6% Molybdenum Super-Austenitic Stainless for Seawater Service
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UNS NUMBER
N08367
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ALLOY FAMILY
Super Austenitic (6Mo)
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STRUCTURE
100% Austenite
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UTS (MIN)
≥655 MPa
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MAX TEMP
~400°C
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DENSITY
8.06 g/cm³
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Stainless Steel AL6XN (UNS N08367) is a 6% molybdenum super-austenitic stainless with nitrogen strengthening, reaching PREN 45–48 and yield strength near 310 MPa. It is the standard upgrade from 316L/904L for seawater, desalination, and FGD where chloride pitting and crevice corrosion are severe.
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Cr
20–22%
Oxidation + pitting
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Ni
23.5–25.5%
Austenite; SCC resistance
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Mo
6–7%
Pitting / crevice (6Mo)
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N
0.18–0.25%
Strength + PREN
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UTS
≥655 MPa
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YS 0.2%
≥310 MPa
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Elong.
≥30%
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Service
-196~400°C
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What is Stainless Steel AL6XN?
AL6XN (Allegheny Ludlum 6Mo, now part of the 6Mo family) is a super-austenitic built around 6–7% molybdenum, 20–22% chromium, and 23.5–25.5% nickel, with 0.18–0.25% nitrogen for solid-solution strengthening. That combination pushes PREN to 45–48 — far above 904L (36–38) — and roughly doubles the yield strength of 316L.
It is the go-to material for seawater cooling, reverse-osmosis desalination, and FGD equipment where 316L and even 904L pit or crevice-corrode. The high nitrogen also makes it stronger than 904L, allowing thinner walls. It is more expensive than 904L but generally far cheaper than a nickel alloy like C-276. For the very highest chloride/acid extremes, 254SMO (similar PREN) is the close alternative.
Chemical Composition (Weight %)
| Element |
Min (%) |
Max (%) |
Role in Alloy |
| Chromium (Cr) |
20.0 |
22.0 |
Oxidation + pitting resistance |
| Nickel (Ni) |
23.5 |
25.5 |
Austenite; SCC resistance |
| Molybdenum (Mo) |
6.0 |
7.0 |
Pitting / crevice (6Mo) |
| Nitrogen (N) |
0.18 |
0.25 |
Strength + PREN boost |
| Copper (Cu) |
— |
0.75 |
Minor |
| Carbon (C) |
— |
0.030 |
Low; no sensitization |
| Manganese (Mn) |
— |
2.00 |
Deoxidizer |
| Silicon (Si) |
— |
1.00 |
Deoxidizer |
| Phosphorus (P) |
— |
0.040 |
Impurity |
| Sulfur (S) |
— |
0.030 |
Impurity |
| Iron (Fe) |
Balance |
— |
Matrix former |
★ 6–7% molybdenum + 0.18–0.25% nitrogen give PREN 45–48 — the defining 6Mo super-austenitic profile. Yield (~310 MPa) is about double 316L. Carbon ≤0.030% avoids sensitization.
Mechanical Properties
Values are for the solution-annealed condition per ASTM B688 (plate), B675/B676 (pipe), B564 (forgings), and B366 (fittings).
| Condition / Form |
UTS (MPa) |
YS 0.2% (MPa) |
Elong. % (min) |
Hardness (max) |
| Minimum (ASTM B688 / SB-688) |
≥655 |
≥310 |
30 |
30.5 HRC (240 HBW) |
| Typical mill-annealed |
690–900 |
310–450 |
35–50 |
95–100 HRB |
Modulus of Elasticity 195 GPa. ASME allowable stresses run up to 40% higher than 316L. Not hardenable; strength from alloy + nitrogen.
Physical Properties
| Property |
Value |
Note |
| Density |
8.06 g/cm³ |
Above 316L |
| Melting Range |
1370–1398°C |
Solidus ~1370°C |
| Thermal Conductivity (100°C) |
~14 W/m·K |
Lower than 316 |
| Thermal Expansion (20–100°C) |
14.2 ×10⁻⁶ /K |
Below standard austenitics |
| Specific Heat |
500 J/kg·K |
At 21°C |
| Electrical Resistivity |
0.89 µΩ·m |
At 21°C |
| Magnetic Property |
Non-magnetic |
Fully austenitic |
| PREN (Pitting Resistance) |
45–48 |
PREN = %Cr + 3.3×%Mo + 16×%N |
Corrosion Resistance
Seawater & Desalination
PREN 45–48 gives AL6XN practical immunity to chloride pitting and crevice corrosion in seawater, RO desalination, and ballast systems — the main reason to select it over 316L/904L.
