6% Moly Alloys

What Are They, and Why Do I Need Them for My Processing Application?

If you work in the food & beverage industry, you've learned that not all process fluids are created equal. Say, for example, that you own a small, niche producer of specialty flavored ketchups and barbecue sauces. You just installed a new 316L stainless steel line in your plant less than two years ago, and it's already developing pit corrosion. What do you do?

Do you replace the line and hope for better results the second time around?

Is there another solution?

In this article, we'll look at what are called 6% molybdenum alloys, a special-purpose category of metals used in sanitary processing lines as an alternative to traditional stainless-steel alloys.

6% moly alloys offer vastly improved corrosion resistance against certain types of abrasive fluids, including those with high acid and high salt concentrations.
6mo - Super Alloys

We'll discuss:

  • A review of the sanitary processing industry's de facto standard, stainless steel, and the advantages it offers over carbon steel.
  • Specific details on 6% moly alloys, including their chemical composition, what's called the PRE Number (and why it's important)
  • The various options in 6% moly alloys on the market.
  • Two of these moly Super Alloys™, AL-6XN® and Hastelloy® C-22®, available from CSI.

Stainless steel: the sanitary benchmark for processing systems

Originally developed in the early 20th Century while searching for a rust-resistant metal for use in gun barrels, stainless steel is not a single alloy but rather a family of metals with different properties for each option.

By definition, stainless steel must contain at least 10.5% chromium to provide adequate resistance to rust.

The more chromium the compound contains, the greater its corrosion resistance. And, by definition, stainless steel must contain a minimum of 50% iron.

Other elements may be added to stainless steel for different purposes.

  • Nickel and copper increase stainless steel's resistance to acids.
  • Sulfur improves its weldability and resistance to chlorides
  • Adding titanium to the compound helps prevent the formation of chromium chloride.

Five different classes of stainless steel.

1. Austenitic stainless steel

The most popular compound due to its ductility, ease of working, and good resistance to corrosion. It is also the most widely used alloy in sanitary processing applications. The most common grade of austenitic stainless, Type 18-8 or 304 stainless steel, comprises 60% of the stainless steel made in the United States. 316 is also an austenitic stainless-steel alloy.

2. Ferritic stainless steel

Used for highly corrosive applications, such as those found in marine environments. These alloys are hard and brittle in nature, and are resistant to chloride stress corrosion cracking, where the metal tends to crack along its grain line under tensile stress.

6mo - Tubing
6mo - Facility

3. Duplex stainless steel

Came into use in the 1960s. These compounds are characterized by having both austenite and ferrite in their microstructures, and exhibit high strength — due to the ferrite — along with good corrosion resistance and ductility — due to the austenite.

4. Martensitic stainless steel

A common compound used in applications such as knife blades and is not particularly corrosion-resistant (but better than carbon steel).

5. Precipitation hardening stainless steel

One of the most recently developed compounds. It is soft and ductile in its solution-annealed state but becomes very strong and hard when subjected to relatively low precipitation hardening temperatures of around 1000°F.

As alluded to earlier, stainless steel is the primary metal typically used in sanitary processing systems, including in process pipes and fittings and the working surfaces of pumps, valves, heaters, and tanks.

There are two primary reasons for this.

1. Compared to alternatives, including carbon steel and high-grade plastics, stainless steel offers a smooth, polished surface that is easy to clean and eliminates places for pathogens and other contaminants to linger.

2. Stainless steel is also resistant to the corrosive effects of many processing fluids commonly found in both food & beverage processing and biopharmaceutical manufacturing. Stainless steel helps keep processes clean.

What is 6% molybdenum, and why use it as an alternative to stainless steel?

6% moly alloys belong to a family of alloys known as Super Alloys. 6% moly alloys provide far superior corrosion resistance compared to stainless steel and high tensile strength.

6% moly alloys contain between 6% and 7% of an element called molybdenum in their makeup, compared to around 2% to 3% in most grades of stainless steel.

When added to stainless steel, molybdenum reduces the intensity of the oxidizing effect required to keep the passive layer. It also prolongs the possibility of breaking the passive film formed by chromium oxide. Simply put, more molybdenum will take longer to break the passive film hence more corrosion resistant.

Molybdenum significantly increases the corrosion resistance of the various austenitic stainless steels used in sanitary processing applications. For example, when chlorides come in contact with chromium, it will break the passive film, and molybdenum prolongs that process and increases the corrosion resistance. For these reasons, these super alloys are widely used as an alternative to stainless steel in manufacturing processes involving high levels of chlorides and acids.

Applications include:

  • Ketchup
  • Barbecue Sauce
  • Vinegar
  • Buffer Solutions (found in pharmaceutical manufacturing)

To gain an understanding of the increased corrosion protection afforded by the 6% molys, it helps to have an appreciation of what’s called the PREN. The PREN (Pitting Resistance Equivalent Number) is used as a guide in the processing industries to quantify, compare, and rank the corrosion resistance of a material.

