Alloy 20 vs Alloy 28: Selecting the Right Superaustenitic for Acid Service

Alloy 20 is optimized for sulfuric acid service (especially <50°C concentrated H₂SO₄) and mixed-acid environments; Alloy 28 excels in phosphoric acid, sour gas (H₂S/CO₂/Cl⁻), and high-chloride environments. The choice depends on the dominant corrosive medium and operating temperature.

Alloy 20 vs Alloy 28

Both Alloy 20 (UNS N08020) and Alloy 28 (UNS N08028) are superaustenitic stainless steels-nickel-iron-chromium alloys with enhanced chromium, molybdenum, and copper additions that outperform standard austenitic grades in aggressive acid environments. Their key difference lies in composition emphasis: Alloy 20 prioritizes copper (3–4%) and niobium stabilization for sulfuric acid resistance, while Alloy 28 maximizes chromium (26–28%) and molybdenum (3–4%) for superior pitting resistance and phosphoric/sour gas service.

Superaustenitic stainless steels occupy a tier above standard austenitic grades (304, 316) in both alloy content and corrosion resistance. They are defined by PREN (Pitting Resistance Equivalent Number) values exceeding 40, significantly higher than 316 stainless steel's ~25. Both Alloy 20 and Alloy 28 belong to this class, offering resistance to aggressive acids, chlorides, and stress corrosion cracking that conventional stainless steels cannot provide.

Alloy 20: Sulfuric Acid Specialist

Alloy 20 (UNS N08020, also known as Carpenter 20Cb-3, NS143, 0Cr20Ni35Mo3Cu4Nb) was developed in 1963 specifically to resist sulfuric acid corrosion in chemical processing. Its defining features are the addition of copper (3–4%) for sulfuric acid resistance and niobium (Nb ≥ 8×C) for sensitization resistance. It remains the benchmark material for sulfuric acid environments.

Alloy 28: Phosphoric Acid and Sour Gas Champion

Alloy 28 (UNS N08028, also known as Sanicro 28, 1.4563, 00Cr27Ni31Mo3Cu) was developed specifically for wet-process phosphoric acid heat exchangers. Its higher chromium (26–28%) and molybdenum (3–4%) content provide exceptional resistance to phosphoric acid, sour gas environments (H₂S/CO₂/Cl⁻), and halide-containing media. Its carbon content is restricted to ≤0.03% for inherent intergranular corrosion resistance.

Alloy 20 contains higher copper (3–4%) and niobium stabilization (Nb ≥ 8×C), optimized for sulfuric acid. Alloy 28 contains higher chromium (26–28% vs 19–21%) and molybdenum (3.5% vs 2.5%), providing superior PREN (~36 vs ~28) and better pitting/crevice corrosion resistance.

Element	Alloy 20 (N08020)	Alloy 28 (N08028)	Key Difference
Carbon (C)	≤0.07%	≤0.03%	Alloy 28: ultra-low carbon
Chromium (Cr)	19.0–21.0%	26.0–28.0%	Alloy 28: 7% higher Cr
Nickel (Ni)	32.0–38.0%	30.0–34.0%	Alloy 20: 2–4% higher Ni
Molybdenum (Mo)	2.0–3.0%	3.0–4.0%	Alloy 28: ~40% higher Mo
Copper (Cu)	3.0–4.0%	0.6–1.4%	Alloy 20: 3× more copper
Niobium (Nb)	≥ 0.8×C (~1.0%)	-	Alloy 20: Nb stabilized
Manganese (Mn)	≤2.0%	≤2.5%	Similar
Silicon (Si)	≤1.0%	≤1.0%	Similar
Iron (Fe)	Balance	Balance	Similar

PREN Calculation

PREN = %Cr + 3.3 × %Mo + 16 × %N (nitrogen ~0.15% in both grades):

Alloy 20 PREN: 20 + 3.3 × 2.5 + 16 × 0.15 = ~28.5

Alloy 28 PREN: 27 + 3.3 × 3.5 + 16 × 0.15 = ~36.5

Alloy 28 PREN advantage: ~8 points higher = significantly better pitting and crevice corrosion resistance.
Industry threshold: PREN > 40 for seawater service; Alloy 28 approaches this with its 36.5 rating.
Alloy 20 PREN 28.5 is substantially better than 316 (PREN ~25) but well below Alloy 28.

