Most of the stainless steels we use are austenitic and contain at least 16% chromium. We know these as the 300 series (303, 304, 316L etc). When the chromium reacts with oxygen a passive layer of Cr²O³ is formed. This protective layer is what gives SS it's awesome corrosion resistance. Generally speaking he more chromium you have the more corrosion resistant the alloy will be.
Passivation
Whenever you fabricate with stainless steel you run the risk of leaving impurities in the alloy. These impurities will inhibit the formation of the protective oxides and can lead to corrosion.
The passivization of stainless steel is an important part of maintaining it's corrosion resistance. The process removes contaminates and restores the self-repairing oxide layer after machining or joining.
The basic procedure involves cleaning off contaminates such as grease and oil and then soaking the piece in a acid solution. Nitric acid is usually used, though I've been hearing more about citric acid as a 'green' alternative.
Welding
Welding stainless steel can be difficult if you aren't prepared for it. If I can help it I like to use one of the low-carbon varieties designated by the letter L after the number (304L). The lower carbon content improves weldability.
While stainless can be successfully welded using stick, MIG or TIG I prefer TIG due to it's excellent weld qualities. I use DCEN with pure argon and 2% thoriated tungsten. Filler material is also important and can be recommended by a welding supply house.
You must make sure the joint is clean from contaminates. If you need to use a wire brush to clean make sure it's made of stainless to prevent foreign bodies from being embedded. If you use solvents make sure they are completely evaporated before you begin and that all moisture is gone.
I've been told that one amp per thousandth of thickness is a good place to start when selecting your current, though your mileage may vary. I generally don't bother preheating or post heating on material this thin. If I were going to weld a thick piece to a thinner piece I may preheat the larger piece a bit.
I went out to setup the still for a cleaning run and I discovered that the lid had some rust where the SS meets the copper. I knew better. This isn't a complete guide by any means. Just a little food for thought.
Shame on me
RD