In the realm of industrial materials, 904L vs 316L stainless steel is a comparison frequently encountered by engineers, metallurgists, and procurement specialists.
Both alloys belong to the austenitic stainless steel family, prized for their corrosion resistance, mechanical integrity, and adaptability across industries.
However, while 316L is a widely used, cost-effective solution for many general-purpose applications, 904L is a premium-grade stainless steel specifically engineered to perform under highly corrosive and aggressive conditions.
Choosing the correct material between 904L and 316L can significantly affect not only the initial investment but also long-term operational costs, safety, and reliability,
particularly in sectors such as chemical processing, oil & gas, marine, power generation, and pharmaceuticals.
1. What is 316L Stainless Steel?
316L is a low-carbon version of 316 stainless steel (the “L” stands for “low carbon,” typically ≤ 0.03%).
It is one of the most widely used stainless steels in industrial and commercial environments due to its balance of corrosion resistance, weldability, and affordability.
Key Features of 316L:
- Austenitic Structure: Provides excellent ductility, toughness, and non-magnetic behavior in the annealed condition.
- Corrosion Resistance: Good resistance to general corrosion, especially in chloride environments like marine or coastal regions.
- Weldability: Excellent. Low carbon content minimizes the risk of carbide precipitation during welding, avoiding intergranular corrosion.
Common Standards:
- ASTM A240 / A276
- EN 1.4404
- UNS S31603
2. What is 904L Stainless Steel?
904L is a super austenitic stainless steel designed for exceptional corrosion resistance in highly aggressive environments.
It contains significantly higher levels of nickel, molybdenum, and copper, which enhance its resistance to acids, particularly sulfuric and phosphoric.
Key Features of 904L:
- Super Austenitic Structure: Extremely stable and non-magnetic, even after cold working. Maintains toughness across a wide temperature range.
- Corrosion Resistance: Excellent, especially against sulfuric acid, phosphoric acid, and chlorides. Often used in conditions where standard stainless steels like 316L would fail.
- Weldability: Good, but requires skilled technique. Post-weld heat treatment (solution annealing) may be necessary for optimal corrosion resistance.
Common Standards:
- ASTM B625 / A276 / A312
- EN 1.4539
- UNS N08904
3. Chemical Composition of 904L vs 316L Stainless Steel
| Element | 316L (%wt) | 904L (%wt) | Function in Stainless Steel |
| Carbon (C) | ≤ 0.03 | ≤ 0.02 | Reduces risk of intergranular corrosion; improves weldability by minimizing carbide precipitation. |
| Chromium (Cr) | 16.0 – 18.0 | 19.0 – 23.0 | Provides corrosion and oxidation resistance by forming a stable passive film on the steel surface. |
| Nickel (Ni) | 10.0 – 14.0 | 23.0 – 28.0 | Stabilizes the austenitic structure; improves ductility, toughness, and resistance to stress corrosion cracking. |
| Molybdenum (Mo) | 2.0 – 3.0 | 4.0 – 5.0 | Enhances resistance to pitting and crevice corrosion, especially in chloride environments. |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 | Acts as a deoxidizer; improves hot-working properties and resistance to scaling. |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 | Improves oxidation resistance; also used as a deoxidizer in the melting process. |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 | Kept low to avoid embrittlement and maintain ductility. |
| Sulfur (S) | ≤ 0.03 | ≤ 0.035 | Improves machinability in small amounts; excess can reduce toughness and corrosion resistance. |
| Copper (Cu) | ≤ 0.5 | 1.0 – 2.0 | Increases resistance to reducing acids, especially sulfuric acid; enhances corrosion resistance in harsh chemical environments. |
| Nitrogen (N) | ≤ 0.10 | ≤ 0.10 | Strengthens the austenitic structure and improves pitting resistance and tensile strength. |
| Iron (Fe) | Balance | Balance | Base metal of stainless steel, contributes to strength and magnetic properties. |
4. Corrosion Resistance Performance
General Corrosion
- 316L stainless steel offers reliable corrosion resistance in neutral or mildly corrosive environments like seawater or food processing.
