6% Molybdenum Superaustenitic Alloys

This class of alloys was introduced in the early 1970s, largely in response to the need for a cost effective alloy with resistance to brackish water or seawater environments.  The main driving force was power generation and the booming nuclear industry.  Stainless 316L was not resistant to warm chloride bearing waters and more resistant nickel base alloys like 625 were considered too expensive.

AL-6X® and its successor, AL-6XN®, were developed by Allegheny Ludlum in response to this need.  Millions of feet of AL-6X and AL-6XN surface water condenser tubing are still in service after over 35 years of operation.  Avesta developed 254SMO® around the same time, and other mills later produced similar versions, but owing to a market development relationship with Rolled Alloys, AL-6XN Alloy (N08367) became the most widely used and accepted 6% Mo alloy.  The invention of adding nitrogen to improve corrosion resistance and strength also meant the alloy could be produced in heavier cross sections.  Rolled Alloys was instrumental in developing the sources to ensure the supply of all product forms and still maintains an inventory of these products in a wide range of sizes.

The power industry found that this alloy that worked so well in seawater also performed extremely well in the flue gas desulfurization (FGD) systems of coal fired plants.  Other industries took notice when presented with the advantages of a strong, acid-chloride and stress cracking resistant alloy.  With useful resistance to organic and inorganic acids, the chemical process industry (CPI) expanded its usage.  It was found to resist microbiologically influenced corrosion (MIC).  The food and pharmaceutical industries began to utilize the alloy for situations where 316L was inadequate.  It is also being used in off-shore oil and gas environments, pulp bleaching applications, service water piping and LNG transportation and storage.  It is one of the more widely used alloy materials in reverse osmosis desalination systems.

AL-6XN Alloy is slightly stronger that 254SMO (S31254) and is accepted for ASME B&PV Code work up to 800° F (vs. 700° F).  The higher nickel content of AL-6XN Alloy improves general corrosion resistance and results in an alloy more resistant to sigma formation during fabrication.  Copper content is maintained low, which improves chloride pitting resistance and is preferred by some users for environmental reasons. Independent studies by SINTEF showed that AL-6XN Alloy consistently outperformed other 6% Mo alloys in terms of pitting and crevice corrosion resistance in seawater.


Only AL-6XN Alloy produced under the authority and expertise of Allegheny Ludlum (ATI) has this extensive history of excellent performance.  In recent years, other producers have attempted to supply substitute materials that meet the broad chemistry range of the UNS number, but don’t necessarily offer the same performance or integrity.  Rolled Alloys inventories only true AL-6XN Alloy products produced to the highest quality standards.  Don’t make your system out of an inferior product.