Alloy 20 fasteners for sulfuric acid corrosion

Q: I’m working with boiling sulfuric acid and tried Alloy 20 for some of my bolting but am having corrosion. I’m confused as I thought that is what they are best for. Any suggestions.

A: You are right Alloy 20 does typically perform very well in sulfuric acid and even boiling sulfuric acid but only for a specific range.Not knowing your specific situation, it is most likely the concentration of acid you are working with that is the issue. Alloy 20 provides great protection in boiling sulfuric acid at specific range of 20% to 40% concentration. If you are over this concentration, even at 45%, if may cause corrosion issues. For high concentrations of sulfuric acid, you may want to look at Inconel 625 or Hastelloy C276. Both Inconel 625 and Hastelloy C276 bolts should provide you protection in all concentrations of sulfuric acid up to 80%. For anything higher than 80%, I would suggest Tantalum which can handle all concentrations and temperatures.

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