 |
Stainless steel 904L alloy is a super austenitic stainless steel. High levels of chromium and nickel, plus molybdenum and copper, ensure the product's excellent corrosion resistance. With a high alloy chemistry of 25% nickel and 4.5% molybdenum, stainless steel 904L outperforms molybdenum-containing stainless steels 316L and 317L in resistance to chloride stress corrosion cracking, pitting and corrosion. 904L steel was originally developed to resist corrosion in dilute sulfuric acid and has very good corrosion resistance. Stainless steel 904L is easy to weld and process in standard workshop manufacturing practices.
Rolex 904l, also known as rolex 904l steel, is the material used by Rolex to make watch cases. In fact, Rolex also used 316L stainless steel before. It was not until 1985 that stainless steel 904L was first used, and a solid watch strap was used. It was not until 2003 that all watches were replaced with 904L steel. So, why did you choose 904L?
In fact, both 316L and 904L are super austenitic stainless steels. Both are non-magnetic and have no effect on the accuracy of the watch. 904L steel contains more chromium, so it is more corrosion-resistant than 316L. In addition, Rolex 904L steel is easier to grind and polish than other brands, and even serious bumps can be restored by grinding.
In the professional field, 904L is the second best stainless steel material after 2507 super duplex stainless steel. Compared with 316L, its advantage is not only corrosion resistance. 904L steel is much more stable than 316L in extreme chemical environments, and its bending strength and tensile strength are also better than 316L.
Grade | Alloy 904L |
Density | 7.95 g/cm3 |
Melting Point | 1300 – 1390°C |
Electrical Resistivity | 33.5 Microhm-in at 68°C |
Thermal Conductivity | 12.9 W/m-°K |
Modulus of Elasticity | 190 GPa |
Specific Heat | 450 J/kg-°K (0 – 100°C) |
Grade | N≤ | Cr | Mo | Cu | P≤ | C≤ | Mn≤ | S≤ | Si≤ | Ni |
904L | 0.1 | 19.0-23.0 | 4.0-5.0 | 1.0-2.0 | 0.045 | 0.02 | 2.0 | 0.035 | 1.0 | 23.0-28.0 |
Grade | Tensile, min, ksi[MPa] | Yield, min, ksi[MPa] | Elongation, %(min) |
N08904 | 71【490】 | 31【220】 | 35 |
STANDARD | WERKSTOFFNR. | UNS | JIS | BS | AFNOR | EN |
904L | 1.4539 | N08904 | SUS 904L | 904S13 | Z1CNDU20.18.06Az | X1NiCrMoCu25-20-5 |
Product | Standard | Show |
Pipe SMLS & Welded | ASTM A312 |  |
Tube SMLS & Welded | ASTM A269/A249 |  |
Sheet /Plate | ASTM A240 |  |
Bars | ASTM A479 /ASTM A276 |  |
Forging | ASTM A182 |  |
Fittings | ASTM A403 |  |
904L is the acid specialist — its 1.5% copper gives it superior resistance to sulfuric and phosphoric acids that even the higher-molybdenum 254SMO cannot match.
254SMO is the chloride champion — its 6% molybdenum and PREN of ~43 make it the strongest choice for pure seawater and extreme chloride pitting.
316L is the general-purpose workhorse — adequate for mild environments but fails rapidly in concentrated acids or hot chlorides.
Chemical Composition Comparison
| Element | 316L (S31603) | 904L (N08904) | 254SMO (S31254) | Key Difference |
|---|---|---|---|---|
| Chromium (Cr) | 16.0–18.0% | 19.0–23.0% | 19.5–20.5% | 904L & 254SMO: higher Cr → better oxidation resistance |
| Nickel (Ni) | 10.0–14.0% | 23.0–28.0% | 17.5–18.5% | 904L: highest Ni → best SCC resistance |
| Molybdenum (Mo) | 2.0–3.0% | 4.0–5.0% | 6.0–6.5% | 254SMO: highest Mo → best chloride pitting resistance |
| Copper (Cu) | — | 1.0–2.0% | 0.5–1.0% | 904L: highest Cu → best reducing acid resistance |
| Nitrogen (N) | ≤0.10% | ≤0.10% | 0.18–0.22% | 254SMO: N adds strength + pitting resistance |
| Carbon (C) | ≤0.030% | ≤0.020% | ≤0.020% | All low-C for weldability |
PREN and Corrosion Resistance Comparison
| Parameter | 316L | 904L | 254SMO |
|---|---|---|---|
| PREN (Cr + 3.3×Mo + 16×N) | ~25 | ~35 | ~43 |
| Steel Type | Austenitic SS | High-alloy Austenitic SS | Super Austenitic SS (6Mo) |
| Critical Pitting Temp (CPT) | 15–25°C | 40–50°C | >70°C |
| Chloride Resistance | Moderate | Very High | Extremely High |
| SCC Resistance | Moderate | Excellent | Excellent |
| Sulfuric Acid Resistance | Poor (>10% conc.) | Excellent (to 95% conc.) | Good (but less Cu than 904L) |
| Phosphoric Acid Resistance | Moderate | Excellent | Good |
| Relative Cost | ★ (baseline) | ★★ (2–3× cost of 316L) | ★★★★ (4–5× cost of 316L) |
Selection rule of thumb: Choose 316L for general-purpose service where cost matters and corrosion is moderate. Choose 904L for sulfuric acid, phosphoric acid, and moderate chloride service where its copper content provides a decisive advantage. Choose 254SMO for extreme chloride pitting, seawater, and desalination where maximum pitting resistance is critical.
