Dissimilar metal threaded joints-SDSS to CS & Cu-Ni to CS 1

[cjabhijit]

Dear Experts,

Pls help me with this, I have a threaded connection in the Fire Fighting system where we are connecting CS Blind 2" flange with a 3/4" threaded hole where we are connecting SDSS/CuNi 3/4" Nipple/Pipe NPT(M).

I believe we will have a corrosion issue if we do this because of dissimilar metal joining together.

Can anybody support me here quickly?


Thanks & Regards,
Abhijit

 

[EdStainless]

In either case the SS or the CuNi will be more corrosion resistant than the CS.
The result will be accelerated corrosion of the CS.
How accelerated depends on the water.
If the water is high conductivity the situation will be worse.


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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Isolate the contact (Teflon or barrier coating on threads) between the dissimilar materials and you have no galvanic corrosion concerns.

 

[cjabhijit]

Finally I believe my concern is right this should not be done. I will take it up with materials to conclude the same.

Thanks & Regards,
Abhijit

 

[EdStainless]

I would not try to rely on Teflon tape to isolate, you will cut through it some place in the joint.
These joints are made all of the time, in fresh water.
If this water is more corrosive then you have bigger issues (the CS piping).

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

I second Ed's recommendation. The galvanic issue is only a problem if your carbon steel selection is already marginal. And nobody kid yourself- Teflon tape used on an NPT joint is not at all a reliable means of galvanic isolation. If you need isolation you need to use a galvanic union.

 

[metengr]

Just a point of clarification, Teflon tape was not mentioned, there are thread seal compounds which contain Teflon and would do very well in this application. I do agree with a galvanic union.

 

[moltenmetal]

I've never met an NPT thread sealant which would act as a reliable galvanic isolation method. Very interested to know what you're talking about metengr- obviously you have some experience with this.

What is often forgotten in galvanic corrosion issues in pipe is earthing/grounding. If both ends of the pipe are earthed by, say, intimate contact with pipe shoes or the like, then isolating the joint between dissimilar materials does no good anyway.

 

[CoryPad]

NPT joints are difficult to isolate the two members because the thread form uses an intentional interference fit at the major diameter.

 

[moltenmetal]

CoryPad: your experience matches mine. There is always at least some metal to metal contact in an NPT joint (unless one of the parts is plastic), or else it is insufficiently tightened and likely to come loose with thermal cycling or vibration.

 

[metengr]

We have used a similar sealant below for our plant fire protection piping systems containing small dissimilar threaded metal connections. No leaks or galvanic corrosion has been reported.

http://www.fppi.com/assets/documents/data/PipeFit_AS.pdf

 

[cjabhijit]

metengr,

Thanks so much for the info. However, why just can't we eliminate the route cause which creates the possibility of leaks due to corrosion.

So I am thinking of elimination this threaded joint itself.

I have asked Material Engg for the same, will keep you all updated.

Thanks & Regards,
Abhijit

 

[moltenmetal]

Metengr, that's just ordinary anaerobic pipe thread sealant, which we use in large quantity. The likelihood such a joint ends up without metal to metal contact is near zero.

Btw we ignore the advice not to use it in combination with Teflon tape, based on the advice of a Dow spec. Twenty years in, we have found that the combination of dense Teflon tape top dressed with anaerobic sealant is the best thread sealant combination- everything else we've tried, and we've tried them all, is inferior.

 

[metengr]

moltenmetal
I will have to try the combination.

 

[cjabhijit]

As per the advice from Material Engineer, we are eliminating this threaded joint in between 2 dissimilar materials (i.e. SDSS to CS) as it is a definite source of corrosion. Instead of having a having a Blind flange with threaded hole to connect the threaded SDSS pipe we are now using Reducing flange of SDSS & welding pipe to it.

This is making it a better joint-No corrosion issues.

Thanks for all the replies, it helped me a lot.

Thanks & Regards,
Abhijit