AL-6XN alloy is identified by the Unified Numbering System (UNS) designation of N08367. It is an austenitic stainless steel alloy with high nickel (24%), chromium (22%), molybdenum (6%), and nitrogen (0.18%) content and is commonly known as super austenitic stainless steel (alloys such as 254 SMO and 904L also fall under the super austenitic designation). AL-6XN alloy has a face-centered cubic crystal structure similar to other austenitic stainless steels such as 304L and 316L. The alloy is non-magnetic, and its magnetic permeability remains low even after severe cold forming.
AL-6XN alloy is commonly used because of its excellent mechanical, high-temperature, and corrosion properties.
There are many advantages to utilizing the superior corrosion resistance of AL-6XN alloy.
AL-6XN offers many benefits like cost effectiveness, product range, commercial availability, and fabrication capability.
AL-6XN alloy is available in a wide range of product forms complying to specifications set by ASME and ASTM.
Answers to frequently asked questions about installation, appropriate applications, and expected life for AL-6XN alloy.
AL-6XN is ideal for ketchup, tomato paste, vinegar-based products, sports drinks, hot water systems, shampoos, toothpaste, and cleaning supplies.
Learn more about alloys by watching these videos
All AL-6XN alloy tubing and fittings supplied by CSI will meet CSI's standards for surface finish and dimensions.
| Element | UNS N08367 |
|---|---|
| Carbon (C) | 0.03 maximum |
| Manganese (Mn) | 2.00 maximum |
| Phosphorus (P) | 0.040 maximum |
| Sulfur (S) | 0.030 maximum |
| Silicon (Si) | 1.00 maximum |
| Chromium (Cr) | 20.00 – 22.00 |
| Nickel (Ni) | 23.50 – 25.50 |
| Molybdenum (Mo) | 6.00 – 7.00 |
| Nitrogen (Ni) | 0.18 – 0.25 |
| Copper (Cu) | 0.75 maximum |
| Iron (Fe) | Balance |
| Super Austenitic | Duplex | Traditional | |||||
|---|---|---|---|---|---|---|---|
| AL-6XN
(UNS N08367) | 254 SMO
(UNS S31254) | 904L
(UNS N08904) | 2507
(UNS S32750) | 2205
(UNS S32205) | 1.4435
(UNS S31603) | 316L
(UNS S31603) | |
| % Carbon | 0.030 max. | 0.020 | 0.020 | 0.030 | 0.03 max | 0.030 max | 0.030 |
| % Chromium | 20.0-22.0 | 19.5-20.5 | 19.0-23.0 | 24.0-26.0 | 22.0-23.0 | 17.0 – 19.0 | 16.0 – 18.0 |
| % Nickel | 23.5-25.5 | 17.5-18.5 | 23.0-28.0 | 6.0-8.0 | 4.5-6.5 | 12.5-15.0 | 10.0 – 14.0 |
| % Molybdenum | 6.0-7.0 | 6.0-6.5 | 4.0- 5.0 | 3.0-5.0 | 3.0-3.5 | 2.5 – 3.0 | 2.0 – 3.0 |
| % Nitrogen | 0.18-0.25 | 0.18-0.22 | 0.15 | 0.24-0.32 | 0.14 | 0.11 | 0.10 |
| % Iron (Fe) | 48 Balance | 54 Balance | 47 Balance | Balance | Balance | Balance | Balance |
| % Manganese | 2.0 max. | 1.0 | 2.0 | 2.00 | 2.0 max | 2.0 max | 2.0 |
| % Phosphorous | 0.040 max. | 0.030 | 0.045 | 0.030 | 0.030 | 0.045 max | 0.045 |
| % Sulfur | 0.030 max. | 0.010 | 0.035 | 0.02 | 0.02 | 0.025 | 0.030 |
| % Silicon | 1.00 max. | 0.80 | 1.0 | 1.00 | 1.00 max | 1.00 max | 0.75 |
| % Copper | 0.75 max. | 0.5-1.0 | 1.0-2.0 | 0.5 | – | - | – |
Due to the stable, face-centered cubic crystal structure, commonly referred to as austenitic structure, AL-6XN alloy can be easily used in fabrication. AL-6XN alloy is a good alternative to titanium in condenser tubing applications, as a result of the excellent physical properties listed below.
