Ferralium 255 (UNS S32550) is a Cu-bearing super duplex stainless steel (Fe-base, austenite-ferrite microstructure, PREN ≥ 40); Inconel 625 (UNS N06625) is a Ni-Cr-Mo nickel-base alloy with Nb stabilisation (fully austenitic, single-phase). Both are corrosion-resistant alloys used in marine, oil & gas, and chemical processing applications, but they sit at different cost-performance tiers and serve overlapping but not identical service envelopes.
| Element | Ferralium 255 (S32550) | Inconel 625 (N06625) |
|---|---|---|
| Iron (Fe) | balance (~63%) | max 5.0% |
| Nickel (Ni) | 4.5–6.5 | min 58.0 |
| Chromium (Cr) | 24.0–27.0 | 20.0–23.0 |
| Molybdenum (Mo) | 2.9–3.9 | 8.0–10.0 |
| Niobium (Nb + Ta) | none | 3.15–4.15 |
| Copper (Cu) | 1.5–2.5 | none |
| Nitrogen (N) | 0.10–0.25 | none |
| Carbon (C), max. | 0.04 | 0.10 |
Ferralium 255 PREN ≈ 40–42 (Fe-base PREN formula). Inconel 625 PREi ≈ 50+ (using the modified Pitting Resistance Equivalent for nickel alloys including Nb). Inconel 625 has substantially higher chloride pitting resistance, particularly at elevated temperatures > 80 °C where super duplex grades approach their service ceiling.
| Property | Ferralium 255 | Inconel 625 (annealed) |
|---|---|---|
| Yield strength (Rp0.2), min | 550 MPa | 415 MPa |
| Tensile strength (Rm), min | 760 MPa | 825 MPa |
| Elongation (A5), min | 15% | 30% |
| Density | 7.81 g/cm³ | 8.44 g/cm³ |
| Magnetic | Slightly magnetic (ferrite phase) | Non-magnetic |
Inconel 625 is approximately 3–5× the material cost of Ferralium 255 due to high Ni content (~62% vs ~5% for Ferralium). For applications within Ferralium 255 service envelope (PREN ≥ 40 sufficient, service temperature < 80 °C, NACE MR0175 sour service), super duplex is the cost-effective choice. Inconel 625 is reserved for applications where Ferralium 255 service limits are exceeded or where the application specifically requires Ni-base alloy properties.
Choose Ferralium 255 for: standard seawater service, oil & gas downhole within NACE MR0175 limits, chemical processing involving reducing acids (Cu advantage), most NORSOK M-650 offshore applications, cost-sensitive corrosion-resistant flange and fastener bolting.
Choose Inconel 625 for: high-temperature corrosive service > 80 °C, the most aggressive sour-gas environments (H2S partial pressure beyond super-duplex limits), high-temperature oxidising environments (jet engines, gas turbines), seawater service at > 50 °C with high chloride concentrations, or where non-magnetic behaviour is critical.
Ferralium 255 specs: ASTM A479 / A789 / A790 / A815 / A182 / A276 (UNS S32550). Inconel 625 specs: ASTM B443 (plate), B446 (bar), B444 (pipe), B704 (welded tube), B705 (welded fittings), B564 (forgings) under UNS N06625. Both grades qualify under NORSOK M-650 (Ferralium per MDS D55, Inconel 625 per MDS R15) and NACE MR0175 / ISO 15156-3 for sour service.
