Coating Material for Sulfuric Acid

[qaisarabbas]

Hello folks,

We have a situation where a plunger coating of a PD pump is found demaged. Details are as below:

Service: 95~100% Sulfuric Acid
Operating Conditions: 100 psi & 50C
Plunger Material: Casted SS-316
Origional Coating: Ceramic (Ti based)

Now we are intending to carry out metallic coating (thru weldoverlay) on the plunger instead of cemaic coating. Few options to our knowledge are:
1. Hastealloy
2. Stellite
3. Nitronic 50
4. Inconel

Please suggest the suitable option in our case.

Regards

[arcpro]

Without knowing the properties of the original coating (hardness, corrosion resistance, strength, bonding), any comment on any of the suggested option would not prove to be helpful.
I would recommend to have a word from vendor before deciding anything.

[ben]

Austenitic stainless steels would survive the service conditions you have mentioned.
Check the wear resistance required to have good surface hardness of the coating.

[qaisarabbas]

We have requested the OEM to suggest the alternate option.
However, the hardness of Ceramic coating is measured around 400 BHN while the substrate (316) is around 200 BHN.
No doubt, austenitic SS (like Alloy 20) can be the best option as far as corrosion resistance is concerned. However, its wear resistance can not be commented upon.

[mechcolor]

Abbas,

All the four you mentioned would not 100% meet the requirements because of one reason or the other.

Thermal spray coatings would be the way to go for achieving both required wear and corrosion resistance.
For normal components to operate at the given concentration of sulfuric acid, it is not a big problem to suggest some good materials based on experience but for plunger coating, it's better you seek the opinion of vendor. He knows best about what's in the heart of his machine.

[qaisarabbas]

Thank you all.

[qaisarabbas]

OEM did not suggested any metallic weld overlay in place of Ti-based ceramic coating as a permanent solution. However, to meet the urgency at plant, we have to carry out a temporary short-term solution.
Now, if I consider the basis on two factor: Corrosion resistance & Hardness, following weld overlay options seems to be shortlisted:
1- Alloy-20
2- Hastelloy C
3- Titanium Alloy (ASTM B348 grade 5)
Among these, Ti-alloy seems most appropriate with the highest hardness value of around 335 BHN (compared to ceramic hardness of 350 BHN) with acceptable corrosion resistance against concentrated sulfuric acid at ambient temperature. Should I recommend weldovery of around 5~6mm using GTAW (with preheating & without PWHT)? What are your comments?

[mechcolor]

You want to overlay SS316 plunger with Titanium?

[qaisarabbas]

Yes. Is it the acceptable option? Please advise.

[mechcolor]

Welding steel with titanium is very difficult due to the low solubility of iron in alpha titanium at room temperature. When titanium is welded with steel the intermetallic phases TiFe and TiFe2 form, which are very hard and brittle and prevent the production of technically useable welds.

One way to achieve ductile welds of steel and titanium is to use intermediate layers of materials capable of being welded with both titanium and steel, without brittle phases occurring. One such material is vanadium. Titanium/vanadium/steel joints have been produced successfully by resistance spot, electron beam and diffusion welding.

Another way of making welds between titanium and other metals such as stainless steel or aluminum is friction welding. In most cases however a loss of strength in the weld must be expected.