Q: We are running a steel pickling process are considering switching from a sulfuric acid process to hydrochloric acid. Our system currently uses Inconel 625 bolting. With this be sufficient?

A: Inconel 625 is a good material for acid corrosion but for large system of hydrochloric acid, especially with the corrosion that can occur with fumes and vapors, you may want to try a Hastelloy C22 or even tantalum if the Inconel 625 does not hold up. Both perform very well in hydrochloric acid. Tantalum fasteners have the best corrosion resistance and are essentially immune to HCl but limited strength. Hastelloy especially grade C22 while not as corrosion resistant as tantalum has very good corrosion resistance to HCl and good strength as well.

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.

Q: I have a waste water purifying situation where I’m using chlorine to treat the water. In addition to the acidic solution produced the fastener will also experience high loads, so I’m also concerned with stress corrosion cracking. Currently Super Duplex 2507 is not doing the job. Any recommendations on materials?

A: One material worth considering is Hastelloy C22 fasteners. These fastener are excellent in wet chlorine environments and can be used if ferric chloride is produced incidentally (Hastelloy B2 and Hastelloy B3 cannot handle ferric chloride). In addition, Hastelloy C22 is also know for providing resistance to stress corrosion cracking as well as pitting and crevice corrosion and has a typical ultimate tensile of 111ksi and yield strength of 52ksi