UNS S31254 vs 904L Super Austenitic Stainless Steels: Ultra-Harsh Chloride Environment Specialists

Two Super-Austenitic Specialists for Aggressive Media

UNS S31254 and 904L (UNS N08904) are fully austenitic stainless steels developed for environments beyond the reach of 316L and the duplex grades. They are often compared because both serve the chemical and offshore industries, but their alloying strategies differ: S31254 is a 6% molybdenum super-austenitic grade strengthened against chloride attack, while 904L is a high-nickel grade optimized for strong acid service.

Chemical Composition per ASTM A240

Element (wt%)	S31254	904L UNS N08904
C	0.020 max	0.020 max
Mn	1.00 max	2.00 max
P	0.030 max	0.045 max
S	0.010 max	0.035 max
Si	0.80 max	1.00 max
Cr	19.5-20.5	19.0-23.0
Ni	17.5-18.5	23.0-28.0
Mo	6.0-6.5	4.0-5.0
N	0.18-0.22	not specified
Cu	0.50-1.00	1.00-2.00

PREN = Cr + 3.3 Mo + 16 N, computed from nominal composition, reaches about 42-43 for S31254 versus about 35 for 904L. The gap comes from the higher molybdenum and, decisively, the nitrogen addition of S31254, which the 904L specification does not require.

Corrosion Resistance in Service

S31254 excels in hot chloride environments: seawater above ambient temperature, brine solutions, desalination plant heat exchangers, offshore subsea equipment and pulp bleaching systems. Its high PREN resists pitting and crevice corrosion where 904L would show attack. 904L excels in strong acids, notably sulfuric and phosphoric acid and acid-laden chemical processes, where the high nickel content keeps the passive film stable and the grade also resists stress corrosion cracking in sour gas service. In mixed acid-chloride streams, S31254 is the more reliable choice at elevated temperature.

Mechanical Properties

Annealed plate per ASTM A240: S31254 has minimum tensile strength 650 MPa, yield strength 300 MPa and elongation 35%. 904L has minimum tensile strength 490 MPa, yield strength 220 MPa and elongation 35%. Both grades are non-magnetic and retain good toughness, including at low temperature.

Fabrication and Welding

Both grades are weldable with GTAW, GMAW and SAW using low heat input and over-alloyed nickel-based filler metals such as AWS ERNiCrMo-3 to keep weld metal corrosion resistance above that of the base material. Post-weld pickling and passivation are mandatory to remove weld oxide scale and restore the passive film. Forming forces are higher than for 316L, and S31254 in particular work-hardens more rapidly, so slower speeds and stronger tooling are needed.

Selection and Cost

Select S31254 for hot chloride and seawater service where pitting and crevice attack govern: seawater reverse osmosis, offshore wellheads, coastal condensers, bleach plants. Select 904L for strong acid service: sulfuric and phosphoric acid handling, chemical tanker linings, acid storage and nuclear waste processing. S31254 typically carries a 20-30% cost premium over 904L, so for mild or low-temperature chloride service the economical answer is 316L or a duplex grade, not either of these specialists.

FAQ

What is the main difference between S31254 and 904L?

Alloying strategy. S31254 uses 6-6.5% molybdenum plus 0.18-0.22% nitrogen to defeat chloride pitting and crevice attack, reaching a PREN of about 42-43. 904L uses a higher nickel content of 23-28% with 4-5% molybdenum and no nitrogen requirement, reaching a PREN of about 35, and excels in strong acids.

Why is the PREN of S31254 so much higher?

Because both molybdenum and nitrogen are strong contributors to the PREN formula Cr + 3.3 Mo + 16 N, and S31254 carries more of both. Nitrogen also strengthens the alloy and is absent from the 904L specification.

Which grade should I choose for seawater heat exchangers?

UNS S31254. Hot seawater, brine and chloride-laden cooling water demand the higher pitting and crevice resistance of the 6% Mo grade; 904L is not the first choice for chloride pitting duty.

Which grade is better for concentrated sulfuric or phosphoric acid?

904L. Its high nickel content stabilizes the passive film in strong acids, and it is the conventional choice for acid storage, chemical tankers and acid processing equipment. S31254 also performs well but is usually not needed there.

Can these grades replace 316L?

They can outperform 316L by a wide margin, but at several times the cost. Use 316L for mild service and reserve S31254 or 904L for environments where 316L and the duplex grades are not adequate.

What filler metals are used to weld them?

Over-alloyed nickel-based fillers such as AWS ERNiCrMo-3 are typical, applied with low heat input and controlled interpass temperature. Weld metal should always be at least as corrosion resistant as the base material, and pickling plus passivation completes the joint.