| Property | Value |
|---|---|
| Density | 0.291lb/in3 8.06 g/cm3 |
| Modulus of Elasticity | 28.3 x 106 psi 195 GPa |
| Melting Range | 2410 – 2550°F 1320 – 1400°C |
| Thermal Conductivity 68 – 212°F 20 – 100°C | 6.8 Btu/hr • ft • °F 11.8 W/mK |
| Coefficient of Expansion 68 – 212°F 20 – 100°C | 8.5 • 10-6/°F 15.3 • 10-6/°C |
| Specific Heat Capacity | 0.11 Btu/lb • °F 500 J/kg • K |
| Electrical Resistivity | 535 Ohm • circ mil/ft 0.89 µohm • cm |
| Magnetic Permeability Fully annealed 0.5" plate 65% cold-worked plate | 1.0028 Oersted 1.0028 (µ at 200H) |
| Scaling Temperature | 1885°F 1030°C |
AL-6XN alloy is easily welded using similar weld techniques and equipment as 316L stainless steel, including travel speed (IPM) and weld current. Use weld insert rings for additional alloying when orbital or manual welding in the field. The insert ring alloy must have higher molybdenum content than the AL-6XN alloy to compensate for alloy dilution on cooling. Typically Hastelloy® C-22® (13% Mo) is used, but if C-22 is not available, Alloy 625 (9% Mo) or Alloy C-276 (15% Mo) may be substituted. When using weld ring inserts, simply place the weld ring between the two sections to be welded and fusion weld as usual. The weld current must be increased slightly to compensate for the increased thickness of material contributed by the insert ring.
Use inert gas for both the weld cover and backing. Either helium or argon may be used, although argon is more commonly used. Never use filler wire in place of weld rings for sanitary tubing; welding techniques that apply filler wire to the weld face are not recommended, due to the possibility of insufficient alloying in the weld root.
Consumable weld rings made from Hastelloy® C‑22® (or another alloy containing more Molybdenum than AL‑6XN) are used to over-alloy each weld. This maintains the high corrosion resistance you expect from AL‑6XN.
AL-6XN weld appearance can be somewhat misleading when compared to hygienic welds in 316L stainless steel. A typical AL-6XN weld will have non-uniform freeze lines and slag islands in the weld bead. The slag islands are dull or blue-gray in color and adhere to the surface. Light and dark spots on both the inside and the outside of the weld are also common. The heat-affected zone can also have discoloration and is generally a little darker than typical 316L stainless steel welds.
Alloys that have more than 3% molybdenum typically require over alloying, because iron-nickle-chromium-molybdenum (Fe-Ni-Cr-Mo) alloys have a tendency to deplete molybdenum when welded. Typically, 2/3rd of the molybdenum content of the base metal gets reduced when welded.
316L has molybdenum in the 2-3% range, so 2/3rd of the molybdenum is reduced to approximately 2%, which is a minimal difference. With 316L and similar alloys with 3% or less molybdenum, corrosion resistance at the weld is minimally affected.
However, in superaustenitics, duplex alloys, or any other alloys that have more than 3% molybdenum, having a consumable weld insert will restore the corrosion resistance at the weld. Alloys such as AL-6XN®, 254SMO, 904L, 2507, 2205 have more than 3% molybdenum on average, so overalloying the weld, with at least 9% molybdenum, will keep the corrosion resistance at the weld.
Excellent results have been achieved in milling, turning, and drilling AL‑6XN alloy when appropriate tools are used. Since AL‑6XN alloy has a hardness of approximately 88 RB, selection of the proper tools and machine set-ups is the key to achieve adequate machinability. The high strength and work hardening capability of AL‑6XN alloy require more power to cut.
