Comparison of Steel 254SMO and 904L

Before knowing the difference between 254SMO and 904L, here is a quick summary of the most important conclusions:

 

• 254SMO has a higher PRE (Pitting Resistance Equivalent) of approximately 42.5 vs. 32 for 904L—making it significantly more resistant to pitting and crevice corrosion in chloride environments.

 

• 904L has higher nickel content (23–28%) compared to 254SMO (17.5–18.5%), giving it better resistance to reducing acids such as sulfuric acid at moderate concentrations.

 

• 254SMO offers higher mechanical strength—yield strength of 310 MPa vs. 220 MPa for 904L

• 904L is generally less expensive than 254SMO due to lower molybdenum content.

 

• For seawater, chloride-heavy environments, and FGD systems: 254SMO is the better choice.

 

• For sulfuric acid, phosphoric acid, and mixed acid service, 904L is often more suitable.

 

Both grades are available in pipe, tube, plate, flange, and fitting form to ASTM/ASME standards.

 

What Is 254SMO?

 

254SMO is a super-austenitic stainless steel developed specifically for use in highly aggressive chloride-containing environments. Its UNS designation is S31254, the European material number is 1.4547, and it is also known as F44 under ASTM A182 for forgings and flanges.

 

The "SMO" in its name stands for stainless steel with high Molybdenum content. At 6.0–6.5% molybdenum — roughly three times the amount found in 316L — 254SMO achieves a level of pitting resistance that puts it well above most austenitic grades.

 

254SMO was originally developed by Outokumpu (formerly Avesta Sheffield) in Sweden in the 1970s. Today it is one of the most widely specified super-austenitic grades globally, particularly in offshore, desalination, and pulp and paper applications.

 

What Is 904L?

 

904L is a low-carbon, high-alloy austenitic stainless steel with UNS designation N08904 and European material number 1.4539. It is sometimes referred to as "Alloy 904L" because its nickel content places it at the boundary between stainless steel and nickel alloy territory.

 

904L resists corrosion in moderately aggressive acid environments, particularly dilute sulfuric acid across a wide concentration range. Its combination of chromium, nickel, molybdenum, and copper gives it broad corrosion resistance across both oxidizing and reducing media.

 

The alloy was developed in the 1970s and quickly found its home in chemical processing, oil refinery gas scrubbers, and phosphoric acid equipment. More recently, its non-magnetic and corrosion-resistant properties made it the alloy of choice for high-end watch cases — Rolex has used 904L for its steel watches since 1988.

 

Steel 254SMO and 904L: Chemical Composition Comparison

 

 

Summary: 254SMO has more molybdenum and more nitrogen. The result is a higher PRE value and better performance in chloride environments.

 

904L has more nickel and more copper. Nickel improves resistance to reducing acids like sulfuric acid, and copper specifically suppresses the corrosion rate in dilute to moderate H₂SO₄. The result is better performance in acid environments where 254SMO's advantages are less relevant.

 

Steel 254SMO and 904L: Pitting Resistance Equivalent (PRE)

 

For materials used in chloride environments, the PRE value is the single most useful number for comparing corrosion resistance. It is calculated as:

 

PRE = %Cr + 3.3 × %Mo + 16 × %N

 

 

Note: PRE values vary depending on the actual chemical composition within the specification range. Values above use nominal midpoint compositions for comparison purposes.

 

Summary: A PRE above 40 is generally considered the minimum threshold for continuous immersion in seawater. 254SMO clears this threshold with a meaningful margin. 904L, with a PRE around 36–37, falls short of this threshold and is not recommended for continuous seawater immersion service.

 

For brackish water, industrial cooling water, and similar environments with lower chloride concentrations, 904L's PRE is adequate and its lower cost makes it competitive.

 

Steel 254SMO and 904L: Mechanical Properties Comparison

 

254SMO's higher nitrogen content gives it a clear strength advantage over 904L. Nitrogen is one of the most effective solid-solution strengthening elements in austenitic stainless steels—at roughly 30 times the strengthening effect of carbon but without the sensitization problems.

 

 

Note: 254SMO's yield strength (310 MPa) is 41% higher than 904L's (220 MPa). Both can be cold-formed, deep-drawn, and bent without special precautions.

 

Steel 254SMO and 904L: Physical Properties Comparison

 

 

Note: Both grades are fully austenitic and non-magnetic in all standard conditions. The thermal conductivity difference (14 vs. 12 W/m·K) is minor for most applications.

 

Steel 254SMO and 904L: Corrosion Resistance

 

This is where the two grades diverge most significantly. 254SMO is clearly superior for chloride environments. Its critical pitting temperature of ≥50°C means it can handle heated seawater and hot concentrated chloride solutions that would pit 904L within months.

 

Pitting and Crevice Corrosion in Chloride Media Data

 

 

Sulfuric Acid (H₂SO₄) Resistance

 

904L is significantly better in dilute to moderate sulfuric acid service. The higher copper content in 904L directly suppresses the corrosion rate in H₂SO₄ by a mechanism that molybdenum does not replicate. For H₂SO₄ service above 70%, neither grade is appropriate — Hastelloy B grades are required.

 

 

Phosphoric Acid (H₃PO₄) Resistance

 

Both grades perform well in phosphoric acid, but 904L has a slight edge in wet-process phosphoric acid that contains fluoride and chloride impurities due to its higher nickel content:

 

 

Hydrochloric Acid (HCl) Resistance

 

Neither grade is suitable for HCl service except at very low concentrations and ambient temperatures. For HCl service, Hastelloy C276 or C22 should be specified instead.

 

Organic Acid Resistance

 

Both grades perform similarly in organic acids (acetic, formic, citric). 904L's higher nickel gives it a marginal advantage in some reducing organic acid environments, but the difference is rarely significant enough to drive material selection.

