AL6XN Seamless Pipe

AL6XN Seamless Pipe is the super-austenitic stainless steel that marine and chemical engineers specify when seawater is the medium and 316L is a maintenance problem. The alloy, UNS N08367, carries roughly twice the molybdenum of 316L, a deliberate nitrogen addition, and nickel at 23.5-25.5% - the combination that lifts it out of the ordinary austenitic family and into the class of materials that hold up in warm seawater, chloride brines, and oxidizing-reducing process streams. 

ASTM A312 AL6XN Stainless Steel Pipe Specifications

Specifications	ASTM A312 / ASME SA312
Dimensional specification	ANSI/ASME B36.19M, ANSI/ASME B36.10M
Seamless / Welded Pipe Size	1 / 8″ NB – 24″ NB
EFW Pipe Size	6″ NB – 48″ NB
Outside Diameter	6.00 mm OD up to 914.4 mm OD, Sizes up to 24" NB
Manufacturing Shapes	Hydraulic, Round, Rectangle, Square, Hex, Honed
Manufacturing Techniques	Seamless / Welded / ERW / Fabricated / CDW / 100% X-Ray Welded
Schedule Thickness Range	SCH 5, SCH10, SCH 40, SCH 80, SCH 80S, SCH 160, SCH XXS, SCH XS
Pipes End Types	Plain End, Beveled End, Treaded One End, TBE (Threaded Both Ends)
Equivalent Materials	00Cr20Ni24Mo6N, AL6XN, AL6XN plus(ATI)

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Where AL6XN sits: the 6Mo super-austenitic family

The shorthand "6Mo" names the family: super-austenitic stainless steels with 6% molybdenum, of which AL6XN, 254SMO, and Alloy 926 are the best-known members.

 

What distinguishes AL6XN within the family is its balance - nickel at 24% with molybdenum at 6.3% and nitrogen at 0.22% gives it a fully austenitic, stable structure that resists the sigma-phase problems of higher-alloyed grades, and it is supplied in an annealed condition that keeps the alloy ductile and weldable.

 

The practical result is an alloy that beats 316L by a wide margin in pitting and crevice corrosion, that approaches the nickel-based alloys in seawater performance, and that costs a fraction of the nickel alloys.

 

The numbers that engineers quote for AL6XN are its corrosion indices. The PREN (pitting resistance equivalent) is approximately 47, calculated as Cr + 3.3Mo + 16N - well above the 22-25 of 316L and above most duplex grades - and the critical pitting temperature in the standard chloride test runs far above the austenitic grades.

 

In practice this means AL6XN pipe can carry warm, aerated seawater where 316L pits within months, and it resists the crevice attack under gaskets and deposits that is the usual failure mode of stainless piping in marine service.

 

Chemical composition per ASTM B804

The composition limits below follow ASTM B804, the seamless pipe specification for UNS N08367. Iron is the balance element.

 

Element	Composition limit, %
Cr	20.0 - 22.0
Ni	23.5 - 25.5
Mo	6.0 - 7.0
N	0.18 - 0.25
C	0.03 max
Mn	2.0 max
Si	1.0 max
P	0.04 max
S	0.03 max
Cu	0.75 max
Fe	Balance

 

Three rows set the corrosion performance. The molybdenum row at 6.0-7.0% is the primary defense: it stabilizes the passive film in chloride media and is the main driver of pitting and crevice resistance.

 

The nitrogen row at 0.18-0.25% works with the molybdenum - nitrogen raises the pitting resistance and, in solid solution, adds strength: the minimum yield of 310 MPa is roughly 40% higher than 316L without any loss of ductility.

 

The nickel row at 23.5-25.5% keeps the structure fully austenitic and gives the alloy resistance to chloride stress-corrosion cracking far beyond the ordinary austenitic grades. The low carbon at 0.03% maximum means the alloy does not sensitize during welding, so it can be put into chloride service in the as-welded condition.