Processes with chloride-induced corrosion in the forms of pitting, crevice, and stress corrosion cracking in 300 series stainless steels are often better managed with AL‑6XN alloy. It is also resistant to corrosion from various acids and salt solutions.
Products such as ketchup, tomato paste, vinegar based products, sports drinks, hot water systems, shampoos, toothpaste, and cleaning supplies contain a high percentage of chlorides. Other times, clean-in-place (CIP) chemicals can contribute to corrosion. AL-6XN is an excellent choice in these cases.
| Alloy | CPT*
(°C) | CPT*
(°F) | CCCT**
(°C) | CCCT**
(°F) |
|---|---|---|---|---|
| AL-6XN | 70 | 158 | 45 | 113 |
| 2507 | 67.8 | 154 | 43.9 | 111 |
| 254 SMO | 60 | 140 | 40 | 104 |
| 904L | 54.4 | 130 | 20 | 68 |
| 2205 | 45 | 113 | 20 | 68 |
| 316L | 20 | 68 | <2 | <35 |
These values are based on basic raw material testing. Values may change based on the product forms.
*Critical Pitting Temperatures (CPT) is the lowest temperature at which pitting corrosion begins in a 24-hour immersion.
**Critical Crevice Corrosion Temperatures (CCCT) is the lowest temperature at which crevice corrosion begins in a 100-hour immersion.
The Pitting Resistance Equivalent Number (PREN) is a quantitative way of measuring corrosion resistance. The higher the number, the better the corrosion resistance. PREN = % Cr + 3.3% Mo + 30% N
Corrosion is an expensive problem and can cost billions. Selecting the right materials for process systems is the most efficient and economical means for controlling corrosion and adding life to a piping system.
AL-6XN has a high return on investment compared to 300 series stainless steels. Considering factors such as downtime, product loss, and replacements, AL-6XN can provide long-term savings. Larger minimum quantity requirements with limitations in product forms can make duplex grades such as 2205, 2507, and the other 6 moly grades such as 254 SMO and 904L more expensive. AL-6XN is significantly less expensive than most of the titanium and nickel based alloys.
It is also a cost effective substitute to non-metallic materials such as PVDF, PTFE, TPE, or PVC. These non-metallics could be difficult to install and maintain.
AL-6XN alloy has a higher up-front cost than 300 series stainless steels. However, life-cycle costs for systems utilizing AL-6XN alloy can be far less than the comparable costs of the initial installation, maintenance, and subsequent replacement of lesser alloys used in aggressive environments.
While quoting a project, it is important to consider the form and type of product. Comparing price per pound of a round bar is not the same as comparing price per foot with a tube.
In addition to material replacement, labor costs, and production downtime caused by system failures in corrosive environments, consideration must also be given to the associated costs of product contamination caused by corrosion. Studies have shown the initial cost comparison of AL-6XN raw material to other alloys is as follows:
Field welding can be easily achieved when suitable over-matched filler rings are used and the material has been properly cleaned and prepared for welding. Low carbon and high nitrogen content minimizes the precipitation of carbides and secondary phases that can occur during welding.
Manufacturers in the personal care industry often use aggressive chemicals in the production of deodorants, shampoos, body washes, toothpastes, detergents, etc. Most of these products use chlorides (salt) as the main ingredient. While these chemicals are good for the end products, they sometimes damage the material that is used for making them.
After multiple failures and replacements, a well-known deodorant manufacturer chose to save money and extend the life of their system by replacing their 316L piping with AL-6XN alloy.
