AL6XN vs Monel bolts for salt water and marine use

Q: Is AL6XN a steel or nickel alloy? Trying to figure out if it would perform as well as Monel in seawater.

A: To answer your first question, AL6XN bolts are actually both a nickel and a steel alloy. AL6XN is often described as a high nickel content stainless alloy. Though AL6XN bolts are prodominently iron based with >40% iron, it also contains a large percentage (24%) of nickel and performs like a nickel alloy, meaning it offers high temperature and high strength corrosion resistance. AL6XN.

In comparing AL6XN to Monel 400 they are fairly similar with the same usable temperature limit of 1000°F and comparable yield strength (though AL6XN has a slight advantage of 53ksi to Monel’s 45ksi). Both perform excellent in seawater, yet Monel can show some pitting in stagnant seawater, where AL6XN Molybdenum content actually helps prevent pitting and crevice corrosion in seawater. If your application is not prone to stagnant water and you don’t need the added strength I would actually suggest Monel 400 bolts as they would be a more economic fastener.

Duplex bolts in sea water contaminated steam turbine

Q: I’m running a steam surface condenser which is installed on a steam turbine in a thermal power station. I’m using sea water that could be contaminated with chemical pollutants cooling water side of the condenser. Any ideas on a good fastener for the tubes and water boxes?

A: Duplex 2205 screws are commonly used in sea water situations as the provide excellent protection from chloride stress corrosion cracking, as well as pitting and crevice corrosion. However Duplex 2205 does have a temperature limit of 300°C, after which you would experience embrittlement which might be limiting for your application.

A more fitting option worth considering as you are not only dealing with salt water corrosion but also corrosion from chemical pollutants is AL6XN. AL6XN screws are often used in steam surface condensers. This alloy was developed for salt water corrosion applications and offers excellent resistance to stress corrosion cracking, pitting and crevice corrosion in a broad range chloride environments. In addition to its resistance to chloride based corrosion it has also be widely used in chemical processing for resistance to various acids. Plus it has a usable temperature limit over 500°C. Since you have both salt water and chemical pollutants, AL6XN may be a better fit.

Duplex 2507: Bolts for geothermal drilling exposed to sulfur and salt

Q: I’m working on a on a geothermal energy drilling project. Carbon steel we’re currently using to bolt drilling components is failing from corrosion. We have corrosive an environment with high levels of sulfur and salt, as well as temperatures to 260C. Any suggestions?

A: Super Duplex 2507 is often used in geothermal explorations. It is a steel alloy that contains chromium, molybdenum, and nickel which greatly improves its strength, corrosion resistance and temperature capabilities compared to most carbon steels. Super Duplex 2507 fasteners are known to provide very high resistance to pitting, erosion and crevice corrosion, as well as corrosion fatigue and stress corrosion cracking (SCC) in chloride environments. Also, Super Duplex fasteners have good general corrosion resistance to moderately acidic environments. Not knowing your exact conditions Super Duplex is certainly worth looking into more as well as other higher performing alloys like Inconel or Hastelloy, which are known to handle hot concentrated solutions of sulfuric acid much better.

Hastelloy C22 fasteners used in wet chloride

Q: I have a waste water purifying situation where I’m using chlorine to treat the water. In addition to the acidic solution produced the fastener will also experience high loads, so I’m also concerned with stress corrosion cracking. Currently Super Duplex 2507 is not doing the job. Any recommendations on materials?

A: One material worth considering is Hastelloy C22 fasteners. These fastener are excellent in wet chlorine environments and can be used if ferric chloride is produced incidentally (Hastelloy B2 and Hastelloy B3 cannot handle ferric chloride). In addition, Hastelloy C22 is also know for providing resistance to stress corrosion cracking as well as pitting and crevice corrosion and has a typical ultimate tensile of 111ksi and yield strength of 52ksi

Duplex 2205 and Hastelloy fasteners in pulp & paper digester

Q: We have a pulp & paper digester that is used to cook wood chips in an alkaline solution. The fasteners will be exposed to high temperatures and abrasive conditions. Any suggestions?

A: Without knowing the specifics of your process, I can make two general material suggestions. Duplex 2205 fasteners are often used in pulp and paper digesters for their high resistance to chloride stress corrosion cracking, as well as pitting and crevice corrosion. But keep in mind Duplex 2205 has a usable temperature limit of 572F. If you have harsher conditions or are are running at higher temperatures, Hastelloy C276 may be a better fit. C276 fasteners are also commonly used in digesters and are a good fit for more severe conditions. They can be used at operating temperatures up to 1900 F and are very resistant to chlorides, as well as hot, extremely caustic media.