Q: I’m using sodium hypochlorite to control slime and bacteria formation in a power plant water system. It’s in about a 15% solution. I tried using Duplex fasteners thinking they are good with chlorides but I’m having corrosion issues. Any suggestions?

A:  I think the problem you have is the fact that sodium hypochlorite is a strong oxidizer. Duplex can experience corrosion in chloride solutions where oxidizers are present. In this case, I would suggest either Titanium grade 2 or grade 5 depending on the level of strength you need. Both titanium grades are nearly immune to hypochlorites. Titanium provides excellent resistance to oxidizers because of its stable oxide film, which regenerates almost instantly in the presence of water or oxygen. Both grade 2 or 5 will exhibit these properties. Grade 5 is only necessary if you require higher strength.

Q: I’m working on a structural project that not submerged in salt water but exposed to moist salt air. I’m not only concerned about the supports having fasteners that won’t corrode but also having a fastener that is high strength. Any suggestions.

A: Though I don’t know the specific strength requirements, I do know that Duplex 2507 is commonly used in situations that require both salt water corrosion resistance and high strength. Both Duplex 2205 and 2507 offer a high level of resistance to chloride (salt) stress corrosion cracking, as well as pitting and crevice corrosion, but 2507 is much better suited when strength is a concern. Duplex 2507 fasteners provide users yield strength which is twice that of annealed austenitic stainless steels.

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.

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.