Sulfuric Acid ( H₂SO₄ ) is one of the most widely used and important acids, being an essential component to many industries. Equally as important is selecting the right material to survive this environment. This newsletter will provide options.

WHICH IS BEST?
That depends on BOTH concentration and temperature – TOGETHER
When looking at corrosion, the temperature of an application and chemical concentration are interconnected – always. You cannot consider one without the other. If the concentration of a chemical is high, typically the temperature at which a material can handle the acid will be lower, and vice versa – for applications that run at a higher temperature, the concentration will generally need to be lower to prevent corrosion. That said there are still many materials that can handle hot sulfuric acid:

Alloy 20 bolts deserve a first look for basic high temperature resistance. It works well in 20-40% boiling sulfuric acid.

 

 

 

 

 

 

 

Hastelloy cannot work in boiling H₂SO₄, but it can service high concentrations sulfuric acid to high temperatures typically below ~135°F.

 

 

 

 

 

 

 

 

 

Zirconium is excellent for high temperature sulfuric acid up to 300°F and 70% concentration. But beware. Zirconium bolts can readily corrode if exposed to any oxidizers – even trace amounts. (see chart below)

Tantalum is the ultimate protection from H₂SO₄ but comes at very high price. Therefore it is reserved for the most severe services. For ALL concentrations up to 98% at any temperature up to 350°F tantalum bolts will have virtually no corrosion rate.

 

 

 

 

 

 

 

 

 

 

 

 

 

HIGH STRENGTH
•  Hastelloy screws will not only perform excellent in high temperatures and concentrations, but also offers up to 100ksi of tensile strength.
•  For higher strength, you can look at Inconel 625. Inconel bolts offer slightly less corrosion resistance than Hastelloy but can provide 144ksi of strength.

POLYMERS FOR H₂SO₄
•  PTFE and PFA offer the highest level of corrosion resistance for polymers. The only drawback is they lack in strength.
•  For high strength corrosion resistance to sulfuric acid, PEEK screws can offer good corrosion resistance and are the highest strength polymer available.

AND CERAMICS?

Yes, of course we always forget to mention these. Ceramics are usable to 4000°F and are almost inert to most chemicals including sulfuric acid. So they can handle both high temperatures and high concentrations – but not strength. Though ceramic screws are the right material for many applications, they lack yield strength and can be brittle – so they are an option under certain circumstances.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q: I’m operating a FGD (Flue Gas Desulphurization) plant and am experiencing some mild corrosion on my scrubber bolting. I’m currently using Incoloy 825 bolts. It’s mild corrosion but non-the-less I’d like to have it cleared up before I get additional issues. Any suggestions?

A: Since scrubber liquids and gases not only contain sulfuric acid but also chlorides, you have double trouble. Incoloy 825 bolts are great for some parts of flue gas desulfurization systems but sometimes not where there is scrubber slurry because its so potent. Here you may want to try Hastelloy C276 bolts which have excellent resistance to sulfuric acid as well as the ability to withstand higher chloride content than other alloys before the onset of localized corrosion in a simulated scrubber environment. Corrosive conditions are also amplified on shutdowns when acid gasses could condense on corrosion resistant bolting.  I’ve also posted a chart for you which compares steel, Incoloy, Inconel and Hastelloy in terms of flue gas scrubber corrosion.

Q:  I’m running a lithium extraction process where a mineral called spodumene is roasted in a mixture of sulphuric acid (95-97%) at at about 150°C to form lithium sulfate. I’ve to use Hastelloy C276 for some process equipment but I’m still experiencing corrosion. Please help.

A:  Hastelloy C276 bolts typically work well in sulfuric acid but not at this temperature and concentration. Normally the max operating temperature for Hastelloy C276 at with H2SO4 at 97% concentration would be around 70C. For these conditions of sulfuric acid, I would suggest to go with tantalum metal fasteners. Tantalum is the most corrosion resistant option available to industry. It is very capable of handling sulfuric acid at any concentration 150C.

Q:  I’m running a municipal solid waste incinerator and am having trouble corrosion from a combination of hydrochloric acid and sulfuric acid that is derived from chloride and sulfur containing compounds in the waste fuel stream. I was using Duplex steel bolts for the chloride corrosion but they aren’t holding up with the condensation of HCl.

A:  Your biggest problem when dealing with hot chlorine and sulfur compounds is when they cool and combine with moisture to create HCl and H2SO4 acids. These droplets of acid could also get very concentrated as the moisture is evaporated off.  I would suggest a high performance nickel alloy fastener like Hastelloy C276. In regards to chloride corosion, Hastelloy should work very well in this kind of environment and is stable at elevated temperatures.  You learn more about Hastelloy C276 fastener by following this link.

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.