904L vs. 304L

Grade Overview: 304L and 904L Stainless Steel

304L (UNS S30403, EN 1.4307, JIS SUS304L) is the low-carbon version of the 18-8 austenitic grade 304, with a maximum carbon content of 0.030%. It is a general-purpose material used across food processing, dairy, beverage, architectural cladding and water treatment equipment. 904L (UNS N08904, EN 1.4539, JIS SUS890L) is a low-carbon super austenitic stainless steel containing roughly 23-25% nickel, 4-5% molybdenum and 1-2% copper, developed for aggressive chloride and sulfuric acid service.

Chemical Composition Comparison (ASTM A240)

Element (max %)	304L (UNS S30403)	904L (UNS N08904)
Carbon	0.030	0.020
Manganese	2.00	2.00
Phosphorus	0.045	0.045
Sulfur	0.030	0.035
Silicon	0.75	1.00
Chromium	17.5-19.5	19.0-23.0
Nickel	8.0-12.0	23.0-28.0
Molybdenum	-	4.0-5.0
Nitrogen	0.10	0.10
Copper	-	1.0-2.0

The table lists maximum values unless a range is given, balance iron, per ASTM A240. The substantially higher nickel and molybdenum contents of 904L are the main drivers of both its corrosion performance and its price.

Mechanical Properties (ASTM A240, minimum)

Property	304L	904L
Tensile strength, MPa	485	490
Yield strength (0.2% offset), MPa	170	220
Elongation in 50 mm, %	40	35
Hardness	217 HBW max	90 HRB max

Corrosion Resistance and Typical Applications

304L resists atmospheric corrosion, fresh water and many organic media, and is adequate in mild industrial atmospheres. In chloride-containing or acidic media, its performance drops quickly: pitting and crevice corrosion can appear in seawater splash zones. 904L, by contrast, resists pitting in chloride-rich environments and handles sulfuric acid over a wide concentration range, which is why it is specified for chemical processing plants, acetic and phosphoric acid service, seawater piping, heat exchangers, condensers and offshore structures.

Selection Guidance

Choose 304L when the service environment is mild and cost is the priority. Choose 904L when 316L-class materials fail, for example in seawater heat exchangers, acid circulation loops or bleaching plants. The higher initial cost of 904L is usually recovered through longer service life and lower maintenance in aggressive media.

FAQ

1. What does the L in 304L mean?

The L stands for low carbon, 0.030% maximum, which limits chromium carbide precipitation at grain boundaries during welding and reduces the risk of intergranular corrosion.

2. Is 904L the same as 316L?

No. 904L has much higher nickel, molybdenum and copper, giving clearly better resistance to chlorides and sulfuric acid.

3. Can 904L be welded?

Yes. Its low carbon content minimizes sensitization; standard shop practices with matching filler metals are used.

4. Is 904L magnetic?

In the annealed condition it is essentially non-magnetic, like other austenitic grades.

5. What are the equivalents of 904L?

UNS N08904, EN 1.4539, JIS SUS890L.

6. When is 304L not suitable?

In seawater, strong chlorides, sulfuric acid above moderate concentrations, or where high chloride pitting resistance is required.