Tag: Hastelloy Fasteners

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.

Fasteners For Hydrochloric Acid (HCl)

While hydrochloric acid (HCl) is a common and critical industrial chemical, it often causes significant maintenance issues for fasteners. Due to elevated temperatures, fluctuating concentrations and permeation of its vapors, utilizing the correct material for your fasteners is key to withstanding this potent acid. This article will briefly review some of the most common fastener material solutions for dealing with HCl most effectively.

Engineered Polymers
Depending on the load and temperature requirements of the application, polymer fasteners can perform very well in HCl solutions.  Some of the highest performing polymer fasteners are those in the fluoropolymer family which include PVDF (Kynar) and PTFE. In addition, PEEK is also a high performer although not as good as the fluoropolymers. While these polymers are the most corrosion resistant  in HCl from a materials perspective, when it comes to utilizing them in the form of a fastener, the key is understanding if they are strong enough for the application.

Generally speaking, in order of corrosion resistance PTFE would be the best choice followed by PVDF and PEEK. However in terms of strength, the opposite is true with PEEK being the strongest, followed by PVDF and PTFE being the weakest. With polymers, its best to first determine the minimum strength needed and then go up the corrosion resistance scale to select the best material.

Specialty Metals  
In HCl environments, typical steel fasteners generally will not suffice as they are rapidly attacked. For these tough corrosive HCl applications, specialty materials should be considered with one of the most common being Hastelloy C276. This high performance nickel alloy performs well in many HCl environments and far outperforms 316 stainless steel.However, as temperatures and concentrations increase, even Hastelloy C276 has its limitations and will be decimated by corrosion. If the fasteners are hard to replace or cannot fail, more exotic materials likeZirconium or Tantalum fasteners could be considered. While these materials are significantly more expensive, there performance, especially tantalum’s, is second to none.