 

Steel 254SMO and 904L: Temperature Service Range

 

 

Neither grade is designed for high-temperature structural service. Above 400°C, their corrosion resistance declines and sigma phase precipitation becomes a risk. For high-temperature service, Incoloy 800H, Incoloy 800HT, or Inconel 625 are more appropriate choices.

 

Both grades retain excellent toughness at cryogenic temperatures, making them suitable for LNG applications and cryogenic processing.

 

Steel 254SMO and 904L: Welding Comparison

 

Both 254SMO and 904L are weldable by standard processes, but their weldability characteristics differ in important ways.

 

 

When welding 254SMO, using a matching ER385 filler is acceptable for non-critical service, but the weld zone will have a slightly lower PRE than the base metal due to dilution effects.

 

For critical chloride service, ERNiCrMo-3 (Inconel 625 composition, PRE ~52) is preferred as an over-alloyed filler that maintains corrosion resistance at the weld.

 

The same approach applies to 904L in critical service.

 

Available Product Forms and Standards

 

Both grades are available in all standard mill product forms. The applicable ASTM/ASME standards are the following:

 

 

Both grades are also covered under corresponding ASME standards for pressure vessel and boiler applications regulated by ASME codes.

 

Steel 254SMO and 904L: Cost Comparison

 

Both 254SMO and 904L are premium materials, but they sit at different prices. 254SMO costs more per kilogram.

 

 

Steel 254SMO and 904L: Industry Applications

 

254SMO Is Typically Used In

 

Seawater handling systems: Cooling water pipes, ballast water systems, seawater desalination (MSF and RO plants), firefighting systems on offshore platforms.

 

Flue gas desulfurization (FGD): Absorber vessels, spray nozzles, and ductwork exposed to hot, wet sulfurous acid with high chloride concentrations.

 

Pulp and paper bleach plants: Bleaching stages (D, E, H stages) where hypochlorite and chlorine dioxide attack most stainless steels.

 

Chemical process equipment: Heat exchangers, reaction vessels, and piping handling concentrated chloride solutions.

 

Offshore oil and gas: Subsea piping, topsides piping, and injection systems in high-chloride-produced water environments.

 

Hydraulic and instrumentation tubing: Offshore and marine applications requiring high strength in a compact wall thickness.

 

Power generation: Condenser tubes in coastal power stations using seawater cooling.

 

904L Is Typically Used In

 

Sulfuric acid plants and handling: Storage tanks, transfer piping, and heat exchangers in H₂SO₄ service up to 50–60% concentration.

 

Oil refinery acid gas scrubbers: Amine treating units and sulfur recovery units exposed to H₂S and CO₂ in acidic aqueous environments.

 

Phosphoric acid production: Wet-process phosphoric acid equipment, evaporators, and storage.

 

Chemical process piping: Mixed-acid environments with a reducing character where nickel's reducing acid resistance is needed.

 

Pharmaceutical and food processing: Where non-contaminating, easily cleanable surfaces are required alongside moderate corrosion resistance.

 

Watch and precision instrument components: Non-magnetic, corrosion-resistant applications requiring excellent surface finish.

 

Summary

 

 

Frequently Asked Questions

 

Q: Can 904L be used in seawater service?

 

904L can be used in ambient-temperature, moving seawater for short-duration or low-criticality applications, but it is not recommended for continuous seawater immersion. Its PRE of approximately 36–37 falls below the commonly accepted threshold of 40 for reliable seawater service. In stagnant seawater or heated seawater above 35–40°C, pitting and crevice corrosion will occur. For seawater service, 254SMO, super duplex grades S32750/S32760, or titanium should be specified instead.

 

Q: Is 254SMO the same as AL6XN?

 

254SMO and AL6XN are both 6% molybdenum super-austenitic stainless steels with very similar PRE values (approximately 42–46). They are functionally equivalent in most seawater and high-chloride applications.

 

The main differences are: AL6XN further elevates its PRE and is primarily produced and stocked in North America; 254SMO is more globally available and more commonly specified in European and Asian projects. If your project specification calls for one grade specifically, it cannot automatically be substituted for the other without engineering review.

 

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

 

316L has a PRE of approximately 24 and contains only 2.1% molybdenum and 10–14% nickel. 904L has a PRE of approximately 37 and contains 4.0–5.0% molybdenum and 23–28% nickel.

 

In practice, 904L outperforms 316L in sulfuric acid across a much wider concentration and temperature range, resists pitting in higher chloride concentrations, and is less susceptible to stress corrosion cracking.

 

904L costs approximately 2–3× more than 316L per kilogram.

 

Q: Can 254SMO and 904L be welded to each other?

 

Yes. Dissimilar welding of 254SMO to 904L is technically feasible. The recommended filler for this dissimilar weld is ERNiCrMo-3 (Inconel 625 composition).

 

Q: Which grade is better for cryogenic service?

 

Both grades are fully austenitic and maintain excellent toughness and ductility at cryogenic temperatures. Neither grade undergoes the ductile-to-brittle transition that limits ferritic and martensitic steels at low temperatures. For service temperatures down to −196°C, both grades are acceptable.

 

Q: Does 904L pass the requirements for sour service (NACE MR0175 / ISO 15156)?

 

904L is included in NACE MR0175 / ISO 15156-3 for use in H₂S-containing environments, subject to the specific environmental limits defined in that standard. 254SMO is also covered.

 

Equivalent Grade Cross-Reference

 

 

JN Alloy supplies both 254SMO (UNS S31254) and 904L (UNS N08904) in pipe, tube, plate, bar, flange, and fitting form to full ASTM/ASME standards, with mill test reports and third-party inspection available.

 

Contact our sale team with your service conditions, and we will recommend the correct grade and product form for your application.