Fasteners for CHLORIDE CORROSION
Fighting chloride corrosion is one of the most common issues that face industrial processes. From industrial process in water treatment to pulp & paper as well as seawater applications, chlorides are very common and menace to industry. This brief newsletter shares insight into which materials for fasteners work best to defend against chloride corrosion to protect your equipment.

 

 

 

 

 

 

TITANIUM: Premium Chloride Protection
Titanium fasteners offer unparalleled resistance to chloride including chloride solutions, hypochlorites, chlorates, perchlorates, sodium chlorite, and chlorine dioxide. In addition, Titanium grade 5 fasteners also offer high strength of 148ksi UTS in addition to excellent chloride resistance. The only drawback is titanium cannot be used with dry chlorides or it will rapidly corrode.

DUPLEX STEEL: Localized Corrosion
Duplex fasteners offer an economic solution for high strength chloride resistance, especially for localized corrosion such as crevice and pitting, as well as stress corrosion cracking. Available in Duplex 2205 and Super Duplex 2507 for added strength.

AL6XN: Chloride & Reasonable General Corrosion Resistance
AL6XN bolts offers a high level of chloride resistance combined with high temperature usability (1000F), as well as moderate general corrosion resistance to other chemicals.

ALLOY 20: Chloride + Good General Corrosion Resistance
Alloy 20 fasteners are a good mid-range corrosion resistant alloy. They offer decent resistance to chloride but also good resistance to other chemicals such as sulfuric acid. They are also usable to 1000˚F

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 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.