The American Society for Mechanical Engineers (ASME) and American Society for Testing and Materials (ASTM) specifications for the wide range of AL-6XN alloy (UNS N08367) product forms are listed below.
| Product | ASME | ASTM |
|---|---|---|
| Plate, Sheet, and Strip | SB-688 SA-240 | A-240 B-688 |
| Rod, Bar, and Wire | SB-691 | B-691 |
| Welded Pipe | SA-358 SA-409 | A-358, B-804 A-409, B-675 B-691 |
| Sanitary Welded Tubing | A-270 | |
| Welded Tubing | SA-249 SB-676 SA-269 | A-249 B-676 A-269 |
| Billets and Bars for Reforging | B-472 | |
| Forged Pipe Flanges, Fittings, and Valves | B-462 | |
| Wrought Nickel Alloy Welded Fittings | SB-366 | B-366 |
| Nickel Alloy Forgings | SB-564 | B-564 |
| Pipe Welded with Filler | SB-804 | B-804 |
| Castings (CN-3MN, UNS J94651) | A-743 A-744 |
Both AL-6XN (UNS N08367) and 316L (S31603) are corrosion-resistant iron-based stainless steel alloys. However, AL-6XN is a super austenitic alloy with a higher concentration of nickel, chromium, molybdenum, and nitrogen, and a pitting resistance number (PREN) greater than 45. The result is a Super Alloy™ that is more corrosion resistant then 300 series alloys when exposed to chloride-induced media.
All 6 moly materials contains 6% molybdenum, hence, all 6 moly materials are similar in chemistry with lower levels of chromium, nickel, nitrogen, and copper. The key to material selection is component availability and welding compatibility. AL-6XN is readily available in all forms such as sheet, plate, round bar, pharmaceutical and sanitary tubing and fittings.
Examples of 6 moly alloys available in the market:
Due to high concentrations of molybdenum, chromium, and nickel, AL-6XN® is the recommended alloys for the transmission of corrosive fluids through stainless steel in sanitary environments. Any application that experiences chloride-induced corrosion in the forms of pitting, crevice, or stress corrosion cracking may be a good candidate for replacement with AL-6XN. This alloy has been used successfully in a diverse range of industries:
When a plant is calculating the ROI (return on investment) for AL-6XN they are looking for direct losses such as materials and labor costs, loss in production time/shutdowns, product loss, microbiological contamination, and service life. Installing AL-6XN at the beginning of a project will serve as a cost-effective best practice over other 300 series alloys due to its lifetime value. Plants will not have to replace lower grade alloys that experience pitting and corrosion and other related side-effects repeatedly due to equipment failure and sanitary concerns. Instead AL-6XN can offer a one-time solution over the course of the system's lifetime.
Both alloys were developed to resist localized corrosion in seawater applications and are classified as 6 moly alloys. AL-6XN (UNS N08367) and 254SMO (UNS S31254) have a similar composition however, AL-6XN will outperform 254SMO for pitting resistance with a PREN of 45 and 42 respectively. For biotech, pharmaceutical, and hygienic/sanitary applications the main advantage of AL-6XN is the availability of tubing and fittings in appropriate finishes.
Yes, there are two different approaches that should be considered when developing a weld procedure for AL-6XN. For sanitary tubing, GTAW is preferred with heat inputs similar to that of 316L stainless steel. The most common practice is the addition of a consumable insert ring in the weld point. The insert ring is used to over alloy the weld, which helps maintain integrity and corrosion resistance. Insert rings made from C-22 are preferred fr the 13% molybdenum content. An alternative method of welding is done autogenously (without insert ring). Welding done autogenously will require post weld heat treatments in order to maintain corrosion resistance properties.
Weld insert rings are used as filler metal when orbital or hand welding using the GTAW process. When consumed, the inserts purpose is to "over-alloy" the weld area. This over-alloying is needed to compensate for chemical segregation (primarily molybdenum) that takes place during the welding process. The filler alloy must have higher molybdenum content than the AL-6XN alloy to compensate for alloy dilution upon cooling. CSI stocks insert rings produced from 13% molybdenum alloy (C22).
The short answer, is yes! You can weld AL-6XN to 316L.