Why the Composition Differences Matter

Copper (Cu): Forms protective complexes with sulfuric acid, slowing corrosion rate. Essential for H₂SO₄ service.
Niobium (Nb): Prevents chromium carbide sensitization during welding and high-temperature service.
Chromium (Cr): Primary driver of passive film stability; higher = better general corrosion and oxidation resistance.
Molybdenum (Mo): Critical for pitting resistance; significantly improves chloride and phosphoric acid resistance.

Both alloys exhibit fully austenitic microstructures with excellent room-temperature ductility. Alloy 20 has slightly higher tensile strength (≥550 MPa vs 500 MPa) due to niobium strengthening, while Alloy 28 offers superior cryogenic toughness and hot workability. Neither is suitable for sustained use above 450°C without careful evaluation.

Property	Alloy 20 (Annealed)	Alloy 28 (Annealed)	Difference
Density	8.08 g/cm³	8.0 g/cm³	Nearly identical
Melting Point	1370–1400°C	1311–1366°C	Alloy 28: slightly lower
Ultimate Tensile Strength	≥550 MPa	≥500 MPa	Alloy 20: ~10% higher
Yield Strength (0.2%)	≥240 MPa	≥214 MPa	Alloy 20: ~12% higher
Elongation at Break	≥30%	≥40%	Alloy 28: better ductility
Hardness (Annealed)	135–220 HB / 28–32 HRC	80–90 HRB / ~33 HRC	Similar
Elastic Modulus	~200 GPa	~200 GPa	Identical
Thermal Conductivity	13.5 W/m·K	~13–14 W/m·K	Similar

Cold-Worked Condition

Alloy 28 cold-worked: Tensile strength up to 896 MPa, yield strength up to 758 MPa, elongation reduced to ~15%.
Alloy 20: Also responds to cold working; strength increases significantly with moderate deformation.
Application note: For high-pressure piping requiring high strength, cold-worked Alloy 28 or Alloy 20 may be specified.

Elevated Temperature Limitations

Condition	Alloy 20	Alloy 28
Max Continuous Service Temp	~425°C (with conditions)	~400–450°C
Sulfuric Acid Service Limit	<50°C for concentrated; <25°C for dilute	Superior at moderate temps
Phosphoric Acid Service	Good up to 60–80°C	Excellent up to 80–100°C
Precipitation Risk	NbC at 650°C (sensitization zone)	No significant precipitation (<0.03% C)

Alloy 20 is the preferred choice for sulfuric acid environments, especially concentrated H₂SO₄ at moderate temperatures. Alloy 28 is superior in phosphoric acid, sour gas (H₂S/CO₂/Cl⁻), and chloride-rich environments. Neither is recommended for hydrochloric acid (HCl) service.

Alloy 20 Corrosion Resistance

Sulfuric Acid (H₂SO₄)

H₂SO₄ Concentration	Alloy 20	Alloy 28	Recommended
<10% (dilute), <50°C	Excellent	Excellent	Either
10–60%, <50°C	Very Good	Good	Alloy 20
60–85% (conc.), <50°C	Excellent	Good	Alloy 20
>85%, elevated temp.	Good (limited by temp)	Good	Alloy 20
Mixed acid (H₂SO₄+HNO₃)	Very Good	Good	Alloy 20
H₂SO₄ with Cl⁻	Moderate	Good	Alloy 28

Mechanism: Copper in Alloy 20 forms protective Cu₂S-type passive films that resist sulfuric acid attack more effectively than chromium-molybdenum films alone.
Temperature limit: Both alloys are generally limited to <50–60°C in sulfuric acid. Above this, expensive nickel alloys (Alloy 625, Alloy 825) are required.
Alloy 20 Key Advantage: Copper addition (3–4%) specifically improves resistance to reducing acid conditions like sulfuric acid.

Phosphoric Acid (H₃PO₄)

H₃PO₄ Condition	Alloy 20	Alloy 28	Recommended
Wet-process (with F⁻/Cl⁻ impurities), RT	Very Good	Excellent	Alloy 28
Thermal concentration (52% P₂O₅), elevated	Good	Excellent	Alloy 28
Pure H₃PO₄, <80°C	Good	Very Good	Either
H₃PO₄ + halide contaminants	Moderate	Excellent	Alloy 28

Alloy 28 was specifically developed for wet-process phosphoric acid production, where fluoride and chloride impurities make conventional stainless steels fail rapidly.
Research (Springer, 2024): Alloy 28 in 52% P₂O₅ phosphoric acid with oxidizing agents showed negative corrosion rates (weight gain from passive film formation), confirming excellent passivity.
Mo content (3.5%): Critical for resisting phosphoric acid with F⁻/Cl⁻ impurities that aggressively attack passive films.

