Titanium vs Monel bolts in stagnant salt water

Q: I’m dealing with a stagnant seawater situation. I’ve tried Monel 400 bolts because of its salt water resistance but am experiencing mild pitting corrosion. Any material that would fair better?

A: Monel 400‘s only Achilles heel in terms of salt water situations usually occurs in stagnant waters where is can experience crevice and pitting corrosion. For still seawater, I would suggest Titanium grade 2 fasteners, which is known for its high resistance to sea water and all chloride environments, even at high depths and temperatures. If you need higher strength, you can always go with grade 5 bolts which are twice as strong as grade 2.

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.

Bolt for hot hydrogen sulfide, CO2, hydrocarbons & salt water

Q: I need a material that will be suitable for oil and gas use. Submersion in hydrogen sulfide, carbon dioxide, hydrocarbons, salt water at temperatures up to 325F. Crevice corrosion has been a factor. Also need 170ksi min tensile strength. Plus we’d like to torque a 3/8-24 fastener to 50 ft lbs.

Sound like a good applications for Inconel 718 bolts. This material has a min ultimate tensile of 180 ksi and typically well into the 200+ range. Because its high nickel content it will do well in sour gas/corrosive environments and is often chosen for down hole oil projects. It is resistant to a wide ranges of acidity and alkalinity, including sulfur compounds and chloride environments.

With regards to torque, the value that is required is 100% dependent on the material, so you should not have a pre-determined torque value. With that said the torque for an Inconel 718 bolt 3/8-24 could range from around 50-67+ Ft-Lb, depending on a variety of factors such as: Lubrication, condition of bolt or mating fastener, cleanliness etc. This is a general range but the actual torque could vary significantly depending on the actual bolt conditions.

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.