Alloy 926
Alloy 926, also referred to as UNS N08926, EN 1.4529, or Incoloy 25-6MO, is a 6% molybdenum super-austenitic stainless steel. The defining characteristic of this alloy is its Pitting Resistance Equivalent Number (PREN) of approximately 46, achieved through controlled additions of chromium (20-21%), molybdenum (6-7%), and nitrogen (0.15-0.25%). That combination gives it genuine resistance to chloride pitting and crevice corrosion in seawater and other aggressive chloride environments. We supply Alloy 926 seamless pipe to customers in offshore, desalination, and flue gas desulphurisation (FGD) applications.
1. Chemical Composition
The table below shows the chemical composition of Alloy 926 per ASTM B677 and ASME SB-677.
| Grade | Carbon (C) | Manganese (Mn) | Phosphorus (P) | Sulphur (S) | Silicon (Si) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Copper (Cu) | Nitrogen (N) | Iron (Fe) |
| Alloy 926 | 0.020 max | 1.00 max | 0.030 max | 0.010 max | 0.50 max | 19.0 -- 21.0 | 24.0 -- 26.0 | 6.0 -- 7.0 | 0.50 -- 1.50 | 0.15 -- 0.25 | Balance |
2. Mechanical Properties
Alloy 926 seamless pipe in the solution-annealed and quenched condition meets the following minimum mechanical property requirements.
| Property | Condition | Tensile Strength (Min) | Yield Strength 0.2% Offset (Min) | Elongation in 50 mm (Min) | Hardness (Max) | PREN (calculated) | Service Temperature Range |
| Value | Solution Annealed + Quenched | 650 MPa / 94 ksi | 295 MPa / 43 ksi | 35% | 223 HBW | ~46 | -196 °C to +300 °C (cryogenic to moderate high-temp) |
3. Equivalent Grade
The table below shows equivalent designations for Alloy 926 across major international standards.
| Grade | UNS | GB (China) | JIS (Japan) | DIN/EN (W.Nr.) | GOST (Russia) |
| Alloy 926 / Incoloy 25-6MO | N08926 | 015Cr21Ni26Mo6Cu2N | -- | 1.4529 | -- |
| 254 SMO (Outokumpu) | S31254 | -- | -- | 1.4547 | -- |
3. Key Technical Advantages
Alloy 926 has several specific performance characteristics that make it worth choosing over less expensive austenitic grades or nickel alloys in the right conditions.
- Seawater Pitting Resistance at PREN ~46: Alloy 926’s chromium, molybdenum, and nitrogen mix gives a PREN of about 46, well above the 40 needed for seawater pitting resistance. This outperforms 316L (PREN ~24) without needing a fully nickel alloy.
- Crevice Corrosion Resistance in Chloride Environments: In seawater systems, tight gaps can cause low-oxygen spots where crevice corrosion starts. Alloy 926’s high molybdenum and nitrogen protect its passive layer, so it resists this damage, unlike 316L and 317L, which can fail quickly.
- Cost Position Between 316L and Solid Nickel Alloys: Alloy 926 costs about half to 60% of solid nickel alloys like C276 or C22. For many offshore and desalination systems, it offers enough corrosion resistance with major overall cost savings.
- Good Mechanical Strength with High Ductility: With a minimum tensile strength of 650 MPa and elongation of 35%, the alloy has enough strength for pressure-containing applications and retains ductility for fabrication. The nitrogen addition provides solid-solution strengthening without reducing weldability.
- Resistance to Stress Corrosion Cracking (SCC): Stainless steels with under 25% nickel can crack in hot chloride environments. Alloy 926, with 24–26% nickel, avoids this problem and works reliably in warm seawater and similar process conditions.
4. Common Manufacturing Standards
Alloy 926 seamless pipe is manufactured and tested in accordance with the following standards.
- ASTM B677: Primary standard -- seamless pipe and tube in UNS N08926 and related alloys
- ASME SB-677: ASME pressure vessel and piping code equivalent of ASTM B677
- DIN 17458 / EN 10216-5: European seamless pipe standard for austenitic stainless and nickel alloys
- GB/T 14976: Chinese national standard for seamless stainless steel pipe
- ASTM A262: Intergranular corrosion testing (sensitisation evaluation)
- ASTM G48: Pitting and crevice corrosion testing -- used to verify PREN performance
- NACE MR0175 / ISO 15156: Material qualification for sour service in oil and gas environments
5. Primary Applications
The Stainless Steel 304H are used in the following high-performing industries.
- Offshore Oil and Gas: Seawater injection systems handle unfiltered seawater at various temperatures. Alloy 926 is chosen for headers and distribution pipes because 316L often suffers from early pitting failures here.
- Desalination Plants: In MSF and RO desalination plants, brine circuits are very corrosive. Alloy 926 piping is used in brine heaters and high-pressure feed lines with hot, concentrated chlorides.
- Flue Gas Desulphurisation (FGD) Systems: FGD scrubber systems in power plants handle wet sulphuric acid environments alongside chlorides. Alloy 926 is used for the absorber inlet ducting, slurry piping, and spray nozzle headers where duplex stainless steels are often marginal.
- Chemical Processing: In chlor-alkali plants, bleach production, and HCl handling, Alloy 926 is used for shell-and-tube heat exchanger shells and process piping where the combination of chlorides and oxidising conditions would attack lower grades.
- Marine and Coastal Infrastructure: Ballast water treatment systems handle full seawater at variable temperatures. Alloy 926 is a commonly specified material for the treatment module pipework and the overboard discharge lines on offshore vessels and FPSOs.