Q: I have down hole application containing sour gas and high chloride levels. The temperature the bolts are exposed to is around 150F and I’m having trouble with cracking. I tried Super Duplex 2507 but am still having issues. Any ideas?

A: Super Duplex 2507 does provide good resistance to stress corrosion cracking, pitting and crevice corrosion in warm chloride environments to around 570 F, but I think in combination with the sour gas, and perhaps stress of your operation, it may need something more robust. A great material for a combination of sour gas, chloride, high strength and temperatures to consider is Incoloy 925. Down-hole oil & gas rely on this material for its excellent resistance to sulfide stress cracking and stress-corrosion cracking in H2S environments. It has a high operating temperature of 1200°F which is well beyond what you need. It is also has a much higher yield strength (120 ksi) compared to Super Duplex 2507 bolts of 82ksi.

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: 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.