Silver Solder and Star San

Does star-san dissolve silver solder?

I wanted a thermowell for my corny keg so I bought a Sta-Brite silver solder kit as recommended in the big silver soldering thread. I soldered a stainless-steel screw onto the end of the dip tube so that it would still fit down into the post boss on the keg.

I tested the solder by blowing into the top in with the rubber-tipped blowgun for my air compressor, putting 100PSI into the end while looking for bubbles with the soldered end in a glass of water. None.

I then filled the keg with a weak star-san solution and put the thermowell in it for about 5 hours. When I took the thermowell out of the keg and turned it upside down, water poured out! It was completely full of water, even though I had just tested it with 100PSI compressed air, and it was under no pressure in the keg...just what, 18 inches of water or something (1/2psi according to google). I was able to grab the screw that I had soldered to the end with a rag and yank it off with my bare hands.

The only thing I can think is that the star-san attacked the solder. I know that star-san uses phosphoric acid. Phosphoric acid is used as a soldering flux for soldering stainless steel. Is there something about star-san that attacks or weakens solder joins?

I have seen soldered joints pass a quick pressure test only to fail sometime later. What I think happened is that the solder did not flow properly to completely wet the joint and a small area was sealed only with the flux that flowed there. If something then dissolves the flux, then you have a leak.

If your screw had threads all the way to the head that may causing problems. I would try cutting off a SS bolt that just fits into the ID of the tube so that you have a little more area between the layers of metal to be fully wetted with solder. Use enough heat so that it draws plenty of solder into the joint. If the bolt head is then a little bigger than the OD required, it can be reduced easily with abrasives.