- 904L stainless steel demonstrates superior durability in aggressive environments such as sulfuric acid or chloride exposure, up to six times better performance in acid resistance tests.
Pitting and Crevice Corrosion
- Pitting Resistance Equivalent Number (PREN):
-
- stainless steel 316L: ≈24–27
- stainless steel 904L: ≈36–39
- Higher PREN in 904L makes it far more resistant to localized corrosion, making it preferable for offshore, marine, and chloride-heavy process use.
Stress Corrosion Cracking (SCC)
- SCC susceptibility is markedly reduced in super-austenitic steels.
- 904L stainless steel’s high nickel content enhances resistance to CCS, making it better suited for chemically aggressive environments.
5. Mechanical and Physical Properties of 904L vs 316L Stainless Steel
| Property | 316L Stainless Steel | 904L Stainless Steel | Remarks |
| Tensile Strength (MPa) | 485 – 620 | 490 – 740 | 904L has higher upper-end strength, beneficial for structural applications. |
| Yield Strength (0.2% Offset, MPa) | ≥ 170 – 310 | ≥ 220 – 290 | 904L has slightly better yield strength, with good ductility retained. |
| Elongation (% in 50 mm) | ≥ 40 | ≥ 35 | 316L is slightly more ductile; better for cold forming. |
| Hardness (Brinell HBW) | ≤ 217 | ≤ 220 | Similar hardness levels; both are suitable for machining and fabrication. |
| Density (g/cm³) | 7.99 | 8.05 | 904L is slightly denser due to higher alloying content. |
| Modulus of Elasticity (GPa) | ~193 | ~195 | Nearly identical; determines stiffness under load. |
| Thermal Conductivity (W/m·K) | 16.3 | 12.5 | 316L has better thermal conductivity; important in heat exchangers. |
| Specific Heat Capacity (J/kg·K) | 500 | 450 | 316L retains heat slightly better. |
| Coefficient of Thermal Expansion (μm/m·°C) | 16.0 × 10⁻⁶ | 15.0 × 10⁻⁶ | 904L has lower thermal expansion; better dimensional stability at high temps. |
| Electrical Resistivity (µΩ·cm) | ~74 | ~94 | 904L has higher resistivity; relevant for electrical and EMI-sensitive uses. |
| Magnetic Permeability (μr) | ~1.02 (non-magnetic in annealed condition) | Non-magnetic (μr ≈ 1.0) | 904L is fully non-magnetic; beneficial in sensitive equipment. |
6. Fabrication and Weldability of 904L vs 316L Stainless Steel
Fabrication processes such as welding, machining, forming, and heat treatment significantly affect the final performance and cost of stainless steel components.
Weldability
| Aspect | 316L Stainless Steel | 904L Stainless Steel |
| Welding Method Compatibility | Suitable for all standard welding methods (GTAW, GMAW, SMAW) | Suitable but requires more care; recommended for TIG and MIG processes |
| Filler Metal | Commonly uses 316L or 308L electrodes | Requires matching 904L filler or high-alloy austenitic filler (e.g., ER385) |
| Preheat/Post-weld Treatment | Not typically required; low carbon prevents carbide precipitation | Post-weld solution annealing may be needed to restore corrosion resistance |
| Hot Cracking Susceptibility | Low | Higher due to high alloy content; controlled heat input is critical |
| Heat-Affected Zone (HAZ) | Minimal degradation | Greater risk of corrosion if not properly post-treated |
Key Insight:
316L offers excellent weldability with minimal post-weld treatment.
904L, although weldable, demands higher skill, special filler metals, and often post-weld heat treatment to maintain corrosion performance in aggressive environments.
Machinability
| Property | 316L | 904L |
| Machining Difficulty | Moderate; better than most high-nickel alloys | Difficult; work-hardens quickly |
| Tool Wear | Normal with carbide tools | High; requires coated tools and coolant |
| Surface Finish Quality | Good finish achievable | Can be rough; slower speeds required |
| Recommendations | Use sharp tooling and moderate speeds | Use rigid tooling, slower speeds, higher feed rates |
Key Insight:
904L’s higher nickel and molybdenum content increases its work hardening rate, making it harder to machine than 316L.
