Q: I’m using zirconium fasteners in a sulfuric acid process but they are showing corrosion. I’m not sure why as I’m well below boiling and only using 40% concentration. There may be some downstream contaminants, but not much. Any ideas?
A: It is most likely the contaminants that are causing your zirconium screws to corrode. Though zirconium can easily handle sulfuric acid well beyond boiling and at 70% concentration – a far harsher condition than you have – it also has an Achilles heel which is oxidizing media such as ferric or cupric chlorides. Even if a minute amount of oxidizers are seeping into your process where they zirconium screws are, you will have corrosion. Depending on the exact temperature that you are running at, I would suggest trying Hastelloy C276 or Inconel 625 if your process is 150°F or lower. Both are similar in that they only have 1mpy of corrosion at 150°F in 40% H2SO4. Inconel is traditionally higher strength and Hastelloy with a slight edge on corrosion resistance. If you have a lot of oxidizing contaminants, you may want to look at Hastelloy C22 which offers <1mpy under the same conditions, but has a much better resistance to oxidizing media that C276. For higher temperatures – nearer boiling, tantalum may be the answer.