When selecting stainless steel for aggressive chemical environments, the choice between 904L and 316L represents a critical engineering decision that impacts equipment lifespan, maintenance costs, and operational safety. Both are austenitic stainless steels, but their alloy compositions create dramatically different corrosion resistance profiles, particularly in sulfuric acid, phosphoric acid, and chloride-containing environments.
Chemical Composition Comparison
316L Composition
- Carbon: ≤0.03%
- Chromium: 16.0-18.0%
- Nickel: 10.0-14.0%
- Molybdenum: 2.0-3.0%
- Manganese: ≤2.0%
- Silicon: ≤0.75%
904L Composition
- Carbon: ≤0.02%
- Chromium: 19.0-23.0%
- Nickel: 23.0-28.0%
- Molybdenum: 4.0-5.0%
- Copper: 1.0-2.0%
- Manganese: ≤2.0%
- Silicon: ≤0.75%
The key differences are immediately apparent: 904L contains significantly more chromium, nickel, and molybdenum, plus the addition of copper. This enhanced alloy content gives 904L superior corrosion resistance in reducing acids and chloride-containing environments, but at a substantially higher cost.
Corrosion Resistance Comparison
Sulfuric Acid Resistance
This is the most significant differentiator between the two alloys:
- 316L: Acceptable resistance in dilute sulfuric acid (<5%) at ambient temperature, but rapid attack above 10% concentration or 50°C
- 904L: Resists sulfuric acid up to approximately 50% concentration at moderate temperatures (up to 70°C), and dilute acid at higher temperatures
The copper addition in 904L significantly enhances sulfuric acid resistance by promoting formation of a stable passive film in reducing conditions.
Phosphoric Acid Resistance
For phosphoric acid service (common in fertilizer production):
- 316L: Suitable for dilute, pure acid at low temperatures
- 904L: Preferred for concentrations up to 85% at temperatures up to 60°C, particularly in wet-process phosphoric acid containing chloride and fluoride impurities
Chloride Stress Corrosion Cracking (SCC)
Both alloys offer excellent resistance to chloride SCC compared to standard austenitic grades, but:
- 316L: Good resistance up to approximately 50-80 ppm chloride at 60°C
- 904L: Superior resistance due to higher nickel content (23-28% vs 10-14%), effective to significantly higher chloride concentrations and temperatures
Pitting and Crevice Corrosion
The Pitting Resistance Equivalent Number (PREN) provides a quick comparison:
- 316L PREN: ~25 (16%Cr + 3.3×2.5%Mo + 12%Ni ≈ 25)
- 904L PREN: ~36 (21%Cr + 3.3×4.5%Mo + 25%Ni ≈ 36)
With a PREN of 36, 904L offers significantly better resistance to localized corrosion in chloride-containing seawater and brine environments.
Mechanical Properties Comparison
Room Temperature Properties
| Property | 316L | 904L |
|---|---|---|
| Yield Strength (MPa) | ≥170 | ≥220 |
| Tensile Strength (MPa) | ≥485 | ≥490 |
| Elongation (%) | ≥40 | ≥35 |
| Hardness (HB) | ≤190 | ≤190 |
| Density (g/cm³) | 7.99 | 8.00 |
The mechanical properties are similar, with 904L showing slightly higher yield strength due to its greater alloy content. Both alloys maintain excellent ductility and toughness at cryogenic temperatures.
High Temperature Properties
For continuous service:
- 316L: Maximum 870°C (1600°F) for intermittent, 925°C for continuous in non-stressed conditions
- 904L: Similar maximum temperature capability, though the higher alloy content provides marginally better high-temperature oxidation resistance
Application Comparison
Where 316L Is Sufficient
- General chemical processing with mild corrosive environments
- Food and beverage processing equipment
- Pharmaceutical manufacturing
- Marine architecture (handrails, trim) in moderate environments
- Water treatment plants
- Pulp and paper bleaching sections
- Heat exchangers in freshwater service
Where 904L Is Required
- Sulfuric acid handling and processing equipment
- Phosphoric acid evaporators and concentration equipment
- Flue gas desulfurization (FGD) systems in power plants
- Seawater cooling systems with high chloride content
- Acid pickling plants (steel treatment)
- Chemical tankers and process vessels
- Electroplating equipment
- Oil and gas production in sour (H₂S) environments
Cost Analysis
The cost difference between 904L and 316L is substantial:
- Material cost: 904L typically costs 2-3 times more than 316L per kilogram, reflecting the significantly higher nickel, molybdenum, and chromium content
- Fabrication cost: Similar welding and forming characteristics, though 904L may require slightly more careful heat input control
- Lifecycle cost: In aggressive environments, 904L’s longer service life can justify the higher initial cost, particularly where equipment replacement is expensive or disruptive
Fabrication and Welding
Both alloys are readily weldable using standard austenitic stainless steel procedures:
- Welding consumables for 316L: ER316L filler wire, E316L electrodes
- Welding consumables for 904L: AWS A5.14 ERNiCr-3 (Inconel 82/182) or matching 904L filler, E385 electrodes
- Interpass temperature: Maximum 150°C for both alloys
- Heat input: Controlled to minimize sensitization risk
Selection Decision Framework
Choose 316L when:
- Operating environment is mildly corrosive
- Chloride content is below 200 ppm at moderate temperatures
- No significant exposure to reducing acids
- Cost optimization is a primary concern
Choose 904L when:
- Significant sulfuric or phosphoric acid exposure
- High chloride concentrations (>200 ppm) at elevated temperatures
- Risk of pitting or crevice corrosion with 316L
- Previous experience shows 316L insufficient
CoreMetal Supply for Both Grades
CoreMetal Steel stocks both 316L and 904L stainless steel products including plate, sheet, coil, pipe, and fittings. Our 904L inventory is available in plate form (up to 50mm thickness), sheet, and round bar. All products carry full EN 10204 3.1 mill test certificates.
Contact CoreMetal Steel for quotations on 904L and 316L stainless steel products with competitive pricing and reliable delivery to your project site.