Alloy 20 is the iron-nickel-chromium alloy that chemical plant engineers reach for when the medium is sulfuric acid and the stainless grades are not enough. The alloy, UNS N08020, occupies a narrow but heavily used position in the corrosion table: it resists sulfuric acid across a wide concentration range at moderate temperatures, it shrugs off the mixed acids that defeat 316L, and it costs a fraction of the molybdenum-heavy nickel alloys.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
Where Alloy 20 sits in the corrosion table
The fastest way to understand Alloy 20 is by its neighbors. Below it in alloy content, 316L stainless fails in warm sulfuric acid and in chloride media because its molybdenum level, 2-3%, is too low to hold the passive film.
Above it, the nickel-based alloys such as C-276 carry the full sulfuric acid duty at higher temperatures and concentrations, but at several times the price. Alloy 20 is the middle step: nickel at 32-38% with chromium, molybdenum, and copper, engineered specifically for sulfuric acid service and for the mixed-acid and chloride environments that appear around it in a chemical plant.
The copper content is the unusual row. Copper is not a common alloying addition to corrosion-resistant steels, but in Alloy 20 it is the element that shifts the corrosion behavior in sulfuric acid: it improves resistance in the reducing, dilute-acid region where the chromium oxide film is not stable, and it works with the molybdenum to resist the pitting and crevice attack of chloride-bearing media.
The result is an alloy whose sulfuric acid performance extends to roughly 50% concentration at temperatures up to about 80-100°C - the envelope in which the bulk of acid-handling piping in fertilizer, metallurgical, and specialty chemical plants operates.
Chemical composition per ASTM B729
The composition limits below follow ASTM B729, the seamless pipe and tube specification for UNS N08020. Iron is the balance element.
| Element | Composition limit, % |
| Ni | 32.0 - 38.0 |
| Cr | 19.0 - 21.0 |
| Mo | 2.0 - 3.0 |
| Cu | 3.0 - 4.0 |
| Nb + Ta | 8 x C min to 1.0 max |
| C | 0.07 max |
| Mn | 2.0 max |
| Si | 1.0 max |
| P | 0.045 max |
| S | 0.035 max |
| Fe | Balance |
Three rows matter for the buyer. The nickel at 32.0-38.0% is the matrix and the reason the alloy resists chloride stress-corrosion cracking far better than the austenitic stainless grades. The molybdenum and copper rows together set the sulfuric acid behavior and the pitting resistance.
And the columbium-plus-tantalum row at 8 x C minimum is the stabilizer: it ties up the carbon so the alloy can be welded and used without the sensitization that would otherwise create a chromium-depleted grain boundary in the heat-affected zone. The niobium-stabilized design is what makes Alloy 20 a fabricator-friendly material - it is welded and put into acid service without a post-weld heat treatment.
Mechanical properties per ASTM B729
Alloy 20 pipe and tube are furnished in the stabilized-annealed condition, and ASTM B729 requires the following minimum mechanical properties:
| Property | Minimum value (ASTM B729) |
| Tensile strength | 551 MPa (80 ksi) |
| Yield strength, 0.2% offset | 241 MPa (35 ksi) |
| Elongation | 30% |
Typical production values in the stabilized-annealed condition run about 620 MPa tensile and 280 MPa yield with 40% elongation. The strength is in the same range as annealed austenitic stainless, which means the alloy fabricates with the same shop practice: it bends, rolls, and machines like a 316-type grade, with a slightly higher work-hardening rate.
The mechanical simplicity of the specification - one delivery condition, no heat-treatment matrix - is part of why Alloy 20 is considered a workhorse: the buyer orders stabilized-annealed and the fabricator welds it without special procedure qualification beyond the normal alloy practice.
Physical properties
| Property | Value |
| Density | 8.08 g/cm³ |
| Melting range | 1400 - 1443°C (approx.) |
| Modulus of elasticity | 200 GPa (approx.) |
| Structure | Austenitic, non-magnetic |
| Thermal conductivity, 100°C | 13.5 W/m·K (approx.) |
The sulfuric acid envelope, applied
The practical question on every Alloy 20 enquiry is "will it hold this acid duty?" The answer is a function of concentration and temperature, and the table below is the engineering shorthand that plant designers actually use. Values are indicative service guidance for pipe and tube, not a substitute for a corrosion test on the exact process medium.
| Medium | Concentration / condition | Alloy 20 verdict |
| Sulfuric acid | 0 - 50%, up to ~80-100°C | Good to excellent - the core service envelope |
| Sulfuric acid | Above 50%, or boiling | Rapidly worse - consider a higher alloy |
| Phosphoric acid | Wet-process phosphoric acid | Good - standard choice in fertilizer plants |
| Nitric acid | All concentrations | Good, similar to 304L family |
| Mixed acids | H2SO4 + HCl, HNO3 + HF pickling baths | Good - where 316L fails |
| Chloride media | Salt water, brine, wet chlorine traces | Moderate - pitting resistance above 316L, below nickel alloys |
| Caustic solutions | Dilute | Good |
Two practical rules complete the picture. First, in the critical range of roughly 10-50% sulfuric acid, corrosion rates are strongly sensitive to temperature and to trace oxidizers in the acid; an upset that raises the temperature 20°C can change the corrosion rate by an order of magnitude, so the specification should be made against the worst-case operating condition, not the normal one.