Sour Gas and Oilfield Environments (H₂S/CO₂/Cl⁻)

Alloy 28 is significantly superior for sour gas service due to higher Cr and Mo content providing PREN ~36.5 vs Alloy 20's ~28.5. In H₂S/CO₂/Cl⁻ environments, Alloy 28 is the standard choice for OCTG (oil country tubular goods) and heat exchangers.

Environment	Alloy 20	Alloy 28	Notes
H₂S only (<100 ppm), <150°C	Good	Very Good	Alloy 28 preferred
H₂S + CO₂ + Cl⁻	Moderate	Excellent	Alloy 28 clearly superior
High Cl⁻ (>1000 ppm)	Limited (PREN~28.5)	Good (PREN~36.5)	Alloy 28 required
Seawater service	Not recommended	Suitable (PREN 36.5)	Alloy 28
Sulfidic stress cracking	Good	Excellent	Alloy 28

Alloy 28 application: Production tubing (OCTG) and heat exchangers in sour oil and gas fields with high H₂S/CO₂/Cl⁻ content.
Alloy 28 mechanism: High Cr forms stable passive Cr₂O₃ film; high Mo forms MoO₂ protective layer resistant to chloride attack under H₂S.
NACE MR0175/ISO 15156: Both alloys may be listed for sour service, but Alloy 28's wider acceptance stems from superior PREN.

Pitting and Crevice Corrosion

Condition	Alloy 20	Alloy 28	Winner
PREN Value	~28.5	~36.5	Alloy 28 (+28%)
Critical Pitting Temp (CPT) in 3.5% NaCl	~20°C	~40–45°C	Alloy 28 significantly better
Crevice Corrosion Resistance	Moderate	Very Good	Alloy 28
Stress Corrosion Cracking (Cl⁻)	Good	Excellent	Alloy 28

What Neither Alloy Handles Well

Hydrochloric Acid (HCl): Neither Alloy 20 nor Alloy 28 is recommended for HCl service. Use Alloy B (N10001) or Alloy 400.
High-Temperature (>450°C) Oxidizing Environments: Both alloys lose corrosion resistance; consider Alloy 625 or Alloy 800H.
Aqua Regia (HNO₃ + HCl): Neither grade provides adequate resistance.

Both alloys are readily weldable using standard austenitic stainless steel processes. Alloy 20 requires niobium-stabilized filler (ERNiCrMo-3) for optimal high-temperature service, while Alloy 28 uses ERNiCrMo-3 or ER316L filler. Neither requires preheating.

Parameter	Alloy 20	Alloy 28
Weldability	Excellent	Excellent
Preheating	Not required	Not required
Interpass Temperature	<175°C (350°F)	<175°C (350°F)
Post-Weld Heat Treatment (PWHT)	Not required (stabilized)	Not required (ultra-low C)
Recommended Filler Metal	ERNiCrMo-3 (Alloy 625 filler)	ERNiCrMo-3 or ER316L
Shielding Gas	Ar + 2–3% CO₂	Ar + 2–3% CO₂
Machinability	Fair (similar to 316)	Fair (slightly harder due to Mo)
Cold Formability	Excellent	Excellent
Hot Workability	Good (970–1065°C)	Good (950–1100°C)

Alloy 20 Welding Specifics

AWS A5.14 ERNiCrMo-3 or AWS A5.11 ENiCrMo-3 electrodes recommended.
Niobium in the filler stabilizes the weld metal against sensitization.
Solution annealing after welding: 900–1150°C for 1–2 hours, rapid air or water quench.
Weld metal composition matches parent metal when using proper filler.

Alloy 28 Welding Specifics

ERNiCrMo-3: Preferred for high-corrosion-service welds (H₂S/Cl⁻ environments).
ER316L: Acceptable for standard acid service; provides matching corrosion resistance.
Solution annealing: 1100°C is optimal for both mechanical properties and corrosion resistance (research-confirmed).
The ultra-low carbon (≤0.03%) means no sensitization risk even in the heat-affected zone (HAZ).

Alloy 28 is generally 10–25% more expensive than Alloy 20 due to higher chromium and molybdenum content. However, in phosphoric acid and sour gas applications, Alloy 28's superior corrosion resistance delivers lower total lifecycle cost through extended equipment life and reduced maintenance.