Theoretically, it is suggested that welding AL-6XN alloy to a 300 series stainless steel is not recommended. However, in reality AL-6XN alloy and 316L stainless steel are welded frequently. This can easily be done using standard welding practices for austenitic stainless steel.
Typically, when welding AL-6XN to 316L tube welds, a consumable weld insert ring is recommended. It will be similar to the process of welding AL-6XN to AL-6XN weld shown in this video.
However, keep in mind that the corrosion resistance can be degraded by making this joint if the conditions are extremely corrosive for the 300 series. Dependent on the severity of the service and if indeed it is a borderline application between AL-6XN and 316L, the mixing of the two metals in the melt most likely would not be attacked.
In general, whenever welding two different metals together, it is important to consider the galvanic potential between the two materials.
The higher the potential difference between the two materials, the quicker the failure by consumption of the anode. As the anode is consumed and therefore becomes smaller in comparison to the cathode, the larger the potential difference becomes, further increasing the rate of corrosion.
If the intended service is indeed very corrosive, then yes, the heat affected zone just past the weld on the 316L side is more likely to be preferentially attacked. However, the AL-6XN side remains unaffected as long as the application stands good for AL-6XN.
AL-6XN contains higher levels of chromium, nickel, and molybdenum than 316L stainless steel, making it the cathodic material with a higher potential than the anodic 316L. When these two materials are joined together by welding in the presence of an electrolyte (the liquid product), an exchange of current between the anode and cathode is achieved. Chemical segregation of the molybdenum and chromium will occur in the heat-affected zone of the weld area. As the current flows between the two materials, the anodic material (316L) is consumed and becomes less noble and creates the potential for galvanic corrosion.
Yes, both alloys can be welded together. We recommend developing a weld procedure using C-22 weld insert rings to enhance corrosion resistance properties.
No, 904L (UNS N08904) is in the super austenitic alloy family which requires over alloying filler metals when welding without post weld heat treatment. Reference ASME BPE section MJ 2.1.1 and MM 5.4 for details of welding requirements.
Yes, using the same cleaning, handling, and welding procedures used with 316L stainless steel. Use over-alloying insert rings when welding without post weld heat treatment.
AL-6XN tubing can be cut using the same tools used to cut 316L tubing. We recommend an orbital tube saw such as a George Fischer saw and the same blades used to cut 316L (CSI part number: CSI035). Speed is your enemy when cutting AL-6XN. Run the blade RPM as low as possible and advance the blade more slowly than you would cutting 316L and use plenty of lubrication (CSI part number: GF-PS8).
Yes, according to studies comparing AL-6XN and 316L stainless steel and the benefits of passivation, corrosion resistance is improved by passivating the surface. AL-6XN will form a passive surface on steel just like 316L. As with any system, cleanliness is key. It is important that any oxides from welding, oils, cutting fluids, etc. be removed after installation in order for the passive surface to form to its full potential, therefore giving the best overall protection and corrosion resistance.
It is difficult to give an exact timeframe when asked how much longer a system will last over 316L. There are many variables with formulas and processes changing over time that include product time of contact and cleanab processes. However, if everything remains constant and the system is well maintained, we have seen 316L systems fail within 12 months or last in excess of 9 years when replaced with AL-6XN alloy. We cannot guarantee success in every application, however, proper installation and maintenance is key to extending the life of any system.
A deodorant manufacturer replaced their 316L piping 3 times in under 3 years. They contacted CSI to find a solution to their problem. CSI recommended replacing their pipelines with AL-6XN, which would provide better resistance to the facility's process.
When a soup processing facility experienced leaks in their pipelines that had only been in service for four years, they turned to CSI to find out why. After a variety of tests, CSI concluded that the company should use AL-6XN, a stainless steel grade with a high molybdenum content that would stand up to the chlorides in their application.
A cheese processing plant encountered corrosion in their recently installed 304L piping, they then looked to CSI to find the cause of the pits. Through a series of tests CSI recommended that the facility use a higher grade metal, AL-6XN to solve their problem with Microbial Induced Corrosion.