Specialized tooling and machining strategies are required to avoid tool wear and poor surface finish.
Formability and Cold Working
| Forming Behavior | 316L | 904L |
| Cold Forming | Excellent ductility and formability | Good, but more springback and force required |
| Annealing Needs | Sometimes needed after heavy cold work | Often required due to higher work hardening |
| Bending and Rolling | Performs well without cracking | May need wider radii or multi-pass forming |
Key Insight:
While both are suitable for forming, 316L is easier to bend, deep draw, or roll-form. 904L is stiffer and may require intermediate annealing during complex forming operations.
Casting and Heat Treatment
- Casting:
316L is more commonly used in precision investment casting due to its stable structure and cost-effectiveness.
904L can be cast, but its higher alloy content can lead to segregation and hot tearing, requiring more controlled cooling and refined casting techniques. - Heat Treatment:
-
- 316L: Non-hardenable by heat treatment; solution annealing (~1050–1100°C) used to restore corrosion resistance.
- 904L: Also non-hardenable, but post-weld annealing (1100–1150°C) is often recommended to regain full corrosion resistance, particularly after severe cold work or welding.
7. Applications of 904L vs 316L Stainless Steel
Applications of 316L Stainless Steel
316L (Low-Carbon 316) is a versatile, corrosion-resistant alloy widely used in industries where moderate chemical exposure and good weldability are required.
Its lower carbon content minimizes carbide precipitation during welding, making it especially suitable for fabrication-intensive applications.
| Industry | Common Applications |
| Chemical Processing | Storage tanks, process vessels, pipelines for acids and caustics at moderate concentrations |
| Pharmaceutical | Cleanroom and sanitary equipment, bio-reactors, tablet presses |
| Food & Beverage | Dairy processing, brewing tanks, piping, and valves |
| Marine | Seawater piping (short exposure), boat fittings, and decks in mildly corrosive environments |
| Medical Devices | Orthopedic implants, surgical instruments, diagnostic equipment |
| Architecture | Facades, handrails, fasteners exposed to mildly corrosive atmospheres |
| Oil & Gas | Downhole screens, piping (non-sour service), and separators in offshore platforms |
Applications of 904L Stainless Steel
904L is a super austenitic stainless steel designed for use in extremely aggressive environments, especially those containing chlorides, sulfuric acid, or reducing agents.
Its higher contents of nickel (23–25%), molybdenum (4–5%), and copper give it exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking.
| Industry | Common Applications |
| Chemical & Petrochemical | Acid handling equipment (sulfuric, phosphoric), reactors, heat exchangers, and pressure vessels |
| Desalination Plants | Brine tanks, evaporators, and high-chloride pipeline components |
| Marine & Offshore | Deep-sea pipelines, risers, pumps, and valve housings exposed to long-term seawater |
| Battery & Electronics | Components exposed to sulfuric acid in lead-acid battery production |
| Mining & Metallurgy | Slurry pumps, autoclaves, and leaching systems (especially for nickel and copper ores) |
| Pulp & Paper | Bleaching plant equipment exposed to aggressive chemicals |
| High-End Consumer Goods | Luxury watches (e.g., Rolex uses 904L for its exceptional luster and resistance) |
| Pollution Control | Flue gas desulfurization (FGD) units in power plants |
8. Summary: Choosing Between 904L vs 316L Stainless Steel
Selecting the right stainless steel grade between 904L and 316L requires careful evaluation of the application environment, performance requirements, fabrication considerations, and cost factors.