When to Upgrade from 316L to 904L
Service environment contains sulfuric acid above 10% concentration
Wet-process phosphoric acid with fluoride contamination
Chloride-containing environments above 40°C where 316L pits
FGD scrubber service with SO₂/SO₃ and chloride condensate
Pharmaceutical or food equipment requiring exceptional purity and corrosion resistance
Applications where 316L has failed prematurely due to pitting, crevice corrosion, or SCC
When to Choose 254SMO Over 904L
Hot seawater service above 60°C where crevice corrosion is the primary risk
Desalination plant heat exchangers and evaporator tubes
Offshore splash zone components with high chloride exposure
Bleach plant equipment in pulp and paper (hypochlorite service)
High-chloride brine service where CPT above 70°C is required
Welding performance：
Like general stainless steel, 904 L can be welded by a variety of welding methods. The most commonly used welding methods are manual arc welding or inert gas shielded welding. The electrode or wire metal is based on the composition of the base metal and has a higher purity. The content of molybdenum is higher than that of the base metal. Preheating is generally not required before welding, but in cold outdoor operations, in order to avoid condensation of water vapor, the joints or adjacent areas can be heated evenly.
Note that the local temperature should not exceed 100 ℃, so as not to cause carbon agglomeration and cause intergranular corrosion. When welding, it is advisable to use small line energy, continuous and fast welding speed. Generally, no heat treatment is required after welding. If heat treatment is required, it must be heated to 1100-1150°C and then cooled rapidly.
Machinability：
The machining characteristics of 904L are similar to those of other austenitic stainless steels, and there is a tendency for tool sticking and work hardening during machining. Positive rake angle cemented carbide tools must be used, and vulcanized and chlorinated oil is used as cutting coolant. The equipment and process should be based on the premise of reducing work hardening. During the cutting process, slow cutting speed and feed amount should be avoided.
Corrosion resistance:
904L stainless steel has excellent corrosion resistance and performs well in strong acid, strong alkali and high temperature environments. Especially in chloride ion environments, its corrosion resistance is far superior to traditional 18-8 stainless steel.
Mechanical properties:
904L steel has high yield strength and tensile strength, and can withstand large forces and pressures. In addition, it also has excellent impact toughness, ductility and wear resistance, and is suitable for a variety of complex working conditions.
Magnetism:
904L has extremely low magnetic permeability and is almost unable to sense or maintain magnetism, which is very beneficial for some special application scenarios.
High temperature performance:
904L stainless steel has good high temperature stability and can maintain its excellent corrosion resistance and mechanical properties. Therefore, it is widely used in high temperature environments, especially in chemical, petroleum, pharmaceutical and other industries.
904L is one of the best stainless steels for sulfuric acid service. It resists corrosion across nearly the full concentration range at ambient temperature, and up to 50% concentration at 95°C. The 1.5% copper addition is the key element — copper raises the steel's potential into a range where reducing acids form stable passive films. In 50% H₂SO₄ at 80°C, 904L's corrosion rate is 0.05 mm/year, compared to 5.8 mm/year for 316L.
Sulfuric acid is the most widely used industrial acid, and its corrosive behavior varies dramatically with concentration and temperature. At low concentrations (<10%), it is moderately corrosive; at 50–80% concentration, it is highly aggressive; above 90%, it becomes less corrosive due to passivation. 904L's copper content makes it effective across a broader range than any standard stainless steel.
| H₂SO₄ Concentration | Max Safe Temperature | Corrosion Rate (904L) | 316L Comparison |
|---|---|---|---|
| ≤10% (dilute) | 95°C | <0.05 mm/year | Marginal above 40°C |
| 10–30% | 80°C | <0.1 mm/year | Fails rapidly above 10% |
| 30–50% | 95°C | ~0.05 mm/year | Not recommended |
| 50–70% | 60°C | <0.3 mm/year | Not recommended |
| 70–90% | 40°C | <0.5 mm/year | Not recommended |
| 90–95% (concentrated) | 30°C | <0.1 mm/year (passivation) | Marginal |
| >95% (oleum) | Not recommended | — | — |
Acid storage tanks and piping: 904L is the standard material for dilute to moderate concentration H₂SO₄ storage and transport at temperatures up to 80°C.
