Duplex 2507 bolts for high strength application in salt air

Q: I have a high strength application that requires resistance to damp salt air and occasional sea splash, I’m using Monel 400 but it’s not holding up the the 75ksi of yield I need. Lots of broken bolts? Is is corrosion or strength? Thoughts?

A: Your issue isn’t the corrosion. Monel bolts offer superior salt water protection even in deep sea environments. This is a strength issue. Monel 400 only offers about 45ksi of yield so 75ksi would definitely cause breakage. You could try Monel K500 which has a yield up to 110ksi and is excellent in salt water, but In terms of both corrosion and strength it would probably be overkill. The most economic bolt for your application would be Duplex 2507. Duplex bolts are known for their high yield strength twice that of austenitic stainless steels. In addition, Duplex 2507 bolts contain nickel, molybdenum and chromium providing good resistance to chlorides and salt water environments.

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.

Screws for high velocity salt water with debris

Q: I have a salt water environment with high velocities and a lot of debris including sand, dirt and particles. Because of the amount of debris, I’m concerned about what material would work best.

A: Both Monel and Titanium can be good choices in this situation. Both perform extremely well in rapidly moving seawater. Titanium has an edge in performance if you have stagnant water but Monel may do better in high velocity erosive conditions. Generally speaking both Titanium and Monel will perform very well in high velocity salt water with sand and dirt.

Difference between Monel 400 and Monel K500 bolts

Q: I’m currently using Monel 400 hex head bolts in salt water. Should I be using K500? What’s the difference?

A: Basically, Monel K500 is a precipitation hardened version of Monel 400. This means the material goes through a heat treating process that changes the alloy’s structural matrix to increase its tensile and yield strength. Monel K500 bolts will give you similar corrosion resistance as Monel 400 bolts, but with added strength. Alloy K500 bolts will provide about 3 times the yield strength (110ksi vs 45ksi) and double the tensile strength (160ksi vs 83ksi) when compared Monel 400 bolts. Overall, if the Monel 400 is failing from a strength perspective, then Monel K500 bolts will be a good choice. However if the Monel 400 bolts are failing due to corrosion, a more corrosion resistant alloy may be needed like titanium bolts or a high performance nickel alloy fastener like Inconel.