Excellent Corrosion Resistance of F51 (UNS S31803)
F51, also known as UNS S31803, is a duplex stainless steel combining the favorable properties of austenitic and ferritic steel. It offers excellent resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking. Its high strength, good weldability, and superior corrosion performance make it ideal for harsh environments like seawater, chemical processing, and offshore platforms.
This article explores the composition, mechanical properties, and applications of F51 to support better material selection for engineering projects.
What is F51 (UNS S31803)?
F51, the ASTM A182 designation for UNS S31803, is a widely used duplex stainless steel that contains approximately equal parts of austenite and ferrite phases. It was developed to provide enhanced strength and better corrosion resistance—especially in chloride-bearing and aggressive media—compared to traditional stainless steels like 304 or 316.
F51 Equivalent Grades
| Standard / System | Equivalent Designation |
| UNS | S31803 |
| ASTM / ASME | A182 F51 (for flanges, fittings), A479 (bars), A790 (pipes) |
| EN / DIN | 1.4462 |
| ISO | X2CrNiMoN22-5-3 |
| Trade Names | SAF 2205, 2205 Duplex |
F51 Chemical Composition
| Element | Content (%) |
| Chromium (Cr) | 21.0 – 23.0 |
| Nickel (Ni) | 4.5 – 6.5 |
| Molybdenum (Mo) | 2.5 – 3.5 |
| Nitrogen (N) | 0.08 – 0.20 |
| Manganese (Mn) | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 |
| Carbon (C) | ≤ 0.030 |
| Phosphorus (P) | ≤ 0.030 |
| Sulfur (S) | ≤ 0.020 |
| Iron (Fe) | Balance |
F51 Mechanical Properties
| Property | Typical Value |
| Tensile Strength | ≥ 620 MPa |
| Yield Strength | ≥ 450 MPa |
| Elongation | ≥ 25% |
| Hardness (Brinell) | ≤ 290 HBW |
| Impact Energy | ≥ 100 J (at room temp) |
F51 Physical Properties
| Property | Value |
| Density | 7.8 g/cm³ |
| Melting Point | ~1420 °C |
| Thermal Conductivity | 19 W/m·K (at 20°C) |
| Electrical Resistivity | ~0.85 µΩ·m |
| Modulus of Elasticity | ~200 GPa |
| Service Temperature Range | -50°C to ~300°C (wet chloride), up to 600°C (dry) |
Applications of F51 (2205 Duplex Stainless Steel)
- Offshore & Marine: Ideal for piping systems, pumps, and structural parts exposed to seawater and high salinity.
- Chemical & Petrochemical: Resistant to organic and inorganic acids, chlorides, and stress corrosion cracking in chemical plants.
- Pulp & Paper: Suitable for bleaching systems and digesters in highly corrosive processing.
- Desalination Systems: Used in evaporators, condensers, and reverse osmosis systems.
- Pressure Vessels and Heat Exchangers: For high-pressure and corrosion-sensitive environments.
Advantages of F51
- High strength: Nearly double the yield strength of 316L stainless steel.
- Excellent corrosion resistance: Superior to austenitic grades in chloride-rich environments.
- Good weldability: Suitable for most welding techniques with minimal risk of hot cracking.
- Resistant to SCC: Excellent stress corrosion resistance in neutral and chloride media.
- Cost-effective: Offers superior performance at a lower cost than high-nickel alloys.
Limitations of F51
- Not suitable for very high temperatures: Microstructural instability can occur above ~300°C.
- Requires careful heat treatment: Improper heat input during welding can lead to phase imbalance.
- Moderate machinability: Harder than 316; may cause increased tool wear during fabrication.
- Limited availability in small-diameter bars or tubes compared to standard stainless steels.
F51 vs 316L vs Super Duplex
| Feature | 316L | F51 (S31803) | S32750 (Super Duplex) |
| PREN (Pitting Index) | ~25 | ~35–38 | ~40–45 |
| Yield Strength | ~200 MPa | ~450 MPa | ~550 MPa |
| Chloride Resistance | Moderate | High | Very High |
| Cost | Low | Moderate | Higher |
FAQ About F51 Duplex Stainless Steel
F51 has much higher strength and better chloride resistance than 316L and is often more cost-effective than 904L in aggressive environments.
Yes, due to its ferritic content, F51 exhibits some magnetic properties.
Yes, with proper procedures and filler materials like ER2209, F51 can be welded while retaining corrosion resistance.
Summary
F51 (UNS S31803) duplex stainless steel delivers outstanding corrosion resistance, strength, and weldability, making it a top choice for challenging industrial applications. Its balance of ferrite and austenite phases offers unique advantages in chloride-bearing and high-stress environments.
MT.BOLTS offers a full range of F51 fasteners, including bolts, nuts, studs, and custom components, manufactured to meet ASTM and international standards. Contact us today for quotes or technical consultation.
