Alloy 20 vs Alloy 28 Selecting the Right Super Austenitic Alloy for Phosphoric and Sulfuric Acid Service

Choosing the wrong alloy for an acid-handling tank, pipe spool, or heat exchanger does not just waste budget - it converts a multi-decade asset into a multi-month liability. Alloy 20 (UNS N08020) and Alloy 28 (UNS N08028) are the two most widely specified super austenitic stainless alloys for sulfuric acid and phosphoric acid service, yet the two are routinely confused or treated as interchangeable. They are not.

This guide answers the questions that actually decide the outcome - composition, acid concentration range, chloride and fluoride tolerance, temperature limits, and total installed cost - starting each section with a direct, quotable answer, followed by the technical reasoning behind it.

Key Takeaways

1. Alloy 20 (UNS N08020) is copper-alloyed and purpose-built for sulfuric acid across a wide concentration range at moderate temperatures. It is the lower-cost, more broadly available option.
2. Alloy 28 (UNS N08028) carries higher chromium (26–28%) and molybdenum (3–4%) for superior resistance to chloride- and fluoride-contaminated phosphoric acid, plus oxidizing or mixed-acid conditions.
3. Alloy 28's Pitting Resistance Equivalent Number (PREN ≈ 36–41) is substantially higher than Alloy 20's (PREN ≈ 26–31), meaning far better resistance to pitting and crevice attack in chloride-bearing service.
4. For clean, moderate-concentration sulfuric acid, Alloy 20 is usually the more economical choice. For wet-process phosphoric acid evaporators and chloride-contaminated sulfuric acid, Alloy 28 is the industry-preferred standard.

What Are Alloy 20 and Alloy 28?

Alloy 20 and Alloy 28 are both nickel-iron-chromium austenitic alloys strengthened with molybdenum, but they were engineered around different corrosion-fighting strategies: Alloy 20 relies on a copper addition to defeat reducing sulfuric acid, while Alloy 28 relies on higher chromium and molybdenum to hold a stable passive layer across a wider range of acids and contaminants.

Alloy 20 - also sold as Carpenter 20, 20Cb-3, or Incoloy 20, UNS N08020 - was developed in the 1940s specifically to solve sulfuric acid corrosion problems that standard 300-series stainless steels could not handle. Its iron content (roughly 31–44%) sits right at the boundary between "stainless steel" and "nickel alloy," which is why you will see it marketed under both labels; functionally, it behaves as a super austenitic alloy either way.

Alloy 28 - also sold as Sanicro 28 or UR 28, UNS N08028 - was developed in Sweden in the 1960s for a tougher problem: phosphoric acid and sulfuric acid streams contaminated with chlorides and fluorides, where copper alone is not enough. It answers that problem with more chromium, more molybdenum, and an ultra-low carbon content.

What Is the Key Compositional Difference Between Alloy 20 and Alloy 28?

The defining difference is the corrosion-resistance mechanism each alloy leans on: Alloy 20 uses 3–4% copper as its primary defense in sulfuric acid, while Alloy 28 uses higher chromium (26–28%) and molybdenum (3–4%), with only trace copper (0.6–1.4%), plus an ultra-low carbon content that removes the need for a stabilizing element.

Two things follow directly from this table. First, Alloy 20's copper addition is a targeted antidote to reducing sulfuric acid, which is why it does relatively little for phosphoric acid or chloride-bearing streams. Second, Alloy 28's much larger chromium and molybdenum reserve builds a passive film that holds up across a broader combination of acid type, concentration, contamination, and temperature - at the cost of more expensive alloying elements.

Which Alloy Performs Better in Sulfuric Acid Service?

Alloy 20 remains the more cost-effective, code-proven choice for clean sulfuric acid across most of the concentration range at moderate temperatures. Alloy 28 pulls ahead once chlorides, oxidizing species, or higher temperatures enter the picture.

Alloy 20 was purpose-built for sulfuric acid, and its copper addition suppresses the corrosion rate in reducing, largely chloride-free sulfuric acid from dilute concentrations up to nearly 100%, provided the temperature stays within the alloy's isocorrosion limits for that concentration. This is the classic "sweet spot" that made Alloy 20 the historical workhorse for sulfuric acid piping, pickling tanks, and general process equipment.

Alloy 28's advantage shows up outside that sweet spot. Independent isocorrosion data comparing Alloy 28 to Alloy 904L and standard austenitic and duplex stainless grades shows Alloy 28 maintaining good resistance in chloride-contaminated sulfuric acid, and even outperforming super-duplex grade 2507 above roughly 20% sulfuric acid concentration. In practice: specify Alloy 20 for clean, moderate-temperature sulfuric acid; move to Alloy 28 once the process stream carries chlorides, oxidizing contaminants, or runs hotter than Alloy 20's rated envelope for that concentration.

Which Alloy Performs Better in Phosphoric Acid Service?

Alloy 28 is the industry benchmark for wet-process phosphoric acid - particularly evaporator tubes and heat exchangers - because commercial phosphoric acid is never pure; it carries chloride and fluoride impurities from the phosphate rock, and Alloy 28's higher chromium and molybdenum resist that combination far better than Alloy 20. Alloy 20 remains a sound, lower-cost choice for cleaner, thermal-process or purified phosphoric acid with minimal halide contamination.

