Frequently Asked Questions About AL-6XN®

FAQs

Is AL-6XN stainless steel?

AL-6XN alloy is a corrosion-resistant iron-based austenitic stainless steel alloy. It is a low carbon, high purity, nitrogen-bearing "super-austenitic" stainless steel with high levels of nickel, chromium, and molybdenum to improve resistance to chloride stress corrosion cracking and chloride-induced pitting.

How similar or different is AL-6XN from 254SMO stainless steel?

The physical properties of AL-6XN and 254SMO are basically identical. Because of the similarities between these two grades, both alloys can be used interchangeably in many applications. Often the selection is based on the availability of the material. AL-6XN is more readily available in sanitary and high purity specifications.

Can I weld AL-6XN to 316L stainless steel?

Welding AL-6XN alloy to a 300 series stainless steel is NOT recommended. AL-6XN alloy and 316L stainless steel can be joined easily using standard welding practices for austenitic stainless steel. However, corrosion resistance can be degraded by making this joint. Whenever welding two different materials together, it is important to consider the galvanic potential between the two materials. 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. As the current flows between the two materials, the anodic material (316L) is consumed. 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.

When welding the two materials together, as stated previously, chemical segregation of the molybdenum and chromium will occur in the heat-affected zone of the weld area. This increases the odds for crevice corrosion and pitting to occur under oxides on the surface of the weld areas. Due to the chemical composition of these materials during weld segregation, 316L actually becomes less noble when coupled by welding than if the two materials were joined together by clamping or bolting, and even these unions cannot eliminate the potential for galvanic corrosion. 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. However, if the intended service is indeed very corrosive, the heat affected zone just past the weld in the 316L joining more than likely would be preferentially attacked.

Are there special welding procedures that have to be followed?

If the component can be heat treated, a full solution anneal at the proper temperature may be substituted for over-alloying the welds. Often annealing is not practical, so in order to maintain the integrity and corrosion resistance of the weld area, over-alloying the weld by using a consumable insert ring or weld wire on all welds is imperative. The insert rings used are typically Alloy 22 or Hastelloy® C-22®, a 15% Molybdenum-containing alloy. The minimum alloy recommended if Hastelloy C-22 or Alloy 22 is not available is Alloy 625, or Alloy C-276 may be substituted as well. Contact your CSI sales representative or visit the Working with AL-6XN page for more information.

What is a weld insert ring?

Weld insert rings are used as filler metal for orbital or hand welding AL-6XN alloy. This will "over-alloy" the weld area to compensate for chemical segregation that may take place during the welding process. The filler alloy must have higher molybdenum content than the AL-6XN alloy to compensate for alloy dilution on cooling. CSI stocks rings produced from 12% molybdenum alloy (C-22); rings manufactured from 9% molybdenum alloy (Alloy 625) or 15% molybdenum alloy (C-276) may also be used.

Is one type of insert ring material better than another?

CSI has standardized on the washer style ring fabricated from Hastelloy C-22 material. The material choice was strictly based upon the corrosion resistance properties of Hastelloy C-22 in pitting applications.

What happens if I don′t use the insert rings and don′t anneal after welding?

Expect corrosion to take place in the weld and in the heat affected zone adjacent to the weld. Without following the correct welding guidelines, AL-6XN alloy will have only a little better corrosion resistance than 316L stainless steel. In the corrosive applications where AL-6XN is most commonly used, failure is imminent if the correct installation guidelines are not followed.

Is it true that 904L material doesn't require a weld insert ring?

No. 904L material is similar to AL-6XN and requires a weld insert ring. The simple rule of thumb is that Fe-Ni-Cr-Mo alloys have about 2/3 the Molybdenum content of the base metal. This is approximate, and for 316L, weld metal Mo of about 2.5% matches the corrosion resistance of the base metal — just over 2% Mo. So, technically, 904L has about 4% Mo and by the rule of thumb (2/3 Mo), the welded 904L will have less than 4% Mo in the base material that is more close to 316L. So, over-alloying 904L with a weld insert ring is essential if annealing is not possible.

Can I weld AL-6XN by hand?

Yes, using the same cleaning, handling, and welding procedures as used on 316L stainless steel. Reduce the amperage by approximately 10% and weld about 10% slower.

Can I weld AL-6XN to 904L?

Both AL-6XN and 904L material are superaustenitic stainless steels. So, even if these alloys have slightly different chemical composition, the potential difference between the two alloys is not huge enough to affect the corrosion resistance. Both alloys can be welded together using a weld insert ring of Hastelloy C-22.

How do I cut AL-6XN tubing?

AL-6XN tubing can be cut using the same tools used to cut 316L tubing. We recommend a George Fischer saw, with gear reduction and the same blades that you use to cut 316L (CSI part number: CSI035). Speed is your enemy when cutting AL-6XN. It is necessary to run the blade RPM as low as possible and advance the blade more slowly than you would if cutting 316L. It is also suggested to use plenty of lubrication (CSI part number: GF-PS8).