The PREN number is calculated from a formula based on chemical composition. The higher the PRE Number that a material has, the more resistant it is to corrosion. By way of an example, the austenitic stainless steel 316L compound common in sanitary processing applications has a PRE Number of 23. A common moly compound, AL-6XN, has a PREN of 46. Therefore, AL-6XN is considerably more corrosion-resistant.
6mo - Applications

This superiority to corrosion resistance leads to the popularity of the moly-dense super alloys in the sanitary processing industries, especially for challenging applications involving harsh fluids such as acids and alkalis.

Overview of the 6% moly alloys available on the market

There are several different 6% moly alloys that are commercially available.


AL-6XN is a low carbon, high molybdenum super-austenitic stainless steel with excellent resistance to corrosion; high strength; and with good formability and weldability. Its chemical composition includes:

  • 23-25% nickel
  • 20-23% chromium
  • 6-7% molybdenum
  • 0-2% manganese
  • less than 1% each of carbon, nitrogen, silicon, phosphorus, sulfur, and copper.
  • Balance Iron, at 47%, makes up the balance of the compound

AL-6XN is generally used in harsh conditions where good strength and excellent resistance to chloride pitting and stress-corrosion cracking are required. It is commonly used with high temperature and low pH fluids in the food processing industry.

6mo - Al-6XN

Ultra 6XN

The significant elements found in Ultra 6XN include:

  • 25% nickel
  • 20% chromium
  • 6.5% molybdenum

This super alloy exhibits extremely high resistance to both uniform and localized corrosion. Ultra 6XN is commonly used in food processing applications with fluids containing a high percentage of sodium.

Alloy 926

This compound is made from:

  • 24-26% nickel
  • 19-20% chromium
  • 6-7% molybdenum
  • Its iron composition ranges from 42-50%

Alloy 926 exhibits excellent resistance to a variety of highly corrosive materials. The addition of nitrogen in the compound improves yield and tensile strength, while the metal's combination of molybdenum and nitrogen resists pitting and corrosion. An addition of 0.5-1.5% copper improves Alloy 926’s resistance to sulfuric acid.

Stainless Steel 254 SMO

Stainless steel 254 SMO is comprised of:

  • 18% nickel
  • 20% chromium
  • 6% molybdenum
  • 56% iron

This is a high-end austenitic stainless steel that is highly resistant to chloride stress cracking. It also offers pitting and crevice corrosion twice that of the stainless steel 300 compounds. Stainless steel 254 SMO is a cost-effective alternative to high nickel and titanium alloys, with excellent workability.

The 6% molys available from CSI

CSI offers two corrosion-resistant Super Alloys for hygienic processing applications in the food & beverage processing, biotech and pharmaceutical manufacturing, and personal care products industries.


When subjected to sodium chloride in both hot and cold solutions, the AL-6XN compound from CSI provides superior resistance to pitting, crevice, and stress corrosion in comparison to 316L stainless steel and many other 6% moly alloys. This compound carries a tested PRE number of 46; in contrast, 316L stainless steel is 23, and stainless steel 254 SMO is 42. AL-6XN is available in ½" through 4" diameters and in random lengths of no less than 17'. Available fittings comply with ASME BPE Table DT-3-1.

Hastelloy® C-22®

A high-end nickel alloy, Hastelloy C-22 is the most versatile Ni-Cr-Mo alloy available today. It outperforms other nickel alloys, including C-276, Alloy 675, and C-4, in various corrosive environments. Hastelloy C-22, with a PREN of 46, has been successfully used with extremely aggressive media where standard 316L is insufficient and AL-6XN is borderline. It comes in ½" through 3" diameters and in random lengths of not less than 17'. Fittings comply with ASME BPE Table DT-3-1. The material also meets Specifications SB622 and SB626 as recognized by ASME BPE and is acceptable for pharmaceutical applications.

Next Steps

As you've seen, some good sanitary processing choices are available as alternatives to the standard 300-grade stainless steel compounds when processing conditions involving pitting, corrosion, and stress cracking are a problem.

The 6% moly compounds, including AL-6XN and Hastelloy C-22, are more expensive than 300 Series stainless steel when compared on an initial cost-per-foot basis. However, the return on investment is high when you consider the repair and replacement costs for the damaged and pitted stainless-steel alternatives.

These Super Alloy compounds may be ideal when processing corrosive fluids, including buffer solutions, saline solutions, ketchup, soy sauce, sports drinks, deodorant, shampoo, and detergents containing a high percentage of chlorides. To learn more, contact CSI at (417) 831-1411.

Contact Us


Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.