel — standard austenitic fabrication equipment and techniques apply. Cold work increases strength but reduces ductility; interstage annealing is recommended after cold reductions exceeding 15%.

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

Physical Properties — UNS N08020

Alloy 20’s physical properties are comparable to other austenitic stainless steels. The moderate density (8.05 g/cm³) reflects the iron-base composition with nickel and chromium additions. The alloy is non-magnetic in all conditions.

Heat Treatment Procedures

Alloy 20 requires solution annealing to restore full ductility after hot or cold working. The niobium stabilization eliminates the need for ultra-low carbon control or post-weld solution annealing in most service conditions:

Corrosion Resistance — Sulfuric Acid Specialist

Alloy 20 was engineered specifically for sulfuric acid service and remains the cost-effective benchmark for this environment. The combination of copper (for H‍₂SO‍₄ passivation), chromium (for oxidizing acid resistance), molybdenum (for pitting resistance), and nickel (for chloride SCC immunity) creates a balanced corrosion profile that outperforms 316L across a wide range of chemical environments.

Sulfuric Acid (H‍₂SO‍₄) ★ — DEFINING STRENGTH

Alloy 20’s signature capability. The 3–4% copper addition self-polarizes the alloy surface in sulfuric acid, shifting the corrosion potential into the passive range where a protective Cr‍₂O‍₃ film is stable. Resistance extends from dilute concentrations up to ~50% H‍₂SO‍₄ at temperatures up to 80°C, and to higher concentrations at reduced temperatures. In boiling 20–40% H‍₂SO‍₄ — a condition that destroys 316L — Alloy 20 achieves corrosion rates below 0.1 mm/yr.

Phosphoric Acid (H‍₃PO‍₄)

Good resistance to phosphoric acid across a wide concentration range. Used extensively in wet-process phosphoric acid plants for heat exchangers, pumps, and piping. Contaminants (chlorides, fluorides) in wet-process acid may require upgrade to Hastelloy G-30 or Alloy 31.

Nitric Acid (HNO‍₃)

Good resistance at moderate concentrations and temperatures. The 19–21% Cr provides adequate oxidizing acid protection. For pure HNO‍₃ service at high concentrations or boiling, 304L stainless remains more cost-effective. Alloy 20 is preferred when HNO‍₃ is mixed with H‍₂SO‍₄ or chlorides.

Chloride Stress-Corrosion Cracking (SCC) ★

The 32–38% nickel content provides complete immunity to chloride SCC — the failure mode that limits 316L service life above 60°C in chloride environments. This makes Alloy 20 the standard upgrade from 316L when chloride SCC is the governing failure mode and full nickel-alloy cost is not justified.

Pitting & Crevice Corrosion

Moderate pitting resistance with PREN ≈ 28. Adequate for many chemical process environments but inferior to 904L (~36), Alloy 31 (~50), and Ni-Cr-Mo alloys (C-276 ~66). For severe chloride pitting service, upgrade to Alloy 31 or C-276.

Intergranular Corrosion (IGC)

Passes ASTM G28 Method A and A262 Practice E in the annealed condition. The niobium stabilization provides excellent as-welded IGC resistance — Nb ties up carbon as NbC, preventing Cr‍₂‍₃C‍₆ precipitation at grain boundaries. This is the key advantage over non-stabilized grades like 316L, which requires L-grade or 316Ti for welded service.

Corrosion Rate Comparison — Alloy 20 vs 316L vs C-276 (mm/yr)

Green = excellent; Red = unacceptable. Values are typical — actual performance depends on specific process chemistry, aeration, velocity, and contaminants.

Grade Selection Guide — Alloy 20 Positioning

Alloy 20 occupies the sweet spot between standard stainless steel and premium nickel alloys. It delivers 70–80% of Ni-Cr-Mo alloy corrosion performance at 40–60% of the cost — optimal for moderate sulfuric acid and chloride SCC service:

Key Applications

Alloy 20 is the workhorse material for sulfuric acid processing, pharmaceutical manufacturing, and chemical process industries where 316L fails prematurely. Its balanced chemistry and cost-effective positioning make it the default specification for moderate acid service:

Applicable Standards & Specifications

Available Product Forms from HT PIPE

We supply Alloy 20 in annealed condition with EN 10204 3.1 certification on every shipment. ASTM G28 IGC testing per heat is available on specification. Alloy 20 is a cost-effective alloy with good availability — contact us for current stock and project quotations.

HT PIPE Project Experience — Alloy 20

For additional project references matching your specific process chemistry, contact our technical sales team. We can provide anonymized case studies for sulfuric acid, phosphoric acid, pharmaceutical, and chemical processing applications.

Frequently Asked Questions — Alloy 20