SS to teflon wear question

[johnv929]

We are installing a teflon rotary seal on a piece of pharmaceutical processing equipment. Preliminary test results showed some wearing of the seal and possibly of the shaft, since there were white and grayish particles being shed after very few revolutions. There also was a noticeable amount of heat buildup. The shaft is currently 316L (our standard metal for the interior of processing equipment). The seal people recommended a shaft hardness of Rc 35 or greater, so I'm concerned that the wear may be due to the shaft being too soft. I'm looking for other alloys that might work.

This is a one off project (we are the user, not the manufacturer of the equipment), so price is not a huge factor, although a somewhat standard material would be better.

The material does not necessarily have to be weldable.

We will need to machine it. A 8 micro inch finish is recommended.

It does need to see wet service (water plus KOH cleaning solution), so corrosion resistance is important. I have tried martensitic stainless parts in this machine before and they were not acceptable.

Coatings would be OK as long as I could be sure that they would remain adhered.

Thanks in advance.

 

[Metalguy]

What was the failure/corrosion type of the martensitic SS, and which one (and its heat treatment) was tried?

316/316LL doesn't make a good rubbing surface at all. Much better is something like Nitronic 60, which has about the same corr. res. as 304. What is the Cl conc. in whatever you're processing? If low, with no chance of conc. by evap., 304 equiv. corr. res. should be fine.

 

[kenvlach]

Electroless nickel (typically, Ni~8%P alloy) is very resistant to alkaline solutions.
For the shaft, try electroless nickel plating heat-treated to get desired hardness (Class 2 per MIL-C-26074, AMS 2404 or ASTM B733).
For the seal, use electroless nickel/PTFE composite coating. Since there seems to be heavy loading on the seal, I recommend a 10-15 vol% PTFE co-deposit, rather than a higher % PTFE content.

For a good article on properties and specifications for electroless nickel plating, see

http://pfonline.com/articles/pfd0318.html

For products of one manufacturer of EN + PTFE, see

http://www.macindustrialproducts.com/enproducts.html

 

[TEV]

What are the dimensions of the shaft? Is it complex? I might be able to help.

 

[johnv929]

The previous material was 440C, used in a drive pin. It did rust.

The wet service is during cleaning and our concentration is about 1% KOH and it is immediately rinsed off. The shaft is 1.5" in diameter.

 

[Metalguy]

I think you should follow Kenvlach's advice on this. I've seen HT'd electroless Ni coatings bent around a very small radius, and there was no flaking or disbondment at all. These coating are VERY hard.

Sounds like you can't have even slight rusting, which I am assuming is what really happened to the 440C. The "dry" time of exposure--which appears to be where most of the time is--just how dry is it? Any materials in it which are known to cause corrosion with SS?

 

[johnv929]

Well,
I decided to follow both of your advice, sort of. The seal engineers recommended that we use nickel coating on the shaft, but said that their food grade seals are virgin teflon, so I don't think there would be a possibility of coating the seal??

They also are reducing the seal loading by going to a lighter back up spring.

I have some Nitronic 60 left from a different application on the same machine. I am having a shaft made from that to test, and I'm thinking about testing it without coating, and coating it if it shows wear. Your thoughts??

 

[Metalguy]

I think either solution will work better than what you had before--especially since you don't *seem* to have a Cl problem. But metal wear can be either abrasive (hard particles) or adhesive (micro welding, galling, etc).

Nitronic 60 is very good at resisting the latter, but not so good with the former--which is what I think you have. The Ni plating is very good for abrasive cond., which is why I suggested you try that approach.

 

[johnv929]

Thx, I just realized you were asking about Cl (Chlorine/Chloride) concentration. It is very low to zero, we final rinse with DI water so any residual chlorine from the city water is immediately taken away.

We are going to electroless nickel coat the shaft, so I will have nitronic 60, nickel coated shaft to test.

Thanks to everyone that responded, it looks like our next test opportunity is 3 weeks from today, I will try to post a follow up with the results.

John