AL6XN

Many Grades of Stainless Steel

Most people familiar with lift stations know that stainless steel can and does rust. If you’ve seen lots of lift stations then you’ve probably seen even 316 stainless corrode significantly after just a few years in problem sites.

The FOGRod is constructed with 10 metal contacts exposed on its CPVC surface. These metal contacts are very high grade stainless known as AL6XN.

AL6XN is from a class of stainless steels known as super-austenitic.

If you don’t want to know details, the take away message is that AL6XN is much more expensive than 316. The cost is due to the process involved in creating its exceptional corrosion resistance. This resistance is created by the addition of chromium, molybdenum and nitrogen. If you want something critical in your lift station to keep working then 316 may not be good enough, but AL6XN definitely will be.

AL6XN

Allegheny Ludlum AL-6XN® alloy (UNS N08367) is the most corrosion resistant iron-base austenitic stainless alloy produced by Allegheny Technologies. It is a low carbon, high purity, nitrogen-bearing “super-austenitic” stainless alloy.

The AL-6XN alloy was designed to be a seawater resistant material and has since been demonstrated to be resistant to a broad range of very corrosive environments.

How are higher grade stainless steels produced?

  • Molybdenum is added to enhance corrosion resistance in chloride environments
  • Carbon is reduced to avoid sensitization during welding
  • Nitrogen is added to compensate for the reduced strength of the “L” grades and to improve pitting resistance and phase stability
  • Chromium is increased to enhance pitting and crevice corrosion resistance
  • Nickel is increased to stabilize the austenitic microstructure and to improve resistance to general corrosion in reducing environments
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    Basic stainless steel composition known as Type 304 (18Cr-8Ni) was first used in the 1950s to replace copper-based alloys that had failed due to ammonia stress-corrosion cracking, erosion by steam impingement and pitting in acidic river water. Most of the initial applications were for Type 304 in fresh, industrial and acidic mine waters.

    Type 316, which contains molybdenum and slightly higher nickel levels, was used in more corrosive situations. The alloy content of Type 316 was found to be too lean to prevent pitting and crevice corrosion in certain applications, such as those involving stagnant seawater. But 304 and 316 resisted ammonia stress-corrosion cracking and steam impingement erosion that caused copper-based alloys to fail.

    Stainless alloys with even higher levels of chromium (Cr) and molybdenum (Mo) were developed to resist the high chloride ion concentrations of brackish water and seawater. One of the results of Allegheny Ludlum’s earliest efforts to develop better austenitic stainless alloys, was the introduction of AL-6X alloy in the early 1970s. The alloy contained 20 wt % Cr, 24 wt % Ni and 6 wt % Mo. The relatively high nickel (Ni) content promotes a fully austenitic structure in the annealed condition.

    The AL-6XN alloy was developed as an improvement over the AL-6X alloy. Both alloys have the same basic alloy composition except that the AL-6XN alloy contains an intentional addition of 0.18 to 0.25% N.

    AL6XN vs SMO254

    Another super-austenitic stainless steel is SMO254. They have similar characteristics but our engineering evaluation preferred AL6XN over SMO254 due to superior corrosion resistance. The following data is provided by Rolled Alloys. The PRE(N) table is one measure of corrosion resistance and you can see how AL6NX is somewhat better than SMO254 and a lot better than 316:

    AL6XN-vs-SMO254-1

    Crevice corrosion resistance is a lot better in AL6XN than SMO254:

    AL6XN-vs-SMO254-in-seawater


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