Stress Corrosion Cracking
High nickel (23.5–25.5%) makes AL6XN highly resistant to chloride stress-corrosion cracking, far better than the 300-series austenitics.
FGD & Chemical
Used in flue-gas-desulfurization and aggressive chemical streams where 316L/904L would fail. Sulfuric/phosphoric resistance is good but below specialized nickel alloys.
Weld Sensitization
Carbon ≤0.030% means AL6XN does not sensitize after welding; post-weld annealing is not required.
Stainless Steel AL6XN vs Similar Grades — Selection Guide
| If you need... |
Choose... |
Why |
| Seawater / desalination |
AL6XN |
PREN 45–48, SCC immune |
| Above 904L chloride need |
AL6XN / 254SMO |
PREN 42–48 |
| General chloride, cheaper |
316L / 904L |
Lower cost |
| Max acid + chloride |
Nickel alloy (C276) |
Higher capability |
| Warm seawater struct. |
AL6XN |
High strength + PREN |
Applicable Standards
| Standard |
Product Form |
| ASTM B688 |
Plate, sheet, strip |
| ASTM B675 |
Seamless pipe |
| ASTM B676 |
Welded pipe |
| ASTM B366 |
Butt-weld fittings (WPN08367) |
| ASTM B564 |
Forgings / flanges |
| ASTM A240 / A312 |
Also covered |
HT PIPE Supply — Stainless Steel AL6XN Product Forms
| Product Form |
Specifications |
HT PIPE Supply |
| Seamless Pipe |
ASTM B675 |
✅ Available |
| Welded Pipe |
ASTM B676 |
✅ Available |
| Forged Flanges |
ASTM B564, ASME B16.5 |
✅ In stock |
| Butt-Weld Fittings |
ASTM B366 WPN08367 |
✅ Available |
| Plate / Sheet |
ASTM B688 |
✅ Available |
| Bar / Rod |
ASTM B691 |
✅ Available |
HT PIPE supplies AL6XN in pipe, flanges, fittings, plate, and bar for seawater, desalination, and FGD service, with full MTRs and matching high-molybdenum welding consumables.
Project Experience — Stainless Steel AL6XN
HT PIPE supplies AL6XN for seawater cooling, reverse-osmosis desalination, and FGD systems worldwide.
| Project |
Product |
Specification |
| SWRO Desalination Train (Middle East) |
AL6XN Pipe + Fittings |
B675, B366 |
| Seawater Cooling System (Asia) |
AL6XN Flanges + Plate |
B564, B688 |
| FGD Scrubber (Europe) |
AL6XN Plate + Forgings |
B688, B564 |
We advise AL6XN vs 254SMO and supply matching 625 (ERNiCrMo-3) or AL6XN filler.
| FREQUENTLY ASKED QUESTIONS |
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FAQ — Stainless Steel AL6XN
| What is AL6XN used for? |
| Seawater, reverse-osmosis desalination, and FGD equipment where chloride pitting and crevice corrosion defeat 316L/904L. PREN 45–48 and high SCC resistance are the drivers. |
| How does AL6XN compare to 904L? |
| AL6XN has 6–7% molybdenum (vs 4–5% in 904L) and nitrogen, raising PREN from 36–38 to 45–48 and roughly doubling yield strength (310 vs 220 MPa). It is the stronger, more chloride-resistant upgrade. |
| Does AL6XN need post-weld heat treatment? |
| No. Carbon ≤0.030% prevents sensitization; welded fabrications stay corrosion-resistant as-welded. |
| What filler is used for AL6XN? |
| ERNiCrMo-3 (Inconel 625 type) or a matching 6Mo wire to preserve the high molybdenum/nitrogen and PREN. Plain 316/904L filler would undermatch badly. |
| AL6XN or 254SMO? |
| Very similar (both 6Mo, PREN 42–48). Choice is often availability and mill form; 254SMO adds copper for some acid services. Either outlasts 904L in aggressive chloride. |
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Need Stainless Steel AL6XN for Seawater Service?
HT PIPE supplies AL6XN (UNS N08367) pipe, flanges, fittings, plate, and bar for seawater, desalination, and FGD applications with full MTR documentation. Contact our technical team for material selection and welding advice.
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