Q: I am needing to replace some hex heads and nuts on an HCL tank lid. It appears they were originally coated and due to a previous leak, they are starting to corrode. What would be the best alternative bolt type for this situation? HCL concentration is 37% and is at normal room temperature.

A: Coatings in concentrated HCl is very risky. If the coating gets breached, the bolt will fail rapidly and this could easily happen with HCl as its vapors can usually penetrate most coatings. To have a solution that will be more robust would be to go with a nickel alloy like Hastelloy C276 fasteners. Hastelloy C276 is designed for HCl service and will perform very reliably assuming your HCl is at ambient temperatures. You could see this by looking at the HCl iso-corrosion curve below.

Q: I have a process that utilizes about 10% hydrochloric acid with trace amounts of other chemicals. I tried zirconium bolts because they are known for HCl but we’re burning through them like crazy and I can’t figure out why they are corroding. Any ideas and what other material may work?

A: I think the key to what you are describing is not the hydrochloric acid corrosion but the “other chemicals” in your process. Zirconium bolts offer excellent resistance to HCl corrosion even upwards of 37%. The problem with zirconium fasteners is that if they are exposed to any oxidizing media. Oxidizers, even in trace amounts can cause zirconium to corrode rather quickly. Depending on the temperature of your process, Hastelloy C276 bolts will be a good fit. For a high temperature application, I would suggest tantalum fasteners. They can handle HCl corrosion even up to the boiling point.