304 spot corrosion, why?

[badservo]

I have a 304 stainless dip tube in a 10 gallon pressure pot that has developed a hole and would like to know what could have caused this to happen. The tube is 5/8" OD and .035" wall, it is welded to a 316 fitting to thread into the pot which is also 316. The only liquids that it has been in contact with are water (for initial cleaning) and common PVA glue. The tube was installed 3 months ago ago. My only guess would be that the ground clamp was placed there when welding the tube to the fitting and something on the ground clamp has caused this. The weld and the rest of the tube look to be in perfect condition. So what would be the most likely cause of this?

http://files.engineering.com/getfil...-08c762916823&file=IMG_20140615_134701543.jpg

 

[CoryPad]

The PVA would go through the tube, yes?

This appears to be pitting corrosion initiating from the outside. Are you sure you have identified all possible corrodents? Was the part pickled and/or passivated after welding? How far away from the weld are these pits?

You should have a complete metallurgical evaluation performed on this piece to determine the root cause(s).

 

[badservo]

PVA covers the outside of the tube and flows through the tube. I am 100% sure that city water and PVA glue are the only liquids we have put in contact with the tube, will ask glue manufacturer about possibility of contaminated PVA. The part was not pickled or passivated after welding. The pits are about 15" away from weld. The rest of the tube and the fitting show no signs of corrosion.

 

[Compositepro]

Looks like pitting caused by galvanic reaction. Most likely a spec of carbon on the tube. Perhaps from some charred PVA.

 

[CoryPad]

Temperature of the PVA?

 

[EdStainless]

Or char on the tube from manufacture.
Make a replacement and have it pickled before you install it.
If there is something wrong you will see it then.

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Plymouth Tube

 

[badservo]

Compositepro:
Tube was new when welded. The ground clamp could have cause some surface contamination.

CoryPad:
The PVA is 40F to 100F (ambient conditions).

EdStainless:
I will replace the tube, pickle and passivate.

I will report back in 3 or 4 months with the results. Thanks for the help.