Metal finishing Q&As since 1989

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How to Seal Chrome Plated Pipe Thread Plumbing Components at high pressures

I am having a problem accomplishing gas tight seals with chrome plated brass NPT fittings at high pressures: 3000 psi. The threads are within tolerance and I am beginning to suspect that the problem is the chrome plating: since plain unplated brass fittings work well. I have tried all the usual means of accomplishing the seal: teflon tape and several different types of liquid pipe sealants. At the same time the sealing material must be compatible with 100% oxygen.

Roy Armstrong
Gas Handling - Downingtown, Pennsylvania, USA
2007

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First of two simultaneous responses -- 2007

If plain brass works well and plated ones do not, it has to be the plating.
My first guess is that you are having a high plating buildup on the first thread of the female part which is not allowing it to tighten completely despite the same torque applied.
On male threads, the apex of the thread will plate at least 4 times more than the rest of the thread and will frequently have tiny nodules that you need a scope to see. These will definitely not hold 3,000 psi.
Send pictures of the two parts, both plated and unplated and specific suggestions can be made.

James Watts
- Navarre, Florida

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Second of two simultaneous responses --

Could be a plating related problem. The plating will be thicker at the tops of the threads than at the root, and will change the geometry a bit. It's also harder than brass and may not deform as easily either. If you need the chrome for appearance, a plating shop could mask the threads and not plate them.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2007