Cost Factor	Alloy 20	Alloy 28	Notes
Raw Material Cost	Reference (1.0×)	1.1–1.25×	Higher Cr/Mo in Alloy 28
Ni Content Impact	32–38% Ni	30–34% Ni	Ni price is primary cost driver
Cu Content Impact	3–4% Cu (more expensive)	0.6–1.4% Cu	Cu partially offsets Cr/Mo cost
Availability (Plate/Sheet)	Good	Good	Both widely stocked
Availability (Tubular)	Good	Good	ASTM B729/B668 compliance
Lead Time (Custom Forms)	4–8 weeks	6–10 weeks	Alloy 28 may take longer
Weld Filler Cost	ERNiCrMo-3	ERNiCrMo-3 or ER316L	Alloy 20: Nb filler required
Lifecycle Cost (target application)	Lower if H₂SO₄ service	Lower if H₃PO₄/H₂S service	Match to application

When the Higher Cost of Alloy 28 is Justified

Phosphoric acid with fluoride/chloride impurities: Alloy 20 will suffer accelerated attack; Alloy 28 lasts significantly longer.
Sour gas with high H₂S/CO₂/Cl⁻: OCTG and wellhead failures with Alloy 20 would far exceed material cost savings.
Seawater service: Alloy 20's PREN ~28.5 is inadequate; Alloy 28 (PREN ~36.5) provides reliable service.

Criterion	Alloy 20 (N08020)	Alloy 28 (N08028)	Winner
Sulfuric Acid (<50°C, concentrated)	Excellent	Good	Alloy 20
Sulfuric Acid (dilute, with Cl⁻)	Moderate	Good	Alloy 28
Phosphoric Acid (pure)	Good	Very Good	Alloy 28
Phosphoric Acid (F⁻/Cl⁻ contaminated)	Moderate	Excellent	Alloy 28
Sour Gas (H₂S/CO₂/Cl⁻)	Good	Excellent	Alloy 28
Seawater / High Cl⁻	Not Suitable	Good	Alloy 28
Mixed Acid (H₂SO₄+HNO₃)	Very Good	Good	Alloy 20
Pitting Resistance (PREN)	~28.5	~36.5	Alloy 28 (+28%)
Sensitization Resistance (welding)	Excellent (Nb stabilized)	Excellent (ultra-low C)	Equal
Mechanical Strength (UTS)	≥550 MPa	≥500 MPa	Alloy 20
Ductility (elongation)	≥30%	≥40%	Alloy 28
Weldability	Excellent	Excellent	Equal
Machinability	Fair	Fair	Equal
Cost	Lower (reference)	Higher (10–25%)	Alloy 20
Phosphoric Acid Specific Origin	Not developed for this	Developed for this	Alloy 28
Sulfuric Acid Specific Origin	Developed for this	Not developed for this	Alloy 20

The Alloy 20 vs Alloy 28 decision is fundamentally driven by the dominant corrosive medium in the application. Both are superaustenitic stainless steels with excellent corrosion resistance, but their different compositional emphasis makes each optimal for distinct service environments:

Alloy 20 (N08020, 0Cr20Ni35Mo3Cu4Nb) is the sulfuric acid specialist. Its 3–4% copper addition creates a unique passive film resistance against H₂SO₄ that Alloy 28 cannot match. Developed specifically for chemical processing applications involving sulfuric acid, mixed acids, and acid cleaning, Alloy 20 remains the engineering standard for H₂SO₄ service below 50°C. Use it when your process is sulfuric acid-dominant and the economics favor a copper-bearing superaustenitic over nickel-based alloys.

Alloy 28 (N08028, 00Cr27Ni31Mo3Cu) is the phosphoric acid and sour gas champion. Its 26–28% chromium and 3.5% molybdenum provide PREN ~36.5, offering dramatically better pitting and crevice corrosion resistance than Alloy 20. Originally developed to solve wet-process phosphoric acid heat exchanger failures, Alloy 28 has since become the standard for sour oil and gas production (H₂S/CO₂/Cl⁻), seawater service, and FGD systems. Use it when the threat is halide-contaminated phosphoric acid, sour gas, or any environment where high PREN and resistance to localized corrosion determine equipment life.

For procurement engineers: cross-reference the primary corrosion threat and maximum operating temperature against the comparison tables above. For process engineers: confirm that the selected alloy has been tested in the specific acid concentration, temperature, and impurity profile of your process stream. When in doubt, consult the alloy datasheets and NACE/ASME BPVC guidelines for the relevant service conditions.