317L vs 904L Stainless Steel: High-Performance Corrosion Resistant Alloys

Overview: Two High-Performance Austenitic Grades

When 316L cannot guarantee reliable service in aggressive media, engineers typically step up to 317L (UNS S31703, EN 1.4438, X2CrNiMo18-15-4) or 904L (UNS N08904, EN 1.4539, X1NiCrMoCu25-20-5). Both remain fully austenitic, but they achieve their higher corrosion resistance in different ways and at different cost levels. 317L raises molybdenum to 3.0-4.0 percent, while 904L combines 4.0-5.0 percent molybdenum with 23.0-28.0 percent nickel and 1.0-2.0 percent copper. The extra nickel and copper give 904L a decisive advantage in sulfuric acid and other reducing media, plus markedly better pitting resistance. The selection question is therefore one of corrosion duty, temperature, chloride level and lifecycle cost, not simply of which grade is stronger.

Chemical Composition

Limiting composition per ASTM A240/A240M for UNS S31703 and UNS N08904.

Element (%)	317L (S31703)	904L (N08904)
C	≤0.030	≤0.020
Mn	≤2.00	≤2.00
P	≤0.045	≤0.045
S	≤0.030	≤0.035
Si	≤0.75	≤1.00
Cr	18.0-20.0	19.0-23.0
Ni	11.0-15.0	23.0-28.0
Mo	3.0-4.0	4.0-5.0
Cu	-	1.0-2.0
N	≤0.10	≤0.10

Mechanical Properties

Minimum room-temperature values of annealed plate per ASTM A240/A240M.

Property	317L	904L
Tensile strength	≥515 MPa	≥490 MPa
Yield strength (0.2%)	≥205 MPa	≥220 MPa
Elongation	≥40%	≥36%
Hardness	≤217 HBW	≤220 HBW

Corrosion Resistance and PREN

Using the common formula PREN = Cr + 3.3 x Mo + 16 x N, 317L calculates to about 29-31, comfortably above the roughly 24-25 of 316L. 904L calculates to about 34-37, with the copper addition contributing further to resistance in sulfuric acid: at moderate concentrations and temperatures 904L handles sulfuric acid far better than any conventional molybdenum stainless steel, and it also resists phosphoric, acetic and formic acids, seawater and chloride-rich cooling water. The practical gap is real but narrow: 904L resists pitting and crevice corrosion in environments where 317L still pits, especially at elevated temperature and in high chloride levels above 1000 ppm. For borderline duties, coupon testing in the actual process medium is the recommended basis for the final selection.

Selection Guidance and Cost

317L is best understood as a cost-effective upgrade of 316L: it costs only moderately more, welds with familiar procedures, and is the right answer where 316L fails marginally in chloride service, pulp and paper bleach plants, flue gas desulfurisation scrubbers and pharmaceutical or fine-chemical reactors. 904L is specified where the medium is more aggressive - hot sulfuric acid, concentrated chlorides with crevices, seawater heat exchangers, and equipment that must outlast repeated cleaning cycles with acid detergents. Because 904L contains roughly double the nickel plus copper, its raw material cost is significantly higher. Where weight savings matter and welding can be controlled, duplex grades such as UNS S32205 or UNS S32750 may be evaluated as alternatives, but they are not always suitable for highly reducing acid media or for heavily welded thin-wall equipment.

Typical Applications

317L is used in chemical storage and piping, pulp bleaching equipment, dye and pigment plants, tall-oil and fatty-acid production, and food processing with high chloride washing. 904L is used in sulfuric acid production and storage, phosphoric acid evaporators, seawater cooling and firewater systems, offshore platform utilities, pharmaceutical vessels, textile bleaching lines and pollution-control scrubbers.

Frequently Asked Questions

Q1. Which grade is stronger, 317L or 904L?
Their guaranteed strengths are comparable - 317L has a minimum tensile of 515 MPa and 904L 490 MPa, with yield strengths of 205 MPa and 220 MPa respectively. The deciding factor is corrosion resistance, not strength.

Q2. What is the PREN of 317L and 904L?
317L calculates to about 29-31 and 904L to about 34-37. PREN is a ranking tool for pitting resistance in chloride media; values above 32 are considered necessary for seawater and highly chlorinated service.

Q3. When should 904L be chosen over 317L?
When the medium contains hot sulfuric acid, high chloride levels with crevices, or when 317L has been proven to pit in the actual process. 904L is also preferred for long, maintenance-free service in marine and cooling-water systems.

Q4. Are 317L and 904L weldable?
Yes, both are fully austenitic and weld with standard processes. Matching or over-alloyed filler metal is required; 904L is usually welded with nickel-based or high-molybdenum filler to preserve corrosion resistance in the weld metal.

Q5. Can 904L replace 317L in existing equipment?
Yes, dimensions permitting. Both grades are covered by ASTM A240 for plate and by ASTM A312/A269 for pipe and tube; a replacement must be verified against the design code, thermal expansion and pressure-temperature rating of the original equipment.