Comprehensive Comparison Table
| Aspect | 316L Stainless Steel | 904L Stainless Steel |
| Corrosion Resistance | Good resistance to mild corrosives and chlorides; moderate pitting resistance | Superior resistance to aggressive chlorides, sulfuric acid, and oxidizing agents; excellent pitting resistance |
| Chemical Composition | Ni ~10–14%, Mo ~2%, minimal Cu | Ni ~23–25%, Mo ~4–5%, Cu ~1.5–2% |
| Mechanical Properties | Good toughness, ductility, and strength | Higher strength and hardness; slightly less ductile |
| Fabrication & Welding | Excellent weldability; minimal heat treatment | More difficult welding; often requires solution annealing |
| Cost | Lower raw material and fabrication cost | Higher raw material cost due to alloying elements; higher fabrication cost |
| Applications | Food processing, pharmaceuticals, moderate chemical exposure, water treatment | Harsh chemical processing, marine, offshore, acidic and chloride-rich environments |
| Service Life & Maintenance | Moderate service life with regular maintenance | Extended service life with reduced maintenance |
| Magnetic Properties | Slightly magnetic after cold working | Essentially non-magnetic |
When to Use 316L Stainless Steel
- Applications where excellent corrosion resistance is needed in moderately aggressive environments
- Cost-sensitive projects where nickel and molybdenum contents are balanced for performance and price
- General-purpose marine, food processing, and medical equipment
- Situations requiring easy fabrication and welding
When 904L Stainless Steel is Essential
- Exposure to highly aggressive chemicals such as sulfuric and phosphoric acids
- Environments with high chloride concentrations where enhanced pitting and crevice corrosion resistance is critical
- Industrial equipment requiring long service life under severe corrosion conditions
- When maximum corrosion resistance justifies the higher material cost
11. Conclusion
316L vs 904L stainless steel are proven austenitic stainless steels with distinct characteristics.
SS 316L is reliable and cost-effective for most applications, while SS 904L offers enhanced corrosion resistance, strength, and reliability in more demanding environments.
Engineers and specifiers should weigh material costs against service conditions to determine the best choice.
Often, the higher upfront investment in 904L is offset by reduced maintenance and longer service life, particularly in aggressive settings.
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FAQs
Is 904L better than 316L stainless steel in all applications?
Not necessarily. While 904L offers superior corrosion resistance—especially in highly aggressive environments—it is also more expensive and harder to fabricate.
For general industrial, food-grade, and pharmaceutical uses where corrosion conditions are mild, 316L is often sufficient and more cost-effective.
What makes 904L stainless steel more corrosion-resistant than 316L stainless steel?
Stainless steel 904L has higher contents of nickel (23–25%), molybdenum (4–5%), and copper (1.5–2%), all of which enhance resistance to acidic environments, chloride-induced pitting, and crevice corrosion.
Its super austenitic structure provides excellent stability in harsh media such as sulfuric acid and seawater.
Can 316L and 904L stainless steel be welded together?
Yes, but careful attention must be given to filler material selection and heat input. Typically, a nickel-based filler (such as ERNiCrMo-3) is used to bridge the metallurgical differences.
Post-weld heat treatment may also be necessary, especially on the 904L side, to restore corrosion resistance.
Why is 904L stainless steel commonly used in the marine and offshore industries?
Its excellent resistance to chloride-induced corrosion, especially pitting and crevice corrosion, makes it ideal for saltwater exposure, splash zones, and subsea applications.
Unlike 316L, 904L maintains integrity in stagnant or warm saltwater environments over long service periods.
Is 904L stainless steel magnetic?
No, 904L stainless steel is essentially non-magnetic in all conditions, including after cold working.
This makes it suitable for applications that require low magnetic permeability, such as in certain electronic, instrumentation, and medical equipment.
How much more expensive is 904L compared to 316L stainless steel?
The cost of stainless steel 904L can be 2–4 times higher than that of 316L, depending on market pricing of nickel, molybdenum, and copper.
Additional costs may also arise from specialized fabrication, welding, and post-treatment processes.
Can 904L be used in sulfuric acid environments?
Yes. 904L is one of the few stainless steels that provides excellent resistance to dilute sulfuric acid—even outperforming some nickel alloys in specific concentrations.
It’s widely used in fertilizer plants, acid storage tanks, and chemical reactors.
