I just came across this post and thought I might add my thoughts as I know a little about Metallurgy (Classics Monthly even claimed I was a " boffin" a couple of months back
).
Stainless is more noble than mild steel, so if you put the two together in the presence of an electrolyte (water) you will get a galvanic reaction (think battery) and it's the mild steel that will go all crusty. Aluminium - Stainless is also not a good idea.
Welding Stainless is a specialist job. The main problem is that when welded the Chromium in the steel forms Chromium Carbides at the grain boundaries instead of being nicely alloyed and preventing the steel going rusty. So the Chromium is now not doing it's intended job, on top of that it left nice brittle chromium carbides on the now susceptible grain boundaries. Corrosion can then attack these chromium depleted areas aggressively. Internal stresses resultant from the welding can also result in stress corrosion cracking. If you know what you're doing these effects can be somewhat be addressed by factors such as alloy selection, filler metal, post weld heat treatment etc (but now we're getting really technical)
One way of reducing the issue is to use low carbon Stainless alloys (less carbon = less carbides) e.g. 304L or 316L modifications to the popular 304 and 316 alloys.
As for welding Stainless to mild steel I'd say you need to keep a close eye on that, especially if it's structural.
So stainless is not as ideal as we might hope. Let's not forget, the best way to avoid corrosion is to isolate the parts, a robust paint scheme not only keeps out the environment but electrically isolates one metal from another.
I hope that's of some use to anyone that's interested.
Rich