Alloy 20: Sulfuric Acid Resistance & Pharmaceutical Equipment

← Back to Knowledge Center

Overview

Alloy 20 (UNS N08020 / W.Nr. 2.4660), also known as Carpenter 20 or 20Cb-3, is a nickel-iron-chromium austenitic alloy with additions of molybdenum, copper, and niobium. It was originally developed by Carpenter Technology in the 1950s specifically for use in sulfuric acid environments, and its composition has been refined over the decades to provide exceptional resistance to both general and localized corrosion in a wide range of aggressive chemical media.

The alloy's corrosion resistance stems from a synergistic combination of its constituent elements: chromium provides oxidation resistance, molybdenum enhances resistance to reducing conditions and pitting, copper increases resistance to sulfuric acid, and niobium stabilizes the alloy against sensitization during welding, preventing intergranular corrosion. This multi-element approach makes Alloy 20 one of the most cost-effective corrosion-resistant alloys available, offering performance comparable to much more expensive nickel-based alloys in many common chemical environments.

Chemical Composition (ASTM B473 / ASME SA-473)

The chemical composition of Alloy 20 is optimized for resistance to sulfuric acid across a wide range of concentrations and temperatures. The niobium addition (at least 8 times the carbon content, per the stabilization formula Nb = 8×C minimum) ensures immunity to intergranular corrosion after welding, making the alloy suitable for fabricated equipment without post-weld heat treatment.

Element	Min %	Max %
Nickel (Ni)	29.0	33.0
Chromium (Cr)	19.0	21.0
Iron (Fe)	Balance	Balance
Molybdenum (Mo)	2.0	3.0
Copper (Cu)	3.0	4.0
Niobium + Tantalum (Nb+Ta)	8×C min	1.00
Manganese (Mn)	—	2.00
Silicon (Si)	—	1.00
Carbon (C)	—	0.07
Phosphorus (P)	—	0.040
Sulfur (S)	—	0.035

Physical Properties

Alloy 20 is an austenitic alloy with a face-centered cubic (FCC) crystal structure. Its physical properties fall between those of standard austenitic stainless steels and higher-nickel alloys. The relatively low density compared to nickel-rich alloys such as Inconel 625 makes it a more economical choice for large-scale chemical processing equipment where weight and material cost are significant factors.

Property	Value	Unit
Density	8.05	g/cm3
Melting Range	1380–1430	°C
Specific Heat (21°C)	500	J/kg·K
Thermal Conductivity (21°C)	12.2	W/m·K
Electrical Resistivity (21°C)	1.08	μΩ·m
Modulus of Elasticity (21°C)	197	GPa
Mean Coefficient of Thermal Expansion (21–100°C)	14.5	μm/m·°C
Mean Coefficient of Thermal Expansion (21–400°C)	16.0	μm/m·°C

Mechanical Properties at Room Temperature

The mechanical properties of Alloy 20 are comparable to those of standard austenitic stainless steels such as 304L and 316L. The alloy is not precipitation-hardening and derives its strength from the solid-solution effect of its constituent elements. For applications requiring higher strength, Alloy 20 can be cold-worked to achieve increased yield and tensile strength, although this reduces ductility and may affect corrosion resistance in some environments.

Property	Value
Tensile Strength	621 MPa (90 ksi)
Yield Strength (0.2% offset)	241 MPa (35 ksi)
Elongation in 50 mm	42%
Reduction of Area	55%
Hardness (Rockwell B)	82 HRB
Impact Strength (Charpy V-notch, room temp)	250 J

High-Temperature Mechanical Properties

Alloy 20 is primarily designed for corrosion resistance rather than high-temperature strength. Its mechanical properties decline progressively at elevated temperatures, and it is generally not recommended for structural applications above 500°C. The following data shows the tensile properties at elevated temperatures for annealed material.

Temperature (°C)	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)
21 (Room)	621	241	42
100	570	215	45
200	530	195	48
300	490	180	50
400	450	165	52
500	400	150	55

Corrosion Resistance

The corrosion resistance of Alloy 20 is its defining characteristic. The alloy was specifically developed to resist sulfuric acid, but its multi-element composition provides broad protection against a wide range of aggressive chemical environments. Understanding the specific corrosion mechanisms and the environments where Alloy 20 excels is essential for proper material selection.

Resistance to Specific Media:

• Sulfuric Acid: Excellent resistance across virtually the entire concentration range at temperatures up to 65°C. At concentrations below 50%, Alloy 20 provides superior resistance even at temperatures approaching 80°C. In concentrated sulfuric acid (above 80%), resistance remains good at moderate temperatures. This is the alloy's primary application domain and the reason it was originally developed.
• Phosphoric Acid: Very good resistance in phosphoric acid at concentrations and temperatures commonly encountered in fertilizer production. The copper addition particularly enhances performance in this medium.
• Nitric Acid: Good resistance in nitric acid at all concentrations up to boiling temperatures, owing to the 20% chromium content. However, more economical stainless steels (304L, 310) may be adequate for pure nitric acid service.
• Chloride Stress Corrosion Cracking: Highly resistant to chloride-induced stress corrosion cracking, even at concentrations and temperatures where standard austenitic stainless steels (304, 316) would fail. The high nickel content (29%) is the primary factor in this resistance.
• Pitting and Crevice Corrosion: Good resistance to pitting and crevice corrosion in chloride-containing environments. The molybdenum (2–3%) and chromium content work synergistically to provide this protection, though Alloy 20 is not as resistant as alloys with higher molybdenum content (such as Hastelloy C-276 or Inconel 625).
• Seawater: Good resistance to general corrosion in seawater. The alloy performs well in splash zones and tidal areas. For submerged service at elevated temperatures, alloys with higher molybdenum content may be preferred.

