High temperatures, low pH, and aggressive chemicals call for a material that exhibits the high corrosion resistance that traditional stainless steel alloys fail to attain.
There are many alloys available to address the corrosion issues; but what makes super-austenitic stainless steel alloy AL-6XN® and nickel alloy C-22 the favorite materials for highly corrosive environments? Are the properties more attractive, or is the availability of the material in the required forms more desirable? Is one better than the other?
Both AL-6XN and C-22 have a face-centered cubic lattice structure and contain chromium, molybdenum, and nickel as the major ingredients for providing corrosion resistance. However, C-22 has more of each of these elements to give better resistance to chlorides, mineral acids, and high temperature oxidation. More chromium gives more resistance to pitting, stress corrosion cracking, and crevice corrosion. More molybdenum provides more resistance against reducing media such as hydrochloric, formic, and phosphorous acid mediums. More nickel resists more attacks from chlorides and other halides. Both AL-6XN and C-22 are materials that can withstand very corrosive environments, but they each have material design limitations and application restrictions.
Though AL-6XN was originally developed for sea-water applications, it is also an excellent material for food and beverage products such as sports drinks, ketchup, soy, barbeque, and salsa where the corrosive product eats away the material. It is also found to resist corrosive attacks from various personal care products such as deodorant and hair care products. AL-6XN is probably the best choice not only for corrosion resistance bust also for applications that require excellent formability, strength, and weldability that is not achievable by any conventional stainless steel alloy.
C-22 is designed to tolerate extremely corrosive environments. The advantage of this material is that it can withstand both oxidizing and reducing acids at high temperatures exhibiting high resistance to localized corrosion. C-22 has been used in chemical, food, beverage, and pharmaceutical industries where high temperatures and high concentrations are utilized. Some of the processes use extremely high concentrations of chlorides at high temperatures. C-22 works effectively in these aggressive conditions.
Remember, every material has a threshold limit, and AL-6XN will not hold well after reaching its threshold. So, it is beneficial to opt for C-22 beyond this limit.
AL-6XN is a cost-effective alternative to 316L for higher corrosion resistance and strength. It is considerably less expensive when compared to C-22, and yet it provides a comparable level of corrosion resistance.
Not only are the properties attractive, but the material is available in many product forms. AL-6XN and C-22 are readily available in forms such as tube, pipe, sheet, plate, bar, billet, and forgings from which components such as fittings, valves, fasteners, and castings can be obtained. Because many sanitary piping components made from AL-6XN alloy that are readily available you should always check for commercial availability of the form of product (tube and fittings) you may need before planning a project.
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“AL-6XN” is a registered trademark of ATI Properties, Inc., licensed to Allegheny Ludlum Corp.
“Hastelloy” and “C-22” are registered trademarks of Haynes International.