Pickling line equipment: Steel pickling uses 5–20% H₂SO₄ at 60–90°C. 904L heating coils, containers, baskets, and chains provide long service life.
Heat exchangers: 904L plate and tube heat exchangers handle sulfuric acid cooling and heating duties where 316L would fail within months.
FGD absorber towers: Flue gas desulfurization systems produce dilute sulfuric acid condensate. 904L is used in absorber tower internals, spray systems, and dampers.
Fertilizer production: Phosphate fertilizer plants use sulfuric acid in the digestion process. 904L reactors and piping resist the combined acid-chloride environment.
Important: 904L is not suitable for hot concentrated sulfuric acid above 95% or for oleum (fuming sulfuric acid). For these services, cast iron or silicon iron is typically used. Always verify the specific concentration and temperature against isocorrosion charts before final material selection.
904L is a standard material for wet-process phosphoric acid (WPA) production equipment. It handles evaporators, agitators, pump casings, and piping at temperatures up to 80–90°C. The combination of high nickel, molybdenum, and copper provides resistance to both the phosphoric acid itself and the fluoride and chloride contamination typical of wet-process acid.
Phosphoric acid is produced by two main routes: the thermal process (pure, high-concentration H₃PO₄) and the wet process (impure, 28–42% H₃PO₄ with fluoride, chloride, and sulfate contaminants). The wet process is far more corrosive because the impurities — particularly fluoride ions from fluosilicic acid — break down passive films on standard stainless steels.
| Factor | Effect on Corrosion | How 904L Handles It |
|---|---|---|
| Fluoride contamination (F⁻) | Breaks passive film; accelerates pitting | High Cr + Mo maintains passivation; Cu enhances film stability |
| Chloride contamination (Cl⁻) | Causes pitting and crevice corrosion | PREN ~35 provides 40% better resistance than 316L |
| Free sulfuric acid | Reducing acid attack | Cu (1.5%) enables stable passivation in reducing environments |
| Temperature (80–90°C) | Accelerates all corrosion mechanisms | High Ni (25%) maintains austenite stability at temperature |
| Slurry erosion (gypsum crystals) | Mechanical wear removes passive film | Higher alloy content enables rapid repassivation |
WPA evaporators: Concentration of 28% to 42–54% P₂O₅ at 80–90°C. 904L tubes and vessels provide 10+ year service life.
Digestion reactors: Sulfuric acid attacks phosphate rock at 70–90°C, producing phosphoric acid and gypsum. 904L reactor vessels and agitators resist the combined acid + slurry environment.
Pump casings and impellers: Centrifugal pumps in 904L handle phosphoric acid slurries with abrasive gypsum crystals.
Filter wash systems: 904L filter cloth washing systems and filtrate piping resist the acidic chloride-fluoride environment.
Heat exchangers: 904L plate heat exchangers for phosphoric acid cooling and concentration duties.
| Grade | WPA Service Performance | Typical Life | Notes |
|---|---|---|---|
| 316L | Poor — pitting within 1–2 years | 1–2 years | Inadequate Mo and Cu for fluoride service |
| 317L | Moderate — better than 316L but limited | 3–5 years | Higher Mo (3–4%) but no Cu |
| 904L | Excellent — standard WPA material | 10–15 years | Best balance of Cu + Mo + Ni for WPA |
| 254SMO | Good — higher Mo but lower Cu | 10–15 years | More expensive; overkill for most WPA |
| Alloy 20 (N08020) | Excellent — highest Cu (3–4%) | 15+ years | More expensive; preferred for highest fluoride |
As a general-purpose corrosion-resistant material, 904L stainless steel sheet can be cold-rolled to obtain high-performance products, which can be used in many fields including food industry equipment, kitchen utensils and electronic industries. The use of each field is different, and the performance requirements of cold-rolled stainless steel 904L stainless steel are also different.
Especially when the thickness of the stainless steel strip is small, it is extremely important to seek its comprehensive cold working performance. By analyzing the influence of rolling and annealing processes on its properties and microstructure, and studying the influence of production process parameters on its microstructure and properties, experimental data can be provided for on-site production optimization and performance improvement.
Petroleum, petrochemical equipment, such as reactors in petrochemical equipment, etc.
Storage and transportation equipment for sulfuric acid, such as heat exchangers, etc.
The flue gas desulfurization device of power plant is mainly used in the following parts: the tower body, flue, baffle plate, internal parts, spray system, etc. of the absorption tower.
Scrubbers and fans in organic acid treatment systems.
