Discoloured PTFE lined piping??? 3

[daviedalton]

Folks,

Don't think this is a corrosion issue as such, but I'm failing to find a satisfactory explanation anywhere else.....

We have what used to be virgin white PTFE lined piping, which would be exposed to almost everything you can think of , solvents, acids, bases, liquids and vapours at elevated temperatures. During a recent inspection, the internals of some lines were found to be a rust brown colour. The discolouration was distributed pretty evenly over the surface of the lining and could not be cleaned off.
One theory here is that the discolouration is a result of the PTFE being permeated by something. However after several hours surfing I failed to find anything on the internet that even suggested that PTFE could become brown. Is this a credible cause? Has anyone seen this before? I'm a little worried about the impact on the mechanical/chem resistance of my brown teflon....

PS this isn't a new issue for us, but we've never properly examined the cause behind the phenomenon..

cheers
dave

 

[EdStainless]

Shave a sample and test it. I'll wager that it is rust in the PTFE. PTFE will have micro-holes in the surface that can trap stuff. I doubt that the PTFE is deteriorating. You could check that with DSC and make sure that it is still sound.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[ApC2Kp]

daviedalton,
While PTFE has resistance to a wide range of corrosives, it is not perfect. The porosity of the sintered grade of material results in permeation and the absorption of small quantites of various fluids. Most people think of PTFE with its non-stick properties. PTFE is able to marked - there is a manufacturer of expanded PTFE gaskets that labels its products by using an ink with methanol solvent base. If your discolored PTFE does not leak, then liners are probably OK for continued use.
There is sales literature from

http://www.dr-schnabel.com

with photos showing the blistering and discoloration of PTFE linings in pipe. Of course, their product is shown with less blisters.
The PFA Teflon liners have less permeation. If the service temperature requirements are lower that could be met with a Kynar PVDF liner material, then permeation might be reduced to negligible level.

 

[bonzoboy]

PTFE does turn brown, I am told, upon reaching 400 F. Maybe you had a chemical reaction in the pipe that caused a thermal excursion.

 

[kenvlach]

Determine the contaminant by leachate testing:

Find two, separated sections of pipe that can be easily valved off.
Rinse well with DI water, blow dry with filtered air or nitrogen.
To one section, add spectrometer grade nitric acid.
In the other, add spectrometer grade of an organic solvent that hasn't been used in the piping.

After 24 hours, have the nitric analyzed for metals & other inorganics, and have the organic solvent analyzed for organics.

 

[daviedalton]

Thanks for the theories guys.....
Ed, rust should be a big no-no. The only other materials of construction in the equipment are glass, Hastelloy C22 and tiny bit of tantalum. Our raw materials are also rigorously tested.

Bonzoboy, maximum temperatures we would see in that equipment would be 200 F. I'd be surprised if we hit the 400F without knowing it.

Kenvlach, sounds like a plan, but for a number of reasons (some political) I'm not going to be allowed to test the lining. (thats the ostrich syndrome)

ApC2Kp I checked out Dr. Schabel, but couldn't find any pictures of discolored PTFE. Can you help me out? It would be great to compare with our own issues.....



Thanks again guys

 

[kenvlach]

Absent any testing, the limited temperature suggests the color isn't from the PTFE itself. Rather, the coloration is due to a reaction between different fluids within pores of the PTFE. Possibilities include:
Neutralization resulting in a precipitate,
Oxidation or reduction resulting in a change of valence in a metal ion,
Decomposition resulting in a carbonaceous residue,
etc.

Set a two-way matrix (spreadsheet) of everything that had been transported within the piping. Enter Rxn in each cell where the x-item would react with the y-item. Then consider each Rxn product(s) color.

 

[EdStainless]

I think that Ken as this one nailed.
If you want to verify you could shave a small sample from the inside and put it through some extractions and analysis.
I am sure that it is a residue from your process being trapped in the surface pores.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[StoneCold]

daviedalton
We have seen the same thing. Teflon in multiservice chemical use will turn brown. I don't have a why for you. However I can tell you that the properties of the teflon are affected. The teflon will become more brittle and will develop cracks over time. I always figured it was related to tetrahydrofuran use. We keep the temperature below 150C but we still see the problem. Mostly is is a problem on our teflon lined butterfly valves and any expansion joints.
We don't have any teflon lined pipe.

Goodluck
StoneCold

 

[ApC2Kp]

daviedalton,
The photos of blistered PTFE liners were part of a presentation made during a visit. Another presentation had copies of information given at April 1999 NACE Congress at San Antonio, but without photos of blistered PTFE.
The photos showed the difference in blistering between the common isostatically molded PTFE, and the paste extruded PTFE used in their manufacturing process.
Their USA contact Dr.Ing. Marc Plinke was based in Kingsport, TN. but may have had email changed. It was @chartern.net Try contacting him by fax 919-749-1958, or thru their website.