AL-6XN Tubing (UNS N08367)

AL-6XN tubing is OD-sized, round instrumentation tube drawn from super-austenitic stainless AL-6XN (UNS N08367), a 6% molybdenum, nitrogen-alloyed grade running roughly 21Cr-24Ni-6.3Mo with about 0.22% nitrogen. It carries process fluid or gas between components and seats into a compression fitting's ferrule set to make a leak-tight, make-and-break joint. Crestflo supplies AL-6XN tube seamless to ASTM B690 and welded to ASTM B676, in both imperial and metric sizes, shipped solution-annealed so it bends and cones cleanly into a ferrule. You reach for AL-6XN tube when a line runs seawater, brine, or high chloride, the service where 316 pits and cracks and even 904L is reaching its limit, because with a PREN around 45 to 47 this is a genuine seawater-grade alloy, and it stays fully austenitic and non-magnetic doing it. Standard items ship from stock; specials are built to order, with delivery running 6-8 weeks. Request a quote to price your run.

Reviewed by the Crestflo Engineering Team

AL-6XN® is a registered trademark of ATI Properties, Inc. Swagelok® and Parker® are trademarks of their respective owners; Crestflo is not affiliated, and interchange is dimensional.

Why Specify AL-6XN Tubing

Tube is the long, continuous wetted surface of an instrument hookup. Unlike a fitting or a valve, it has no dead legs or packing bores, but it has length, every inch of it exposed to the media, and it has to survive the cold work of bending and ferrule make-up without cracking. On a seawater or high-chloride line that rules out most stainless. A tube that pits or stress-corrosion cracks midway along a run is as dead as one that fails at the fitting, and in raw chloride service standard 316 does exactly that.

AL-6XN is a super-austenitic stainless carrying about 6.3% molybdenum, roughly three times the molybdenum of 316L, paired with a deliberate nitrogen addition and a high nickel content near 24%. That chemistry is tuned for one problem above all others: chlorides. The molybdenum and nitrogen together drive pitting and crevice resistance, taking the grade's PREN to about 45 to 47, well clear of roughly 25 for 316L and about 34 for 904L, and that number tracks a high critical pitting temperature, so the alloy resists localized chloride attack at temperatures where the lower-moly grades fail. Its high nickel content and fully austenitic structure underwrite strong resistance to chloride stress-corrosion cracking, the failure mode that most often retires 304 and 316 in warm chloride service. The very low carbon limit (0.030% max) keeps that corrosion resistance intact in the weld and heat-affected zone of a welded tube, rather than letting the weld become the first thing to attack.

Be precise about where AL-6XN sits, because it is easy to mis-position. It is a fully austenitic stainless, not a duplex and not a nickel-base alloy, and despite the "6XN" in the name it is not a 6-series aluminum grade. Its 0.2% yield strength, around 310 MPa (45 ksi) in the solution-annealed condition, runs well above that of 316, a direct result of the nitrogen, but it is still an austenitic grade and not a super-duplex. So note the strength, but do not claim a duplex-style thinner-wall advantage from it: the case for AL-6XN tube is chloride corrosion resistance, not pressure-per-wall. Being fully austenitic buys two real things. It is essentially non-magnetic in the annealed condition (relative permeability near 1.0), which is itself a reason to specify it where magnetic permeability matters, and it keeps its toughness down to cryogenic temperatures with no ductile-to-brittle transition, so it is not ruled out of cold duty. This is a corrosion grade, not a high-temperature creep grade.

What makes AL-6XN the tube to specify is that it delivers true seawater-grade chloride performance, pitting, crevice, and chloride-SCC resistance, at a cost that usually lands below the nickel alloys and titanium that would otherwise be needed in that service. It is the step past 316 and 904L, not a marginal improvement on them, and it is the ASTM-spec 6-moly most often written into US naval, defense, and desalination work.

The full metallurgical picture, composition ranges, mechanical and physical properties, PREN, and corrosion-by-media data, lives on the AL-6XN alloy grade page. This page stays on the tube: how it's made, sized, finished, tested, and matched to the fitting.