Seawater treatment devices, seawater heat exchangers, paper industry equipment, sulfuric acid, nitric acid equipment, acid production, pharmaceutical industry and other chemical equipment, pressure vessels, food equipment.
Pharmaceutical plants: centrifuges, reactors, etc.
Plant food: equipment
904L is the matching steel grade for the strong corrosive medium of dilute sulfuric acid.
Q: What is 904L stainless steel used for?
904L is used in chemical processing equipment for sulfuric acid, phosphoric acid, and acetic acid service; flue gas desulfurization (FGD) scrubbers in power plants; seawater heat exchangers and cooling systems; pharmaceutical reactors and centrifuges; pulp and paper bleach plant equipment; and oil and gas sour gas service. It is also used by Rolex for watch cases due to its superior corrosion resistance and polishability.
Q: Is 904L better than 316L?
Yes, 904L significantly outperforms 316L in aggressive corrosion environments. 904L has a PREN of ~35 vs 316L's ~25, meaning 40% better pitting resistance. 904L also contains 1–2% copper, which 316L lacks, giving it superior resistance to sulfuric and phosphoric acids. However, 316L is cheaper and adequate for general-purpose applications where extreme corrosion resistance is not required.
Q: What is the difference between 904L and 254SMO?
254SMO (S31254) has higher molybdenum (6% vs 4.5%) and higher PREN (~43 vs ~35), making it superior in pure seawater and high-chloride pitting service. However, 904L contains more copper (1.5% vs 0.7%), giving it better resistance to sulfuric and phosphoric acids. Choose 904L for acid service; choose 254SMO for extreme chloride pitting and seawater applications.
Q: Can 904L be used in sulfuric acid?
Yes. 904L was originally developed for dilute sulfuric acid service. It performs excellently in sulfuric acid up to 95% concentration at 20°C, and up to 50% concentration at 95°C. The 1–2% copper addition is the key element that enables stable passivation in reducing acid environments. Corrosion rate in 50% H₂SO₄ at 80°C is approximately 0.05 mm/year.
Q: Can 904L be used in phosphoric acid?
Yes. 904L is a standard material for wet-process phosphoric acid production, handling evaporators, agitators, and pump casings at temperatures up to 80–90°C. It resists both the phosphoric acid and fluoride contamination typically present in wet-process acid. 904L outperforms 316L and 317L in this service.
Q: Is 904L stainless steel magnetic?
No. 904L is a fully austenitic stainless steel with a face-centered cubic (FCC) crystal structure. It is non-magnetic in the annealed condition, with very low magnetic permeability (μᵣ < 1.005). This property is beneficial for watch cases (Rolex) and instruments where magnetic interference must be avoided.
Q: What is the PREN of 904L stainless steel?
The Pitting Resistance Equivalent Number (PREN) of 904L is approximately 35, calculated as Cr% + 3.3×Mo% + 16×N% = 21 + 3.3×4.5 + 16×0.1 ≈ 36. This is 40% higher than 316L (PREN ~25) but lower than 254SMO (PREN ~43).
Q: What welding consumables are used for 904L?
The recommended filler metal for 904L is ER385 (AWS A5.9) or ERNiCrMo-3 (Alloy 625 filler) for higher corrosion resistance. For covered electrodes, use E385 or ENiCrMo-3. Use low heat input, interpass temperature below 100°C, and no preheating. No post-weld heat treatment is normally required due to the low carbon content (0.02% max).
Q: What is the maximum service temperature of 904L?
For corrosion service in acids, 904L is typically used up to 95°C. For structural/oxidation service, it can be used up to approximately 400°C. Prolonged exposure above 400°C risks sigma phase precipitation due to the high alloy content, which reduces toughness and corrosion resistance.
Q: Why does Rolex use 904L steel?
Rolex uses 904L (marketed as "Oystersteel") because it offers superior corrosion resistance compared to 316L, especially in sweat, seawater, and chlorinated pool environments. 904L also takes a higher polish than 316L and maintains its luster longer. Rolex transitioned all watch cases to 904L by 2003.
Q: What is the difference between 904L and 317L?
317L has higher molybdenum than 316L (3–4% vs 2–3%) but contains no copper and less nickel (11–15% vs 23–28%). 904L's copper addition and higher nickel make it dramatically better in sulfuric and phosphoric acid service, while its higher molybdenum gives it better chloride pitting resistance. 904L is the clear upgrade from both 316L and 317L for aggressive environments.
Q: Does 904L require post-weld heat treatment?
No. Due to its ultra-low carbon content (≤0.02%), 904L does not suffer from sensitization during welding and does not require post-weld heat treatment in most applications. If heat treatment is specified for critical service, solution anneal at 1,100–1,150°C followed by rapid cooling (water quench).
