316 SS Retaining Ring Corrosion

[aggie80]

I am on a team that is designing a copper fittings system(elbows, tees etc.) to mechanically join copper tubing. We are considering using a 316 SS ring which would bite into the OD of the tubing. This ring would be subject to a possible 200 F in highly oxygenated domestic water. My concerns are:
- This ring is approx. .5 mm thick
- The system will be rated at 200 psi with a 3 to 1 safety factor
- It is the only structural member in the joint
Thoughts on this type of ring being exposed to those conditions for a 50 year service life?

 

[kclim]

It's possible you could come up with galvanic corrosion issues; 316 is more 'noble' than copper, although they are both pretty close in the galvanic series (I'm not too sure of how much of an issue it would be, but it wouldn't be as bad as 316 to carbon steel).

If the ring 'bites' on to the copper piping and creates a notch on the copper pipe surface, this could weaken the integrity of the pipe, especially as far as fatigue is concerned. Again, I'm not too sure if fatigue would be much of an issue with copper water pipes.

If the ring is subject to significant tensile stress (due to installation, or through operation), SCC may be an issue, given that the water quality isn't great (probably contains chlorides) and that the temperature is quite high (above 60 C).

Crevice corrosion of the copper piping may also be an issue if water can get under the tight gap between the ring and the pipe. I've seen a case where crevices have lead to dealloying attack on an aluminium bronze.

Apologies for the somewhat vague remarks. There's plenty of food for thought here though.

 

[bigc1304]

With the high temperature and oxygen content, the alloying
components of the stainless will probably be attacked as will the copper tube ID. I suspect intergranular and crevice
corrosion will present problems.