Alloy 20 - also known by its original trade name Carpenter 20Cb-3 and the designation UNS N08020 - is a niobium-stabilized, copper-bearing nickel-iron-chromium austenitic alloy developed specifically for resistance to sulfuric acid. First introduced in 1951 to solve the persistent problem of valves and equipment corroding in dilute sulfuric acid, it occupies a distinct position in the valve materials classification system: more corrosion-resistant and more costly than 304 or 316 stainless, but less costly than the true nickel alloys, making it the economical answer wherever sulfuric, phosphoric, or nitric acid resistance is the controlling requirement. Its defining ingredient is copper, which is unusual among corrosion-resistant alloys and is the direct source of its sulfuric-acid performance.
Key Takeaways
- Alloy 20 (Carpenter 20Cb-3, UNS N08020) is a nickel-iron-chromium austenitic alloy with a nominal balance near 35% nickel, 35% iron, and 20% chromium, plus 2-3% molybdenum and 3-4% copper, stabilized with niobium.
- It was developed specifically for sulfuric acid service; the copper addition is what gives it resistance across a wide span of sulfuric acid concentrations and temperatures that 304 and 316 stainless cannot survive.
- Niobium stabilization prevents sensitization and intergranular corrosion in welds, so fabricated and welded valve bodies retain full corrosion resistance without post-weld heat treatment.
- Its higher nickel content also gives strong resistance to chloride stress corrosion cracking, making it a cost-effective upgrade from stainless steel for acid and chloride-bearing valve service.
How It Works
The Copper-Stabilized Austenitic System
Alloy 20 is built on an iron-nickel-chromium austenitic matrix similar in structure to stainless steel, but with the nickel content raised to roughly a third of the alloy and with deliberate additions of molybdenum and, critically, copper. The chromium provides a passive oxide film for general corrosion resistance, molybdenum improves resistance to pitting and to reducing conditions, and the higher nickel content stabilizes the austenite and confers resistance to chloride stress corrosion cracking. This base chemistry alone would make Alloy 20 a capable super-austenitic stainless steel; it is the copper addition that elevates it specifically for acid duty.
Why Copper Resists Sulfuric Acid
The 3-4% copper in Alloy 20 is the element responsible for its outstanding sulfuric acid performance. In sulfuric acid, copper alters the electrochemical behaviour of the alloy surface, slowing the cathodic reaction that drives corrosion and helping the alloy maintain a stable, protective condition across dilute and intermediate acid concentrations where chromium passivity alone is insufficient. This is why Alloy 20 can handle sulfuric acid streams that rapidly corrode 316 stainless, and why it remains the reference material for sulfuric acid valves, pumps, and piping decades after its introduction. The practical concentration-and-temperature envelope for this protection is read from published iso-corrosion charts and summarized for selection purposes in the material for acid service reference.
Niobium Stabilization Against Sensitization
Like the stabilized grades of stainless steel, Alloy 20 contains niobium (columbium, hence the "Cb" in 20Cb-3) deliberately added to combine with carbon and prevent the formation of chromium carbides at grain boundaries during welding. Without this stabilization, the heat of welding would deplete chromium near the grain boundaries - sensitization - leaving those zones vulnerable to intergranular corrosion in service. Niobium ties up the carbon instead, so welded valve bodies and fabricated assemblies retain full corrosion resistance in the weld and heat-affected zone, which is essential for the leak-tight integrity of acid-service valves.
Composition and Standards
Nominal Composition (UNS N08020)
Alloy 20 has a nominal composition of approximately 32-38% nickel, 19-21% chromium, 2-3% molybdenum, 3-4% copper, and niobium stabilization (typically eight times the carbon content), with the balance iron. The deliberate balance of these elements - chromium for passivity, molybdenum for pitting and reducing-acid resistance, copper for sulfuric acid resistance, nickel for chloride SCC resistance, and niobium for weld stability - is what distinguishes it from both ordinary stainless steel and the chromium-molybdenum nickel alloys.
ASTM Specifications
Alloy 20 valve and pressure components are specified to a family of ASTM standards including B462 for forgings and flanges, B473 for bar and wire, B729 for seamless pipe and tube, B463 for plate, sheet, and strip, and B464 for welded pipe; cast valve bodies use the equivalent CN-7M chemistry under ASTM A351 / A744. The alloy can also be supplied to NACE MR0175 for sour-service applications, allowing its use where sulfuric acid resistance must be combined with hydrogen-sulfide exposure.
Is It a Stainless or a Nickel Alloy?
Alloy 20 sits deliberately on the boundary. With nickel near 35% it has too much nickel to be a conventional stainless steel but too little to be a true nickel-based alloy, so it is classified as a super-austenitic stainless steel or nickel-iron-chromium alloy depending on the source. In valve selection this matters because it positions Alloy 20 as the natural intermediate step between 316 stainless and the more expensive duplex stainless and nickel alloys when acid resistance must be improved without paying for a full nickel-alloy solution.