Wet-process phosphoric acid is manufactured by reacting phosphate rock with sulfuric acid, and the resulting acid inevitably carries fluoride and chloride residues from the ore. That combination is exactly the environment Alloy 28 was engineered for: it has become the most widely used metallic material for evaporator tubing in the wet-process phosphoric acid industry, largely replacing older graphite heat exchangers that suffered frequent tube failures. Alloy 20 can still be a reasonable, lower-cost option for phosphoric acid streams with low halide content and moderate temperature, but it is not the first choice for aggressive wet-process duty.

How Do Alloy 20 and Alloy 28 Compare in Pitting, Crevice, and Chloride Resistance?

Alloy 28 has a substantially higher Pitting Resistance Equivalent Number (PREN ≈ 36–41) than Alloy 20 (PREN ≈ 26–31), which translates directly into better resistance to pitting and crevice corrosion wherever chlorides are present - the single most common reason a sulfuric or phosphoric acid alloy fails in the field.

A quick, engineer-friendly formula

PREN is a simplified but widely used screening number: PREN = %Cr + 3.3 × %Mo (nitrogen is negligible in both of these alloys, so it can be dropped from the formula here). Using each alloy's composition range:

Alloy 20 (UNS N08020): Cr 19–21%, Mo 2–3% → PREN ≈ 26 – 31

Alloy 28 (UNS N08028): Cr 26–28%, Mo 3–4% → PREN ≈ 36 – 41

A higher PREN means a stronger, more stable passive oxide film in the presence of chloride ions - the mechanism behind localized pitting and crevice attack. This is precisely why Alloy 28 is favored wherever chlorides and fluorides show up alongside sulfuric or phosphoric acid, and why Alloy 20 is best reserved for streams where chloride content stays low and controlled. Alloy 28's higher nickel-to-iron ratio also gives it better resistance to chloride stress-corrosion cracking, a common failure mode for lower-nickel stainless grades like 316L.

How Do Alloy 20 and Alloy 28 Compare in Cost, Availability, and Product Forms?

Alloy 20 costs meaningfully less than Alloy 28 because it uses less nickel and molybdenum, and it is available in a wider range of product forms - including castings (designated CN7M) - backed by decades of ASME and ASTM code history. Alloy 28's higher alloy content commands a price premium that is justified when it extends service life in more aggressive duty.

How Do Alloy 20 and Alloy 28 Compare in Welding and Fabrication?

Both alloys are readily weldable with matched filler metals, but Alloy 20 needs niobium stabilization to prevent sensitization and intergranular attack near welds, while Alloy 28's ultra-low carbon content (0.02% maximum) resists sensitization on its own, without a stabilizing element.

Alloy 20's niobium (columbium) addition - the source of the "Cb-3" name - ties up carbon as stable carbides so it cannot precipitate at grain boundaries during welding, which would otherwise leave the heat-affected zone vulnerable to intergranular corrosion. Alloy 28 sidesteps this issue at the source: its carbon content is kept so low (0.02% max, versus 0.07% max for Alloy 20) that harmful carbide precipitation is unlikely regardless of heat input, simplifying welding procedure qualification. Both alloys are compatible with matching filler metal (commonly ERNiCrMo-type or ER320-type consumables) and standard GTAW/GMAW practices used for austenitic and nickel-rich alloys.

Which Alloy Should You Choose?

Default to Alloy 20 when the acid stream is clean sulfuric acid at moderate temperature; move to Alloy 28 whenever chlorides, fluorides, oxidizing contaminants, or elevated temperature are part of the operating envelope.

When conditions fall between these lines - for example, moderate chloride content at only mildly elevated temperature - the right call usually comes down to reading the isocorrosion diagram for your specific concentration and temperature and weighing that against the acceptable inspection interval and consequence of failure for the equipment in question.

Frequently Asked Questions

Is Alloy 20 a stainless steel or a nickel alloy?

Both labels are used in practice. Alloy 20's iron content (roughly 31–44%) sits right at the traditional boundary between the two categories; ASTM classification leans toward nickel alloy, while many suppliers market it as a stainless steel. Functionally, it performs as a super austenitic alloy regardless of the label.

What is the maximum sulfuric acid concentration Alloy 20 can handle?

Alloy 20 can handle nearly the full 0–100% concentration range in the absence of chlorides and oxidizing contaminants, but the safe operating temperature drops as concentration moves through the mid-range. Always check the isocorrosion diagram for your exact concentration and temperature before finalizing a specification.

Can Alloy 20 be used in phosphoric acid?

Yes, for cleaner, lower-halide phosphoric acid streams at moderate temperature. For wet-process phosphoric acid with fluoride and chloride contamination, Alloy 28 offers materially better long-term performance and is the more common industry choice.

What does the "Cb" in Alloy 20Cb-3 mean?

"Cb" stands for columbium, the older American name for niobium. It is added to Alloy 20 to stabilize the alloy against carbide precipitation during welding and heat treatment.

Is Alloy 28 the same as Sanicro 28?

Yes. Alloy 28, Sanicro 28, and UR 28 are all trade names for the same alloy, registered under UNS N08028.

Which alloy costs more, Alloy 20 or Alloy 28?

Alloy 28 costs more, because it contains a higher proportion of chromium and molybdenum and a lower iron content than Alloy 20.