I asked a friend of mine at MIT who works with high vacuum systems as part of his research on nuclear fusion. He strongly agrees with the prevailing opinion that zinc in the rebuilding room is a bad idea:
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Well, it looks like the zinc itself is not vacuum-facing, just in the oven surrounding the tube which is being pumped on. The bigger issue is what was already raised in the thread — the zinc melting off and potentially putting nasty vapor in the air and otherwise putting zinc flakes everywhere.
That being said, zinc inside the vacuum is such a bad idea even Wikipedia has a comment on it:
http://en.wikipedia.org/wiki/Materia..._use_in_vacuum
Notably, the zinc will almost certainly sublimate at your temperatures/pressures and can leave conductive deposits in places you probably don’t want them.
See also NASA’s regulation on zinc in vacuum for a more technical/authoritative source:
http://nepp.nasa.gov/npsl/Prohibited...rohibition.htm
Also prohibited are cadmium and (pure) tin:
http://nepp.nasa.gov/npsl/Prohibited/index.htm
This alone may be sufficient reason to keep the zinc flake from being produced in an environment where you are working on tubes — even if it weren’t direct threat in the oven itself, you probably don’t want it floating around your nominally clean workspace and getting into other tubes.
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I am happy to chip in to help pay for the cost of stainless and strongly suggest that we should go ahead and replace the galvanized.
-Matthew