Applications

The combination of excellent sulfuric acid resistance, good mechanical properties, and competitive pricing relative to nickel-based alloys makes Alloy 20 a preferred material for a wide range of chemical processing and pharmaceutical applications:

• Chemical Processing: Reaction vessels, heat exchangers, distillation columns, piping systems, and tankage for handling sulfuric acid across the full concentration range. Alloy 20 is the standard material of construction for sulfuric acid alkylation units in petroleum refineries.
• Pharmaceutical Manufacturing: Process vessels, fermenters, piping, and clean-in-place (CIP) systems. The alloy's resistance to mixed acid environments encountered during pharmaceutical synthesis, combined with its excellent weldability and freedom from intergranular corrosion, makes it ideal for hygienic process equipment that must withstand aggressive cleaning solutions.
• Food Processing: Equipment for processing acidic food products, including citrus juice processing, vinegar production, and pickling operations. The alloy's resistance to organic acids and ease of cleaning make it suitable for food-grade equipment.
• Petrochemical Refining: Sulfuric acid alkylation unit components, including reactors, settler vessels, effluent lines, and heat exchangers. Alloy 20 has decades of proven service in these applications.
• Fertilizer Production: Phosphoric acid evaporators, piping, and tankage in phosphate fertilizer plants. The alloy resists the aggressive mixture of phosphoric acid, fluoride ions, and chlorides encountered in wet-process phosphoric acid production.
• Plastics and Synthetic Fiber Production: Equipment for handling mixed acid media in the production of nylon, rayon, and other synthetic fibers where sulfuric acid is a primary process reagent.
• Pulp and Paper: Bleaching equipment and recovery boilers in paper mills, where chloride-containing environments at elevated temperatures can cause stress corrosion cracking in standard stainless steels.

Welding and Fabrication

Alloy 20 exhibits excellent weldability using all standard arc welding processes (GTAW/TIG, GMAW/MIG, SMAW/stick). The niobium stabilization prevents sensitization during welding, eliminating the need for post-weld heat treatment to restore intergranular corrosion resistance. Key welding considerations include:

• Filler Metal: AWS A5.4 E320LR-15 (SMAW) or AWS A5.9 ER320LR (GTAW/GMAW) are the recommended filler metals. The "LR" designation indicates low-residual elements for maximum corrosion resistance in the weld deposit.
• Preheat: Not required. The alloy should be welded at room temperature.
• Interpass Temperature: Should be limited to 150°C maximum to prevent excessive heat input that could cause microstructural degradation.
• Post-Weld Heat Treatment: Not required for corrosion resistance, as the niobium stabilization prevents sensitization. Solution annealing at 925–980°C may be specified for stress relief in thick-section weldments.

Available Product Forms

Hangbo Alloy Group manufactures and supplies Alloy 20 in the following product forms, conforming to ASTM, ASME, and customer specifications:

• Round Bars: ASTM B473, ASME SA-473, diameters from 6 mm to 250 mm, hot-finished or cold-drawn, annealed.
• Seamless Tubes & Pipes: ASTM B729, ASTM B468, sizes from 6 mm OD to 219 mm OD, various wall thicknesses.
• Plates & Sheets: ASTM A463, thicknesses from 0.5 mm to 50 mm, hot-rolled or cold-rolled.
• Forgings: ASTM B472, custom flanges, fittings, and complex shapes per customer drawing.
• Welding Wire: AWS A5.9 ER320LR, diameters 0.8 mm to 5.0 mm.

Related Standards

Standard	Description
ASTM B473 / ASME SA-473	Bar and Rod
ASTM B729 / ASME SA-729	Seamless Pipe and Tube
ASTM B468	Welded Tube
ASTM A463	Plate, Sheet, and Strip
ASTM B472	Forgings
ASTM B366	Fittings
AWS A5.9 ER320LR	Bare Welding Wire
AWS A5.4 E320LR	Covered Welding Electrode
ASME SB-473	Boiler and Pressure Vessel Code

Contact Us for Alloy 20

Hangbo Alloy Group maintains extensive inventory of Alloy 20 in all standard product forms. We can provide mill test certificates, third-party inspection reports (SGS, TUV), and custom processing services including cutting, machining, and surface finishing for pharmaceutical-grade applications.

For quotations, material certifications, or technical consultation, contact our sales team or call +86-136-1165-6360. We typically respond within 10 minutes.