Many Grades of Stainless Steel
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.
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?
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.