Advantages in Valve Service
Sulfuric Acid Resistance Across Concentrations
The headline advantage of Alloy 20 is its resistance to sulfuric acid over a broad band of concentrations and temperatures, a range in which it remains usable while 304 and 316 stainless corrode at unacceptable rates. This makes it the default body and trim material for valves throughout sulfuric acid production, handling, and consuming processes, and it is frequently the most economical material that will survive the service at all - higher nickel alloys add cost without adding value in many sulfuric streams.
Broad Acid and Chloride Resistance
Beyond sulfuric acid, Alloy 20 resists phosphoric acid, nitric acid, and many mixed-acid and acidic-salt environments, and its higher nickel content provides resistance to chloride stress corrosion cracking that ordinary austenitic stainless steels lack. This combination lets a single Alloy 20 valve handle process streams that mix acids and chlorides - common in fertilizer, pickling, and specialty-chemical service - where stainless steel would be at risk of both general corrosion and chloride cracking.
Weld Integrity and Fabricability
Because it is niobium-stabilized, Alloy 20 can be welded and fabricated into valve bodies and assemblies without losing corrosion resistance at the welds, avoiding the intergranular attack that would otherwise concentrate at the heat-affected zone. It machines and forms more readily than the high-molybdenum nickel alloys, which keeps fabrication cost down and makes it practical for the larger, more complex valve geometries used in acid-plant service.
Typical Applications in Valves
Sulfuric Acid Plants
Alloy 20 is the standard valve material throughout sulfuric acid manufacturing and handling - acid coolers, pump discharge lines, storage and transfer systems, and dilution circuits - where its copper-driven resistance keeps isolation and control valves serviceable in an environment that would rapidly consume stainless steel. It is specified for both cast bodies and machined trim to ensure every wetted surface carries the acid resistance.
Pickling and Metal Finishing
Steel pickling lines and metal-finishing operations use hot sulfuric and mixed acids to clean metal surfaces, and Alloy 20 valves regulate and isolate these aggressive pickling liquors reliably. The alloy's combined acid and chloride resistance suits the variable, contaminated chemistries of working pickling baths better than stainless steel.
Fertilizer and Phosphoric Acid
Phosphoric acid production for fertilizers exposes valves to hot acid containing fluorides, chlorides, and abrasive solids, and Alloy 20 is widely used for valves in these circuits because of its resistance to phosphoric and residual sulfuric acid. Its chloride SCC resistance is an important secondary benefit in the chloride-contaminated wet-process phosphoric acid environment.
Pharmaceutical and Specialty Chemicals
In pharmaceutical and specialty-chemical plants, Alloy 20 valves handle sulfuric and nitric acid process and cleaning streams while avoiding the metallic contamination that corrosion would introduce into high-purity products. Where a stream becomes too aggressive for Alloy 20 - strongly reducing chlorides or hot concentrated acids - the selection moves up to a Hastelloy grade, as described in the Hastelloy valve applications guide, and broader media-based guidance is given in the corrosive media valve selection reference.
Frequently Asked Questions
Is Alloy 20 a stainless steel or a nickel alloy?
Alloy 20 sits on the boundary between the two. With roughly 32-38% nickel it has more nickel than standard austenitic stainless steels but less than true nickel-based alloys such as Hastelloy, so it is often called a super-austenitic stainless steel or a nickel-iron-chromium alloy. Functionally it bridges the gap between 316 stainless and the nickel alloys, offering much better acid resistance than stainless at lower cost than Hastelloy.
Why is Alloy 20 so good for sulfuric acid?
Its corrosion resistance in sulfuric acid comes mainly from its 3-4% copper addition, which suppresses the corrosion reaction across a wide range of sulfuric acid concentrations and temperatures. Combined with about 20% chromium and 2-3% molybdenum, this lets Alloy 20 handle dilute-to-intermediate sulfuric acid where 304 and 316 stainless steels corrode rapidly. It was specifically developed in 1951 for sulfuric acid service.
Can Alloy 20 be used for hydrochloric acid?
Not as a primary choice. Alloy 20 excels in sulfuric, phosphoric, and nitric acid, but for hydrochloric acid and other strongly reducing chlorides a higher-molybdenum nickel alloy such as Hastelloy B-3 or C-276 is required. Alloy 20 does, however, offer good resistance to chloride stress corrosion cracking compared with 304 and 316 stainless because of its higher nickel content.
What is the temperature limit of Alloy 20 in acid service?
Alloy 20 is used across a wide range of sulfuric acid concentrations up to moderately elevated temperatures, but its useful limit falls as both concentration and temperature rise. For hot concentrated sulfuric acid or boiling conditions, higher alloys are required. The exact limit must be read from a published iso-corrosion chart for the specific acid concentration and temperature, not assumed.
Conclusion
Alloy 20 earns its enduring place in acid-service valves through a single, well-targeted design: a copper addition that delivers sulfuric acid resistance no stainless steel can match, backed by chromium, molybdenum, nickel, and niobium for passivity, pitting resistance, chloride SCC resistance, and weld integrity. It is the economical bridge between 316 stainless and the nickel alloys - the right choice when sulfuric, phosphoric, or nitric acid governs selection, and the logical step before moving up to Hastelloy for the most aggressive reducing-chloride streams. As always, the concentration and temperature of the specific service must be checked against published corrosion data using the valve material selection guide.