Also Specified As: AL-6XN Seamless Tubing, UNS N08367 Tube, 6Mo Instrumentation Tubing

Buyers and specifiers call this tube several ways, and each one points to the same 6% molybdenum super-austenitic. A requisition might read "AL-6XN seamless tubing," "AL6XN tube," "6XN tube," "UNS N08367 tube," "N08367 instrument tube," or "6Mo tubing," and every one of them lands here. AL-6XN, AL6XN, AL 6XN, 6XN, 6Mo, and 6-moly stainless are the common forms; the material is registered as UNS N08367. There is no exact European Werkstoff match; the nearest 6Mo grade, EN 1.4529 (UNS N08926), is related but distinct, and should not be treated as the same alloy. The welded form is often written "welded-and-drawn tube," the seamless form "cold-drawn seamless tube." UNS N08367 is the trademark-free number that anchors the grade, and a tube labeled "AL-6XN" is the same super-austenitic stainless.

Scope: this is round, OD-sized instrumentation and fluid-system tube for compression and tube-fitting connections. It is not oilfield coiled tubing for downhole well intervention, not structural or ornamental tube, and not pipe specified by nominal pipe size (NPS) and schedule. Two cautions on naming: despite the "6XN," AL-6XN is a stainless steel, not a 6-series aluminum such as 6061, and it is not the same alloy as 254 SMO (UNS S31254), the sibling 6-moly super-austenitic of Outokumpu origin, close in service but a distinct ASTM-registered grade. Crestflo supplies AL-6XN in instrumentation tube, fitting, and valve forms only, not mill bar, sheet, plate, or pipe.

Seamless vs Welded AL-6XN Tubing

Both constructions are built to their own high-alloy tube specification, and both accept a compression fitting the same way. Seamless AL-6XN tube is cold-drawn from a hollow with no longitudinal weld, which makes it the default where you want maximum integrity for higher-pressure, cyclic, or critical service. Welded-and-drawn AL-6XN tube is formed from strip, seam-welded, then cold-drawn to size; it is dimensionally consistent and cost-effective for general instrument and fluid-system work. The grade's very low carbon limit is what keeps corrosion resistance intact in the weld and the heat-affected zone, so welded AL-6XN holds up in chloride service that would attack a leaner austenitic at the seam. Because AL-6XN is fully austenitic rather than duplex, there is no ferrite-balance check to run on the weld, which is one fewer acceptance step than a 2205 or super-duplex 2507 tube carries.

Not sure which construction to call out? The deep dives on seamless tubing and welded tubing walk through the trade-off before you commit a spec.

AL-6XN Tube Attributes at a Glance

AL-6XN Tube Sizes: OD, Wall & Imperial/Metric

Instrument tube is specified by its actual outside diameter and wall thickness, given in both imperial and metric units, never by NPS and schedule. The common instrument range runs fractional 1/8″ to 1″ and metric 6 to 12 mm; smaller 1/16″ small-bore tube and larger diameters up to 2″ (50 mm) are available to order. Wall thickness is chosen to carry the pressure and to suit the ferrule, and both OD and wall are held to the tolerances of ASTM B690 (seamless) or B676 (welded). The higher yield of AL-6XN is real, but it is still an austenitic grade, so for tube sizing treat it like one: size the wall to the same pressure math you would run for any austenitic stainless rather than claiming a duplex-style thinner-wall credit. Working pressure is not a single blanket figure: it is calculated from OD, wall thickness, and the alloy's allowable stress per ASME B31.3, so it is stated per size and wall rather than as one rating for the grade.

Values per applicable standard; customizable to requirement. For how to work back from a target pressure to a wall thickness, see the tube sizing and wall thickness guide, and for the full dimensional matrix across the range, the tubing size chart.

Temper & Hardness: Solution-Annealed for Bending and Ferrule Seating

The single most important mechanical property of instrument tube is not its peak strength but its workability. AL-6XN is a solid-solution super-austenitic that does not age-harden, so it comes in one working condition, solution-annealed, and that condition is what lets the tube bend around a radius and cone down under a ferrule without cracking, so the joint seats and seals the way a compression fitting is designed to. In the solution-annealed condition AL-6XN carries a tensile minimum around 100 ksi (690 MPa) with a 0.2% yield near 45 ksi (310 MPa) and elongation above 30%, with hardness in the annealed band (typically ≤ ~100 HRB). That yield is well above 316's, but it is the ductility, not the strength, that matters for making up the joint. Being fully austenitic, AL-6XN has no ferrite phase to balance and no ductile-to-brittle transition, so the same temper that makes it workable also keeps it tough from cryogenic temperatures up through its corrosion service range. The anneal matters for one more reason on a 6-moly: after heavy cold or hot work, the solution anneal is what restores corrosion resistance to its best level. Specify the hardness or condition to your own standard on the RFQ and it is confirmed on the certificate.