Do I need to passivate a system constructed of AL-6XN?

Yes, studies have been performed comparing AL-6XN and 316L stainless steel and the benefit of passivation. AL-6XN will form a passive surface on the steel just like 316L. As with any system, cleanliness is key. It is imperative that any oxides from welding, oils, cutting fluids, etc. be removed after installation in order for the passive surface to form to its full extent, therefore giving the best overall protection and corrosion resistance.

Aren't all 6-moly materials the same?

No. All 6%-moly materials are similar in chemistry, however. Some contain lower or higher levels of chromium, some contain higher levels of copper, etc. The key to material selection is component availability and welding compatibility. AL-6XN is readily available in all forms from sheet and plate to sanitary tubing and fittings. When joining materials by welding, it is always best to match the chemistry at each joint with the same material. However, since there is not a huge potential difference between the 6-moly stainless steel materials they can be welded by using a weld insert ring.

How much longer can I expect a system fabricated from AL-6XN to last over 316L?

It is difficult to give an exact number when asked how much longer a system will last over 316L. There are many variables with formulas and processes changing over time. However, if everything remains constant and a system is well maintained, we have seen systems fabricated from 316L fail within 12 months and then last in excess of 8-9 years when replaced with AL-6XN alloy. We certainly cannot guarantee this success in every application, but remember: proper installation and maintenance is key to the life of any system.

Will AL-6XN help or eliminate my rouging problem?

There are three types of rouging. Each type is outlined below with the potential of AL-6XN as a material alternative.

Class I is the most prevalent type found in pharmaceutical systems; it is caused from the erosion of centrifugal pump components, impellers, volutes, and venturi on the back of the pump, as well as cracked valves. You can see the erosion of the softer material (primarily the austenite) away from the heavier material (Nickel, or poor ferrites). Under a scanning electron microscope, the eroded areas display the ghosts of the dendritic structures. The softer parts are blasted out by the higher velocity water (sonic velocities). The higher velocity water erodes the tips of the impellers and all other items in its path. As soon as it gets into the tube, it drops velocity back to standard flow velocity (approx. 5 ft per second), dropping everything it has picked up. The metal particles are attracted to the metal walls by electrostatic actions and gather on the walls. Concentrations are always heavier in the area from the pump outlet to some distance down stream. In these areas the particles will oxidize in the WFI because this is a superior oxidizing agent and begins to develop oxide films. Class I is strictly a function of the pump and the velocities. Type I — the red oxide — is due to the formation of hematites (Fe2O3), while yellow oxide is hydrated hematites (i.e. water stuck on to the molecule). Any material used down stream will rouge where Class I rouging is occurring.

Class II comes from the metal itself, and it is usually caused by a corrosion mechanism eating into the passive layer. Usually this form of rouging occurs in systems that have not been chemically passivated. This is why it is so critical all WFI systems are passivated with nitric or citric acid. Class II rouging forms when little pustules developing on the surface explode, leaving brilliant silver spots under them. These are active sites of rouging, known as "in-situ rouging." Type II is primarily hematite (Fe2O3).

AL-6XN will help this form of rouging tremendously.

Class III is high temperature rouging and develops a heavy black film (magnetite Fe2O3) on the surface. If the surface has been electropolished, this film will appear glossy black. If the system is non-passivated it becomes a powdery black. The glossy black is extremely stable. If you strip it off, re-passivate and pass high temp steam through it again. It will reform in a matter of months. In most cases you don′t worry about this form of rouging. The powdery black surface is not stable and can release particles into the product.

Currently, not enough data has been collected to determine if AL-6XN will help this form of rouging.

In summary, AL-6XN will not help in Class I rouging problems. However, Class II rouging applications will see a definite benefit from the use of AL-6XN. In Class III formations, there is insufficient data available at this time to make a determination; all indications state that AL-6XN will improve Class III rouging problems but will not totally eliminate the problem in all cases.

What type of application is best suited for AL-6XN?

Specifically, any application that is experiencing chloride-induced corrosion in the forms of pitting, crevice corrosion, or stress corrosion cracking may be a candidate for replacement with the AL-6XN Alloy. The alloy has been used successfully in phamaceutical and cosmetic process systems; and food processing systems such as ketchup, soy sauces, BBQ sauces, baby foods, corn syrups, and more. AL-6XN alloy is also used successfully in beverage systems that manufacture sports drinks, as well as the brewery industry. Recently, AL-6XN has been successfully used in the personal care industry, manufacturing deodorants, hair care products, oral care, and even home care products such as detergents and cleaners.

How can I justify the cost of AL-6XN?

Consider the costs of continually replacing 316L stainless steel components, downtime and labor costs associated with those replacements, and the possible product contamination issues. Soon the question becomes "How can you not justify the cost of AL-6XN?"