All AL-6XN tubing and fittings supplied by CSI shall be provided to the following criteria.
The finish designator is indicated in the CSI item number as a suffix. The suffix will define the finishing requirements for the process component and tubing.
| BPE Surface Finish Code | CSI Surface Finish Code | Product Contact Surface
(Max Ra)* | Product Contact Surface Finish Treatment
(ID) | Non-Product Contact Surface
(Max Ra) | Non-Product Contact Surface Finish Treatment
(OD) | Dimensions & Tolerances (BPE) or (CSI) |
|---|---|---|---|---|---|---|
| N/A | PU | N/A | Unpolished/Mill | N/A | Unpolished/Mill | CSI |
| N/A | 7 | 32 µ-inch (0.81 µm) | Mechanical Polish / As drawn | 32 µ-inch (0.81 µm) | Mechanical Polish | CSI |
| SF1 | PL | 20 µ-inch (0.51 µm) | Mechanical Polish/As Drawn | 32 µ-inch(0.81 µm) | Mechanical Polish | BPE |
| SF5 | PO | 20 µ-inch (0.51 µm) | Electropolished | 32 µ-inch(0.81 µm) | Mechanical Polish | BPE |
| SF6 | P25 | 25 µ-inch (0.64 µm) | Electropolished | 32 µ-inch (0.81 µm) | Mechanical Polish | BPE |
| Tubing OD
(in) | Outside Diameter Tolerance | Wall Thickness
(in) | Wall Thickness Tolerance |
|---|---|---|---|
| ½ | ±0.005 | 0.065 | ±10% |
| ¾ | ±0.005 | 0.065 | ±10% |
| 1 | ±0.005 | 0.065 | ±10% |
| 1½ | ±0.008 | 0.065 | ±10% |
| 2 | ±0.008 | 0.065 | ±10% |
| 2½ | ±0.010 | 0.065 | ±10% |
| 3 | ±0.010 | 0.065 | ±10% |
| 4 | ±0.015 | 0.083 | ±10% |
| Nominal Size (in) | OD (in) | Wall Thickness (in) | Squareness Face to Tangent, B (in) | Off Angle, O (in) | Equivalent Angle Degree | Off Plane, P (in) | Centerline Radius (CLR), R (in) |
|---|---|---|---|---|---|---|---|
| ½ | ±0.005 | +0.005/-0.008 | 0.005 | 0.014 | 1.6 | 0.030 | 1.125 |
| ¾ | ±0.005 | +0.005/-0.008 | 0.005 | 0.018 | 1.4 | 0.030 | 1.125 |
| 1 | ±0.005 | +0.005/-0.008 | 0.008 | 0.025 | 1.4 | 0.030 | 1.500 |
| 1½ | ±0.008 | +0.005/-0.008 | 0.008 | 0.034 | 1.3 | 0.050 | 2.250 |
| 2 | ±0.008 | +0.005/-0.008 | 0.008 | 0.043 | 1.2 | 0.050 | 3.000 |
| 2½ | ±0.010 | +0.005/-0.008 | 0.010 | 0.054 | 1.2 | 0.050 | 3.750 |
| 3 | ±0.010 | +0.005/-0.008 | 0.016 | 0.068 | 1.3 | 0.050 | 4.500 |
| 4 | ±0.015 | +0.008/-0.010 | 0.016 | 0.086 | 1.3 | 0.060 | 6.000 |
General Notes:
Create a MyCSI Dashboard to save resources for quick reference, track orders, and talk with experts.
Create a MyCSI Dashboard to save resources for quick reference, track orders, and talk with experts.
It only takes a minute.
Sign in
Sign in to save resource to your dashboard.
Literature for AL-6XN® Alloy have been saved to your dashboard!
Literature for AL-6XN® Alloy are already in your dashboard.
Speak with our team of experts to help you learn how Super Alloy AL-6XN can help lower your downtime.
Contact Us