Finishes & Cleanliness: Bright-Anneal, Electropolish & Oxygen-Clean

AL-6XN tube ships in a bright-annealed or pickled finish as standard. For high-purity gas, analytical, and oxygen service, passivation, electropolishing (EP), and oxygen/UHP cleaning with capped-and-bagged packaging are available. Electropolishing lowers surface roughness and smooths the inner diameter, which matters where particle generation, moisture dry-down, or deposit build-up is the concern. Standard UHP electropolish surface-roughness targets can be specified right on the RFQ to your own standard; electropolished tubing and oxygen and UHP cleaning cover the options in detail. Finish and cleanliness figures follow the applicable standard and are customizable to requirement.

AL-6XN Tube Standards: ASTM B690, B676 & ASME B31.3

AL-6XN tube is covered by the nickel-alloy and high-alloy tube specifications in the ASTM B-series, and the construction decides which applies: seamless falls under ASTM B690 (seamless pipe and tube), welded under ASTM B676 (welded tube), each with its matching ASME SB- designation. Because N08367 carries a UNS N08- number, its product-form specs sit in the B-series rather than the A-series austenitic-stainless tube specs used for iron-based grades. These are the same tube specifications the AL-6XN grade page lists for the alloy. Allowable working pressure for a given size and wall is calculated per ASME B31.3. Where sour service applies, the material can be supplied to NACE MR0175 / ISO 15156 as the application qualifies. Crestflo holds the standard approvals that apply to instrumentation tube and is ISO 9001 / 14001 / 45001 and PED certified.

The full family of AL-6XN product-form specifications, including welded pipe (B675), bar and wire (B691), and plate, sheet and strip (B688), is set out on the AL-6XN grade page; for tube, B690 and B676 are the two that apply.

Working Pressure: Calculated per ASME B31.3, Not Assumed

There is no single pressure rating for "AL-6XN tube," and any supplier who quotes one is skipping the engineering. Allowable internal pressure depends on three things: the tube OD, its wall thickness, and the alloy's allowable stress at the service temperature. ASME B31.3 sets the method, relating wall thickness to pressure, diameter, the allowable stress value S, the weld-joint quality factor E, and the coefficient Y. Seamless tube carries a higher joint factor than welded, which is part of why seamless is chosen for higher-pressure work. AL-6XN's nitrogen gives it a higher allowable stress than a standard austenitic, which is a genuine advantage, but it is a corrosion grade first: do not treat it as a super-duplex, and derate it with temperature the way an austenitic derates. The thin-wall instrument sizes carry ample pressure for analyzer, sample, injection, and hydraulic-control duty. Give us the OD, wall, temperature, and medium on the RFQ and the engineering team returns the calculated working pressure for your configuration. Values per the applicable standard; customizable to requirement.

Testing, NDT & Material Verification

Every length of AL-6XN tube is verified before it ships. ASTM B690 and B676 require each tube to pass either a hydrostatic test or a nondestructive (eddy-current) electric test, and dimensional checks confirm OD, wall, and straightness against the ordered tolerances. Positive material identification (PMI) confirms the alloy on the material, which matters more on a 6-moly super-austenitic than on almost any other instrument tube: the molybdenum, nitrogen, and nickel content is exactly what buys the chloride performance, and an under-alloyed tube supplied as 316 or a leaner grade looks identical but pits in service. On a long seawater run that is an expensive thing to discover in commissioning, so the grade is verified, not assumed. NDT is done in-house; destructive testing, where a job calls for it, goes to ISO/IEC 17025-accredited labs, and third-party inspection is available on request.

For sour service, AL-6XN tube material can be supplied to NACE MR0175 / ISO 15156 where the application qualifies. Confirm the hardness and material-condition limits in the standard against your application, H₂S partial pressure, temperature, and chloride content, and note the sour-service requirement on the RFQ.

