Are 316L and 904L weldable?

Both are readily weldable with standard austenitic stainless procedures. 316L's low carbon avoids sensitization; 904L is also low carbon and can be welded with 904L or higher-alloy (625) filler for maximum corrosion resistance in the weld.

What standards cover 316L and 904L?

316L: ASTM A240 (plate), A312 (pipe), A276 (bar), UNS S31603, equivalent GB 022Cr17Ni12Mo2. 904L: ASTM B625/A240 (plate), B677 (pipe), B649 (bar), UNS N08904, equivalent GB 015Cr21Ni26Mo5Cu2.

In what forms are 316L and 904L supplied?

Both are available as sheet, plate, bar, seamless and welded pipe, tube and fittings. Hangbo Alloy supplies both grades with EN 10204 3.1/3.2 certification.

AISI 316L (022Cr17Ni12Mo2) vs AISI 904L (015Cr21Ni26Mo5Cu2): Acid Resistance

Q: What is the difference between AISI 316L and 904L?

AISI 316L is a standard austenitic stainless steel (about 17% Cr, 12% Ni, 2.5% Mo, low carbon). AISI 904L is a super-austenitic stainless steel (about 21% Cr, 26% Ni, 5% Mo, 2% Cu). The much higher nickel, molybdenum and copper content of 904L gives it markedly better resistance to sulfuric and phosphoric acids and to pitting corrosion.

Q: Which resists sulfuric acid better?

AISI 904L is far superior in sulfuric acid. 316L is limited to dilute sulfuric acid at low temperature; 904L is widely used across a broad concentration and temperature range of sulfuric acid, including concentrations up to about 40-50% at elevated temperature, thanks to its high nickel and copper content.

Q: Which resists phosphoric acid better?

904L is preferred in phosphoric acid, especially in the contaminated process streams found in fertilizer production where chlorides and fluorides are present. 316L is only suitable for very clean, dilute phosphoric acid.

Q: Which has better pitting resistance?

904L has a PREN of about 35 versus about 25 for 316L, so it tolerates much higher chloride levels before pitting. 316L is prone to pitting in warm chloride environments, whereas 904L is suitable for seawater-adjacent and high-chloride service.

Q: Which is more cost-effective?

316L is substantially cheaper (roughly 30-40% of the cost of 904L). For mild, non-aggressive service 316L is the economical default; 904L is justified where sulfuric/phosphoric acid or high chlorides rule out 316L.

AISI 316L and AISI 904L are both austenitic stainless steels, but they occupy very different positions. 316L is the workhorse grade (about 17% Cr, 12% Ni, 2.5% Mo) used across countless mild-service applications. 904L is a super-austenitic stainless steel (about 21% Cr, 26% Ni, 5% Mo, 2% Cu) engineered for aggressive sulfuric and phosphoric acid service and high-chloride environments. This article compares their composition, acid resistance, pitting behaviour and cost.

1. Chemical Composition

Element	AISI 316L	AISI 904L
C	≤0.030	≤0.020
Cr	16.0–18.0	19.0–23.0
Ni	10.0–14.0	23.0–28.0
Mo	2.0–3.0	4.0–5.0
Cu	—	1.0–2.0
Mn	≤2.0	≤2.0
P	≤0.045	≤0.045
S	≤0.030	≤0.035

Key difference: 904L has about 2.5× the nickel, nearly 2× the molybdenum and an added 2% copper versus 316L. This chemistry is what gives 904L its acid and pitting resistance.

2. Mechanical Properties

Property	AISI 316L	AISI 904L
Yield strength σ₀.₂, MPa	≥170	≥220
Tensile strength σᵦ, MPa	≥485	≥490
Elongation, %	≥40	≥35

3. Acid Resistance

3.1 Pitting Resistance Equivalent (PREN)

Alloy	Cr	Mo	N	PREN
AISI 316L	17	2.5	~0.05	≈25
AISI 904L	21	5	—	≈35

3.2 Performance in Key Acid Media

Medium	AISI 316L	AISI 904L	Recommendation
Sulfuric acid (dilute, <10%, RT)	Limited	Excellent	904L
Sulfuric acid (concentrated, warm)	Not suitable	Good to ~40-50%	904L
Phosphoric acid (pure)	Fair	Excellent	904L
Phosphoric acid (with Cl⁻/F⁻)	Not suitable	Excellent	904L
Acetic acid	Good	Good	316L (cost)
Seawater / chlorides	Pitting risk	Good	904L

4. Weldability

Parameter	AISI 316L	AISI 904L
Welding methods	TIG, MIG, MMA	TIG, MIG, MMA
Recommended filler	ER316L	ER904L or ERNiCrMo-3
Sensitization	Avoided by low C	Avoided by low C

5. Applications

Industry	AISI 316L	AISI 904L
Chemical processing	Mild acid, general	Sulfuric/phosphoric acid plants
Fertilizer	Limited	Phosphate production
Marine	Decorative/structural	Seawater systems
Pulp & paper	General	Bleaching (chloride)

6. Cost

Factor	AISI 316L	AISI 904L
Relative price	100% (baseline)	≈250–300%
Best value when	Mild, non-aggressive	Acid/high-chloride service

7. Conclusion

Need 316L or 904L for Your Project?

Shanghai Hangbo Alloy Group supplies AISI 316L and 904L in sheet, plate, bar, pipe and forgings with EN 10204 3.1/3.2 certification. Tell us your medium and we will recommend the right grade.

📧 Email: serena@hangboalloy.com
📞 Tel: +86 166 2109 2936
💬 WhatsApp: +86 166 2109 2936

Criterion	Choose 316L	Choose 904L
Sulfuric / phosphoric acid	Limited	✓ Optimal
High chloride / seawater	Pitting risk	✓ Resistant
Mild, general service	✓ Cost-effective	Over-spec
Limited budget	✓ Cheaper	Premium

Summary: Use AISI 316L for mild, non-aggressive service where cost matters. Use AISI 904L when sulfuric or phosphoric acid, or elevated chloride levels, exceed 316L's capability.

8. Frequently Asked Questions (FAQ)

1. What is the difference between AISI 316L and 904L?

316L is standard austenitic stainless (17% Cr, 12% Ni, 2.5% Mo). 904L is super-austenitic (21% Cr, 26% Ni, 5% Mo, 2% Cu), giving much better acid and pitting resistance.

2. Which resists sulfuric acid better?

904L is far superior; 316L is limited to dilute acid at low temperature, while 904L covers a broad concentration/temperature range of sulfuric acid.

3. Which resists phosphoric acid better?

904L is preferred, especially in contaminated process streams with chlorides and fluorides; 316L only suits clean, dilute phosphoric acid.

4. Which has better pitting resistance?

904L (PREN ≈35) tolerates much higher chlorides than 316L (PREN ≈25) before pitting.

5. Which is more cost-effective?

316L costs roughly 30–40% of 904L; it is the economical default for mild service, while 904L is justified for acid/high-chloride service.

6. Are both weldable?

Yes. 316L uses ER316L filler; 904L uses ER904L or 625 filler. Both are low-carbon to avoid sensitization.

7. What standards cover these grades?

316L: ASTM A240/A312/A276, UNS S31603, GB 022Cr17Ni12Mo2. 904L: ASTM B625/B677/B649, UNS N08904, GB 015Cr21Ni26Mo5Cu2.

8. In what forms are they supplied?

Both available as sheet, plate, bar, seamless/welded pipe, tube and fittings with EN 10204 certification.