im not discounting your theory but i would be hard pressed to fully believe that, with out being submerged in a solution that would cause this, or not cleaned well that this would happen.
but i might have a potential solution, since stainless steel is a rather noble metal, connecting a piece of aluminum to the stem (even wrapping a piece of wire around the down stem in a way it wont fall off and keeps decent contact) you should be able to prevent this (same idea as using zinc on a ship as a sacrificial anode). i got the idea after reading http://en.wikipedia.org/wiki/Galvanic_corrosion
. with the copper and steel i see how the steel rusts but i am not sure why with a brass/steel one why the steel is the one that suffers when the brass has zinc in it (given it also has copper so idk).
i would bet though that not drying/cleaning the rig leads to more chance of this happening that galvanic corrosion does on its own.
also looking at that weld, its bronze which does change how it reacts. i found this http://en.wikipedia.org/wiki/Galvanic_series
which shows which things corrode more easily, seeing as this is "stainless steel" it depends on the exact alloy of the steel but it could be to high to allow it to corrode before the brass/bronze, but honestly i would bet this steel is not that high quality thus it will break down before the brass. then again looking at this http://www.corrosionist.com/Galvanic...n_Seawater.htm
you could say that it would not break down since 410 stainless is pretty common.
idk i just dont think galvanic corrosion is going to be the cause here since you are usually talking about this happening in salt water or in unpurified water.
any way if you are really worried about it you can always rinse the hookah off after you clean it with distilled water and use distilled water on it and you shouldnt have to worry about it ever causing this