Straight Lengths & Coiled AL-6XN Tube

AL-6XN tube ships as straight lengths for racked and rigidly supported runs, and as continuous coils for heat-tracing, long analyzer and sample lines, and chemical-injection and hydraulic bundles where a joint-free run is the whole point. Coiled tube keeps the count of mechanical joints down, which on a seawater, brine, or chloride line is one fewer place to leak, and AL-6XN's chloride and chloride-SCC resistance makes it a natural choice for the subsea umbilical cores, sample lines, and injection runs that live in raw seawater. Both forms are supplied solution-annealed; call out length or coil length on the RFQ. For long-run and tracing applications, see coiled tubing; for thicker-wall high-pressure control lines, hydraulic tubing.

Material Traceability & Certification

Every length of AL-6XN tube carries its own heat/lot number, and each order ships with an EN 10204 MTC 3.1 certificate as standard, 3.2 available on request. PMI confirms the grade at the part level, every time. That documentation backs the audit trails that offshore, naval, desalination, chemical, and sour-service jobs demand, the kind of work where a mis-specified or under-alloyed tube in a long run is not a small problem. On a 6-moly specifically, under-alloying is the governing risk, so the heat number, the certificate, and the PMI result together are how a seawater-grade tube is never taken on trust. Full detail is on the traceability page.

Applications for AL-6XN Tubing

AL-6XN tube earns its place wherever chlorides outrun what 316 or even 904L can handle, the seawater-grade duty where pitting, crevice attack, and chloride-SCC are the governing failure modes:

• Offshore and subsea instrument hook-ups, platform and subsea instrument lines, hydraulic control lines, and chemical-injection cores exposed to seawater, sea spray, and chloride-bearing produced fluids. See offshore and seawater service.
• Naval, marine and shipbuilding, shipboard seawater and condenser systems, where AL-6XN is a long-established specified grade, plus coastal and marine plant instrumentation. See marine and shipbuilding.
• Desalination and seawater cooling, SWRO, MSF, and MED plant instrumentation and seawater cooling-water lines, where raw chloride service defeats standard stainless; this is the alloy's original home ground.
• Flue-gas desulfurization (FGD) and pollution control, scrubber and absorber instrument lines in wet, chloride-laden acidic condensate.
• Pulp and paper bleaching, chemical and pharmaceutical, chlorine-dioxide and bleach-plant process instrument lines, among the most aggressive chloride environments in process industry, plus high-purity process lines that demand chloride resistance.
• Non-magnetic and cryogenic requirements, lines where low magnetic permeability or low-temperature toughness governs, both of which the fully austenitic structure provides.

Where the service steps past AL-6XN, or across to a different alloy family, see the next section.

Where AL-6XN Fits: 254 SMO, Super-Duplex 2507, 904L & Nickel Alloys

AL-6XN sits near the top of the stainless range, a clear step above 316 and 904L, but it is not the only answer in high-chloride service, so matching it to the duty matters. Its nearest neighbor is 254 SMO (UNS S31254), the other common 6-moly super-austenitic: the two are near-twins in seawater and chloride service, both non-magnetic and both seawater-grade, with AL-6XN running slightly higher molybdenum and nitrogen so its PREN (~45-47) edges just above 254 SMO's (~43). The practical distinction is lineage, AL-6XN is the ASTM-spec grade frequently written into US, naval/defense, and desalination work, while 254 SMO is the Outokumpu-origin equivalent more common in European specifications; in service they are interchangeable on corrosion grounds. Where the chloride resistance has to come with far higher strength, thin-wall, high-pressure runs where yield governs, super-duplex 2507 matches AL-6XN on PREN but roughly doubles the yield, at the cost of being ferromagnetic; it is the better pick when strength-per-wall is the driver. Where the stream turns to wet chlorine or hot contaminated acid, a nickel alloy such as Hastelloy C-276 steps up. And where the duty is lighter, reducing acids or chloride beyond 316 but short of seawater-grade, step down to 904L rather than pay for a 6-moly. For the head-to-head that drives most of these decisions, see 254 SMO vs super-duplex vs AL-6XN. Match the alloy to the media and don't over-specify.

