Ferralium® 255 and Alloy 625 are two high-performance alloys engineered for aggressive corrosion, high temperatures and mechanical stress. We stock both. This article sets out their properties, applications and the trade-offs between them.
PREN: a measure of corrosion resistance
The Pitting Resistance Equivalent Number (PREN) rates the resistance of stainless steels and nickel alloys to pitting and crevice corrosion, based on their chromium, molybdenum and nitrogen content. It is a useful comparative measure, though it does not capture every factor - such as the presence of copper or the specific service conditions.
- Ferralium® 255: PREN around 40. This super duplex stainless steel resists pitting and crevice corrosion well, particularly in chloride-rich environments.
- Alloy 625: PREN around 45. This nickel-based superalloy gives superior corrosion resistance, especially in reducing environments and those containing sulphur compounds.
Ferralium® 255: high-strength super duplex stainless steel
Ferralium® 255 is a super duplex stainless steel developed by Langley Alloys in the 1960s. Its combined austenitic and ferritic phases give high strength and excellent corrosion resistance. The composition includes approximately 25.5% chromium, 5.5% nickel, 3.4% molybdenum and 2.0% copper.
Key properties:
- High strength: a minimum yield strength of 586 N/mm², suiting weight-critical and high-stress applications.
- Superior corrosion resistance: high chromium and molybdenum content, plus copper that forms a protective layer at corrosion sites, resist pitting and crevice corrosion.
Typical applications:
- Oil and gas: pumps, valves, wellhead and subsea equipment.
- Marine engineering: propellers, shafts, rudders, seals, pumps and valves.
- Chemical processing: equipment in sulfuric acid, nitric acid, phosphoric acid and PVC production.
- Pollution control: scrubbers, precipitators, fans and pumps.
- Food processing: grain and vegetable processing plants, sugar cane centrifuges.
Alloy 625: nickel-based superalloy for extreme conditions
Alloy 625 (UNS N06625) is a nickel-chromium-molybdenum alloy known for its resistance to oxidation and corrosion across a wide range of aggressive environments. Its composition includes approximately 61% nickel, 21.5% chromium, 9.0% molybdenum and 3.6% niobium, giving high strength and excellent weldability.
Key properties:
- High-temperature strength: retains its mechanical properties at elevated temperatures.
- Outstanding corrosion resistance: high chromium and molybdenum content, with niobium that improves resistance to carburisation and oxidation.
Typical applications:
- Aerospace: turbine blades, seals and exhaust systems.
- Marine engineering: propellers, drive shafts and thrusters.
- Chemical processing: chemical reactors, heat exchangers and pollution control systems.
- Nuclear power: components in nuclear reactors and waste processing systems.
How they compare with other alloys
- Ferralium® 255 vs Alloy 625: both offer excellent corrosion resistance. Ferralium® 255 is a super duplex stainless steel with higher strength, suiting structural applications; Alloy 625 is a nickel-based superalloy with superior high-temperature strength.
- Ferralium® 255 vs Alloy 20: Alloy 20 has good resistance to sulphuric acid, but Ferralium® 255 offers higher strength and better resistance to pitting and crevice corrosion in aggressive environments.
- Alloy 625 vs Hastelloy C-276: both resist corrosion well in harsh environments, but Alloy 625 gives superior resistance to oxidation and carburisation at elevated temperatures.
Summary
Ferralium® 255 (UNS S32550) and Alloy 625 (AMS5666, UNS N06625) are engineered to perform in some of the harshest conditions on earth. Matching the alloy to the demands of each application - oil and gas, marine, chemical processing or aerospace - ensures long, reliable equipment life.
| Property | Ferralium® 255 | Alloy 625 |
|---|---|---|
| Type | Super duplex stainless steel | Nickel-based superalloy |
| PREN | ~40 | ~45 |
| Chromium | ~25.5% | ~21.5% |
| Molybdenum | ~3.4% | ~9.0% |
| Min yield strength | 586 N/mm² | n/a |