WHAT IS 6% MOLYBDENUM, AND WHY USE IT AS AN ALTERNATIVE TO STAINLESS STEEL?

 

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.

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

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.

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.

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.

AL-6XN

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.

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.

Credits https://www.csidesigns.com/blog/articles/6mo