Second, for acid that must be handled hot and concentrated, the economical answer is not more Alloy 20 wall - it is a step up to a higher-nickel alloy, and the decision should be made on the corrosion table, not on wall thickness.
Specifications and product forms
Alloy 20 is supplied against the following specifications, selected by product form:
- ASTM B729 / ASME SB-729 - seamless pipe and tube
- ASTM B464 / ASME SB-464 - welded pipe
- ASTM B468 - welded tube for heat exchangers
- ASTM B463 / ASME SB-463 - plate, sheet, and strip
- ASTM B473 - bar, rod, and forgings
The seamless pipe under B729 covers the NPS range up to large diameters, with schedules through the heavy wall classes; the welded pipe under B464 is produced from stabilized-annealed strip and is a cost-effective option for large diameters where the seam is radiographed and the joint is accepted. For heat-exchanger tube, B468 governs and the tube is supplied in straight lengths ready for rolling into the tubesheet. The stabilized-annealed condition applies to all forms.
Typical applications
Alloy 20 pipe and tube are specified across the following services:
- Sulfuric acid transfer and recirculation lines in chemical plants
- Wet-process phosphoric acid evaporators and piping in fertilizer plants
- Heat exchangers and condensers handling acid and chloride media
- Pickling bath piping and acid recovery systems in steel mills
- Synthetic rubber, pharmaceuticals, and specialty chemical process piping
- Pump, valve, and instrumentation components in acid service
In each service the alloy is the economic middle answer: it closes the gap where 316L fails, and it does so at a price well below the nickel-based alloys. The recurring design decision is the acid duty - concentration, temperature, and the presence of chlorides or oxidizers - and Alloy 20 wins when the duty sits inside its envelope.
Welding and fabrication
Alloy 20 is welded with matching filler of the same family - ER320 or ER320LR per AWS A5.9 for GTAW and GMAW, and E320 or E320LR per AWS A5.4 for SMAW - and no preheat or post-weld heat treatment is required. The stabilized chemistry means the heat-affected zone keeps its corrosion resistance in the as-welded condition, which is the practical advantage that makes the alloy a shop favorite.
The weld practice follows austenitic discipline: clean joint surfaces, controlled heat input, and low interpass temperature. The alloy cold forms readily, though it work-hardens somewhat faster than 316L, and it machines with standard practices and sharp tooling.
What to specify on the order
To keep an Alloy 20 enquiry clean, state the following:
- Standard and form: "seamless pipe to ASTM B729, UNS N08020" or "welded pipe to ASTM B464"
- Size: NPS + schedule for pipe, OD x wall in millimetres for tube
- Condition: stabilized-annealed
- Length and end finish: fixed or random lengths, plain end or beveled
- Certificate: EN 10204 3.1 with heat chemistry and tensile results
FAQ
Q: What is Alloy 20 used for?
A: Primarily sulfuric acid service - transfer lines, heat exchangers, and process piping in chemical, fertilizer, and metallurgical plants - plus mixed-acid and chloride environments where 316L fails.
Q: What is the difference between Alloy 20 and 316L?
A: Alloy 20 carries roughly three times the nickel of 316L plus 3-4% copper and columbium stabilization. The result is much better resistance to sulfuric acid and chloride stress-corrosion cracking, at a higher price.
Q: Is Alloy 20 the same as 20Cb-3?
A: The names are commonly used for the same chemistry family. Alloy 20, UNS N08020, is the standard stabilized grade; 20Cb-3 is a historical trade name for the same material.
Q: Can Alloy 20 pipe be welded?
A: Yes, with ER320/ER320LR filler, no preheat, and no post-weld heat treatment. The columbium stabilization keeps the weld heat-affected zone corrosion resistant in the as-welded condition.
Q: What is the maximum service temperature for Alloy 20?
A: For structural service, roughly 500-600°C with sharply reduced strength. For acid corrosion service the practical limit is set by the acid concentration and temperature envelope, not by the metal temperature alone.
Q: What is the density of Alloy 20?
A: Approximately 8.08 g/cm³, close to the austenitic stainless grades, so pipe weight is calculated with the standard stainless factor of 0.02466 x (OD - WT) x WT in kg per metre.
