Tantalum vs Monel fasteners in hydrogen fluoride

Q: I’m using hydrogen fluoride used in a manufacturing process to make refrigerants. I know it’s very corrosive and have tried both hastelloy and inconel fasteners, which I thought were super corrosion resistant. What am I doing wrong because the acid is still eating them? Should I try Tantalum?
A: Definitely do not try tantalum. This will not work very well either. Hydrofluoric acid presents an users with an unusual situation. Even the most corrosion resistant materials that can handle boiling HCl and H2SO4 like tantalum or Hastelloy are readily corroded by Hydrofluoric acid.  Even small percentages of HF will increase corrosion in these types of fasteners. If you need to use a metal fastener due the strength, you should consider Monel. Monel 400 is one of the few alloys that can tolerate the presence of fluoride/fluorine or their derivatives because it is able to form a protective fluoride film. This is due to it’s high copper content compared to other nickel alloys, which improves it’s resistance to reducing acids particularly HF.  In non-aerated acid Monel alloy 400 resists all concentrations, up to the boiling point.  Here is a link to how Monel fasteners perform in HF compared to other materials.
 
One precaution is to consider aeration, or the presence of oxidizing salts, which can increases the corrosion rate.
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