 

Mechanical properties per ASTM B804

ASTM B804 requires the following minimum mechanical properties for AL6XN seamless pipe in the annealed condition:

 

Property	Minimum value (ASTM B804)
Tensile strength	655 MPa (95 ksi)
Yield strength, 0.2% offset	310 MPa (45 ksi)
Elongation	30%

 

Typical production values run about 760 MPa tensile and 380 MPa yield with 40% elongation. The nitrogen strengthening is the reason the yield is high for an austenitic grade, which translates directly into thinner pressure walls for the same duty. The elongation is ample for bending and rolling, and the alloy fabricates with standard austenitic shop practice.

 

Physical properties and corrosion indices

Property	Value
Density	8.06 g/cm³
PREN (Cr + 3.3 Mo + 16 N)	Approx. 47
Structure	Fully austenitic, non-magnetic
Modulus of elasticity	195 GPa (approx.)
Service temperature range	Cryogenic to ~540°C continuous

 

Pipe sizes and delivery forms

AL6XN seamless pipe is supplied in the following envelope, with project sizes on request:

 

Parameter	Range
Outside diameter	6.0 mm to 273 mm (larger by enquiry)
Wall thickness	0.5 mm to 25.4 mm, schedules and custom
Length	Fixed 6 m / 12 m, or cut-to-length
End finish	Plain end, beveled, or threaded by enquiry
Delivery condition	Solution annealed and pickled

 

The annealed condition is the standard and the only condition used for chloride service: the low-carbon, fully austenitic structure is what makes the alloy's corrosion resistance dependable, and no further heat treatment is applied by the fabricator. For pressure design, the nitrogen-strengthened yield allows thinner walls than 316L at the same duty, which is part of the alloy's cost equation against the austenitic grades.

 

Typical applications

AL6XN pipe is specified where chloride media and the failure history of lesser grades decide the material:

 

Application	Why AL6XN is used
Seawater piping and intake systems	Pitting and crevice resistance in warm aerated seawater
Offshore platform utilities and firewater	Reliability in chloride service without nickel-alloy cost
Desalination plants (thermal and reverse osmosis)	Resistance to concentrated brine and chlorine dosing
Chemical process piping with chlorides and acids	Broad resistance in mixed oxidizing-reducing media
Heat exchangers and condensers	Crevice performance at tubesheet and gasket locations
Flue gas desulfurization and scrubber piping	Resistance to chloride-rich, acidic condensate

 

In each service the alloy is selected on documented failure data: where 316L pits or SCC-cracks in warm chloride service, AL6XN closes the gap at a cost well below the nickel-based alloys. The alloy is also a standard answer where 6Mo performance is required but 254SMO or 926 sourcing is inconvenient, since the three grades are frequently interchangeable in design.

 

Welding and fabrication

AL6XN is welded with over-alloyed nickel filler, most commonly ERNiCrMo-3 (Alloy 625 type) per AWS A5.14, which produces weld metal with corrosion resistance at least equal to the base material.

 

No preheat is required, and the low-carbon base means the heat-affected zone keeps its corrosion resistance in the as-welded condition. The weld practice follows austenitic discipline: clean surfaces, controlled heat input, and inert gas backing on the root.

 

The alloy cold bends and rolls readily with standard austenitic tooling, and it machines with practices similar to the high-molybdenum stainless grades.

 

What to specify on the order

To keep an AL6XN enquiry clean, state the following:

 

• Grade and standard: "AL6XN seamless pipe, UNS N08367, per ASTM B804"
• Size: OD x wall in millimetres, or NPS + schedule
• Condition: solution annealed and pickled
• Length and end finish: fixed or random lengths, plain end or beveled
• Certificate: EN 10204 3.1 with heat chemistry, tensile results, and the nitrogen analysis reported

 

AL6XN is the 6Mo alloy that marine and chloride-service buyers choose for the middle of the cost-performance table: it is not the cheapest option - 316L is - and it is not the ultimate option - the nickel alloys are - but it is the alloy that holds warm seawater without becoming a maintenance program. The specification decision is a chloride decision: when the medium is seawater, brine, or chloride-laden process water, AL6XN is the alloy to benchmark.

 

 

 

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