What’s the Difference Between SS 904L and Duplex for Corrosive Environments?
When selecting stainless steel plates for use in acidic environments, two grades often compete for attention: SS 904L and Duplex (UNS S31803 / S32205 / S32750). While both are corrosion-resistant, their behavior under acid attack, chloride stress, and pitting differs significantly due to their metallurgical structure and alloy composition.
- SS 904L is an austenitic stainless steel with high nickel and molybdenum.
- Duplex stainless steels are a mix of ferrite + austenite, offering a balance of strength and resistance.
Understanding their real-world performance helps in choosing the most cost-effective and chemically suitable plate for chemical plants, tanks, acid pickling lines, scrubbers, and pressure vessels.
Common Use Cases of SS 904L and Duplex Plates
| Application | Preferred Grade |
|---|---|
| Sulfuric Acid Tanks | SS 904L |
| Hydrochloric Acid & Chloride Brines | Duplex / Super Duplex |
| Phosphoric Acid Processing | SS 904L |
| Flue Gas Desulfurization (FGD) Units | Duplex S32205 |
| Acid Pickling Lines & Scrubbers | Duplex S32750 |
| Urea Production / Fertilizer Plants | SS 904L or Duplex |
| Seawater-Acid Mix Environments | Super Duplex |
Acid Resistance Properties Compared
| Property | SS 904L | Duplex 2205 / S32750 |
|---|---|---|
| Resistance to Sulfuric Acid | Excellent | Moderate |
| Resistance to Hydrochloric Acid | Limited | Better in diluted conditions |
| Pitting Resistance (PREN) | ~35 | Up to 40–45 (S32750) |
| Stress Corrosion Cracking (SCC) | Moderate | Excellent |
| Strength | ~220 MPa Yield | ~450–550 MPa Yield |
| Cost | Higher | More cost-effective |
Chemical Composition of SS 904L vs Duplex
| Element | SS 904L | Duplex S32205 |
|---|---|---|
| Cr | 19–23% | 21–23% |
| Ni | 23–28% | 4.5–6.5% |
| Mo | 4–5% | 2.5–3.5% |
| Cu | 1.2–2% | — |
| N (Nitrogen) | — | 0.14–0.20% |
| Structure | Austenitic | Duplex (50/50 mix) |
Plate Sizes & Availability
- Thickness Range: 3mm to 80mm
- Widths: Up to 2500mm
- Surface Finish: 2B, No. 1, No. 4, Pickled
- Optional: Descaled / Shot blasted / Brushed
Heat Treatment & Post-Fabrication Practices
- SS 904L: Requires solution annealing at ~1100°C
- Duplex: Must be controlled during welding to avoid intermetallic phase formation
- Both require pickling or passivation for acid resistance optimization
Weldability & Fabrication Suitability
- SS 904L is more ductile and easier to weld with standard austenitic techniques.
- Duplex requires skilled welding with interpass temperature control.
- Duplex has better strength-to-weight ratio, ideal for structural acid vessels.
Which Grade Should You Choose?
- Choose SS 904L if:
- Your process involves high-concentration sulfuric/phosphoric acid
- You require a fully austenitic microstructure
- Budget is flexible and corrosion risk is high
- Choose Duplex (S32205/S32750) if:
- You face chloride-rich acidic environments
- You need higher strength with cost efficiency
- You’re concerned about pitting or SCC
Grades Commonly Compared with These
- SS 316L – less acid resistant than both
- SS 317L – improved over 316L but still below 904L
- Super Duplex (S32750) – higher resistance than S32205
- Alloy C276 / Titanium – for extreme acid resistance (higher cost)
Global Export of 904L & Duplex Plates
We export SS 904L and Duplex plates to:
- Chemical processing industries in Vietnam, UAE, Saudi Arabia, South Africa
- Acid tank and scrubber manufacturers
- Marine & Desalination plants
- Sulfuric acid plants and pickling lines in India & abroad
Our packaging ensures:
- VCI wrapping + HDPE + fumigated pallets
- Clear marking of heat number + grade
- Mill Test Certificate (MTC) EN 10204 3.1 included
Buyer’s Guide – Quick Selection Tip
| Condition | Best Choice |
|---|---|
| Sulfuric acid handling | SS 904L |
| Chloride-acid combo | Duplex / Super Duplex |
| Need strength + corrosion | Duplex S32205 |
| Highest corrosion resistance | SS 904L or S32750 |
| Budget-conscious projects | Duplex 2205 |
Test Reports & Certification Supplied
- MTC as per EN 10204 3.1
- Optional: HIC, PMI, IGC, NACE, SCC resistance reports
- Material traceability with heat numbers and grade stamps
- Export compliance: ISO 9001, PED, IBR, CE Mark
Voice-Search Optimized FAQs
Can Duplex steel replace SS 904L?
In many chloride-rich or mechanical strength-driven cases, yes.
Which grade handles sulfuric acid better?
SS 904L performs best in high-concentration H₂SO₄.
Is SS 904L good for hydrochloric acid?
Not ideal; Duplex or Super Duplex offers better chloride resistance
Which is more cost-effective – SS 904L or Duplex?
Duplex is significantly more affordable with higher strength.
What is PREN value in stainless steel?
Pitting Resistance Equivalent Number – higher value means better resistance.
Do both grades resist pitting and crevice corrosion?
Yes, but Super Duplex (S32750) resists better than 904L.
What’s the difference in strength between the two?
Duplex has nearly 2x the yield strength of SS 904L.
Is 904L suitable for high-pressure vessels?
Yes, but Duplex is preferred for both strength and acid resistance.
Can both grades be used in marine acidic environments?
Yes, but Duplex or Super Duplex is usually preferred.
Do you supply both grades in cut-to-size format?
Yes, with optional polishing, plasma cutting, and beveling.
Are these grades export-approved?
Yes, with ISO, CE, PED, and IBR certification support.
Which one is easier to fabricate?
SS 904L is easier to weld and form compared to Duplex.
Do you offer price comparison charts for both grades?
Yes, upon enquiry we provide updated price/kg and corrosion data.
What grade is best for long-term acid storage tanks?
SS 904L for sulfuric/phosphoric; Super Duplex for chlorides.
Request Price or Acid Resistance Datasheet?
We provide:
- Price per kg for SS 904L and Duplex S32205/S32750
- Datasheets comparing corrosion test results
- Shipping schedule & packing options
- Custom thickness or profile on request