Pairing AL-6XN Tube With AL-6XN Fittings and Valves

A tube is one part of the hookup, and the joint is only as corrosion-resistant as its weakest metal. For a matched instrument run, pair this tube with AL-6XN tube fittings and AL-6XN instrumentation valves so the whole assembly, tube, ferrules, and valve body, carries the same seawater-grade resistance and the same traceability. Crestflo supplies the tube alongside compression fittings tested to ASTM F1387, so the ferrule set seats correctly on the exact OD and wall you ordered, and the joint is a dimensional match rather than a mix of two suppliers' tolerances. Putting an AL-6XN tube into a 316 fitting just moves the failure point to the ferrule, which on a seawater line is the opposite of what the alloy upgrade was for. Because AL-6XN is a tough, high-nickel super-austenitic that work-hardens and can gall, anti-galling measures such as silver plating on the ferrule are offered on request where repeated make-up or high torque is expected.

Drop-In Interchange and the Tube-Plus-Fitting System

Crestflo AL-6XN tube is cut and finished to standard OD and wall, so it drops into the compression fittings you already run, and the matching AL-6XN fittings are built as dimensional interchange / equivalent parts for the major brands, with the fitting ends qualified to ASTM F1387. These are not "compatible" or "intermix" parts. Swagelok® and Parker® remain trademarks of their respective owners, and Crestflo is not affiliated; the interchange is strictly dimensional. Moving a run off a major brand? Request your Swagelok®/Parker® cross-reference and the engineering team maps your tube and fitting part numbers to Crestflo AL-6XN equivalents. We don't publish a part-number table.

Origin & US-Serving Positioning

Crestflo manufactures for the US market. Standard product is of Indian origin; US or European melt is available on request, with full material traceability. Tube is engineered as a US drop-in and ships in 6-8 weeks, well under the 20+ weeks typical of the majors. This matters more than usual on AL-6XN: it is the 6-moly most often written into US naval and defense seawater specifications, and those jobs frequently carry domestic-melt or TAA/DFARS clauses. If a project calls for a specific melt origin or TAA/DFARS handling, specify it on the RFQ and we'll address it straight, no hedging. See the US-origin compliance page for how we document it.

Why Crestflo for AL-6XN Tubing

Crestflo operates as the instrumentation division of an export house with four decades behind it and formal recognition from the Government of India, with a proven Middle-East delivery record. That backing grounds a focused instrumentation brand built on a handful of clear advantages:

• Lead time of 6-8 weeks versus 20+ weeks typical of the majors, on standard stock and made-to-order specials alike.
• A matched system, tube plus F1387-tested fittings and ferrules that seat on the exact OD and wall you ordered.
• Both seamless and welded AL-6XN tube, in both imperial and metric, straight or coiled.
• Heat-number marking plus MTC 3.1/3.2 on every order, PMI-confirmed, so a 6-moly super-austenitic is never taken on trust, the specific guard against under-alloying.
• NACE MR0175 / ISO 15156 material options where the sour-service application qualifies.
• ISO 9001 / 14001 / 45001 & PED certified, cost-competitive after US duties and tariffs, with small MOQs and private-label available.

We Engineer Confidence.

Request a Quote for AL-6XN Tubing

Give us the details: seamless or welded, OD and wall, straight length or coil, finish and cleanliness, and the line conditions (media, pressure, temperature, sour-service requirement). We'll calculate the working pressure, confirm the spec, and schedule the run.

• Request a quote, the fastest path to pricing and lead time.
• Request a sample for qualification.
• Request your Swagelok®/Parker® cross-reference and we'll map your tube and fitting part numbers.

Datasheets and CAD are available to customers on request. Reach Crestflo's US sales desk at sales@crestflousa.com or +1 346 594 9005 (Houston, TX). Round out the system with AL-6XN tube fittings and AL-6XN valves, browse the tubing pillar for other grades and constructions, and see where this tube serves in offshore and seawater service.

Frequently Asked Questions

Related Pages

• AL-6XN alloy grade & corrosion data
• AL-6XN tube fittings · AL-6XN instrumentation valves
• Tubing pillar · Seamless tubing · Welded tubing · Coiled tubing
• 254 SMO vs super-duplex vs AL-6XN · 254 SMO tubing · Super Duplex 2507 tubing · 904L
• ASME B31.3 · NACE MR0175 · EN 10204 MTC · Offshore & seawater · Marine & shipbuilding