904L vs 316L Stainless Steel: Key Differences

904L (UNS N08904, EN 1.4539) and 316L (UNS S31603, EN 1.4404) are both austenitic stainless steels, but they serve different performance tiers. 316L is the workhorse of the chemical, food and pharmaceutical industries; 904L is a high-alloy austenitic grade developed for harsher chloride and dilute sulfuric acid service. Selecting between them depends on chloride concentration, temperature, media acidity and equipment budget. This article compares the two grades using composition limits and mechanical requirements from ASTM A240, with equivalent designations from EN 10088 and ISO 15510.

Chemical Composition Comparison (ASTM A240 Limits)

Element (wt%)	316L, UNS S31603	904L, UNS N08904
Carbon, max	0.030	0.020
Manganese, max	2.00	2.00
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.035
Silicon, max	0.75	1.00
Chromium	16.0-18.0	19.0-23.0
Nickel	10.0-14.0	23.0-28.0
Molybdenum	2.0-3.0	4.0-5.0
Copper	-	1.0-2.0
Nitrogen, max	0.10	0.10

The decisive differences are nickel (23-28% against 10-14%) and molybdenum (4-5% against 2-3%), plus a deliberate copper addition in 904L. Molybdenum and copper together stabilize the passive film in reducing acids such as dilute sulfuric acid, while the high nickel content resists chloride stress corrosion cracking.

Mechanical Properties (ASTM A240, Annealed Plate)

Property	316L	904L
Tensile strength, min (MPa)	485	490
Yield strength, min (MPa)	170	220
Elongation, min (% in 2 in)	40	35
Density (kg/m3)	8000	7950

Both grades are non-hardenable by heat treatment and are supplied annealed. 904L offers slightly higher proof strength because of its higher alloy content. Neither grade is recommended for long-term structural service above about 425°C, as both are subject to sensitization in the 425-850°C range unless the section is thin and rapidly quenched after solution treatment.

Corrosion Resistance and PREN

Pitting resistance is commonly ranked by PREN, calculated as Cr + 3.3 x Mo + 16 x N. Taking mid-range compositions, 316L gives a PREN of roughly 25 while 904L gives roughly 34. In practice 904L tolerates higher chloride levels, higher temperatures and lower pH before pitting and crevice corrosion initiate. In seawater and marine splash zones 316L is at risk of pitting and crevice attack, while 904L performs acceptably in many moderate marine duties, although duplex and 6% molybdenum super-austenitic grades remain the preferred choice for permanent seawater immersion. In dilute sulfuric acid up to about 20-30% concentration at ambient temperature, 904L is widely used for pumps and valves where 316L fails.

Welding and Fabrication

Both grades are readily weldable by all conventional processes, including GTAW, GMAW, SMAW and SAW. 316L, with its 0.030% carbon maximum, is essentially immune to sensitization during welding in most section thicknesses and usually needs no post-weld heat treatment. 904L should be welded with low-heat-input procedures and matching filler metals such as EN ISO 14343-A 20 25 5 Cu L or AWS ER385, so that corrosion resistance in the heat-affected zone is preserved. Heat tint must be removed from wetted surfaces by pickling or electropolishing.

Cost and Selection Guidance

904L typically carries roughly two to three times the alloy cost of 316L in international markets, driven primarily by its 23-28% nickel content. Where media are mildly aggressive, such as potable water, food contact, organic acids and dilute alkalis, 316L is the economical choice. Where chlorides exceed a few hundred ppm, where pH drops below 4, or where maintenance shutdowns are extremely expensive, 904L pays for itself through longer equipment life and fewer failures.

FAQ

Q1. What is the main difference between 904L and 316L?

The composition tier. 904L adds molybdenum (4-5%), copper (1-2%) and much more nickel (23-28%) than 316L (Ni 10-14%, Mo 2-3%), giving markedly better resistance to chlorides, pitting and reducing acids.

Q2. Which grade resists chloride pitting better?

904L, with a typical PREN of about 34 versus about 25 for 316L.

Q3. Is 904L the same as EN 1.4539?

Yes. 904L is the common trade name for UNS N08904, designated 1.4539, X1NiCrMoCu25-20-5, in EN 10088.

Q4. When is 316L sufficient instead of 904L?

When chlorides are low, temperatures are moderate and the media are not strongly reducing acids, which covers most food, beverage, pharmaceutical and general chemical duties.

Q5. Can both grades be welded?

Yes, with standard austenitic welding practice. Low heat input is recommended, and 904L requires a high-alloy austenitic or nickel-based filler to match the corrosion performance of the base metal.

Q6. Which grade is used in seawater-cooled heat exchangers?

316L is marginal in seawater. 904L is a common choice for many seawater-cooled exchangers, although duplex and 6% molybdenum super-austenitic grades are preferred for severe or permanent immersion service.