As you have implied, heating the metal is actually changing it near the cut and causing it to discolor. There's not soot or anything you can clean off - the metal is changed. You may be able to sand off the surface of the metal near the cuts to remove the discolored metal on the surface, but this will lead to a cut that doesn't look very clean and some discoloration may remain.
The other choice is to cut the metal without heat. Angle grinders don't really cut - they sand their way through the metal creating a massive amount of heat. One tool that actually cuts through the metal is known as a "nibbler". They come in many forms. Hand operated, air powered, electric, and as a small attachment for an electric drill.
They are basically a small punch that nibbles of small bits of metal. They do this very quickly so the result looks like a straigt cut. The issue is that many of the tools that I found top at at 16ga (1.5875mm) or 14ga (1.984375mm) sheet metal. Your 1.5mm sheet is going to be at the top end of their capabilities, but if you choose a quality tool and don't try to move too fast, I think you'll be ok.
As I'm fond of saying... it's stainless, not stainproof!
Typically an un-passivated surface only incurs cosmetic corrosion -- staining, and does not accumulate rust as such. Or if it does, the rate is extremely low.
Rust staining is still a rough and porous deposit, so it can trap bacteria and hence passivation is obligatory for food handling equipment, for example.
There are stainlessless alloys out there, with less chromium and nickel, still enough to slow corrosion, but not to stop it outright -- COR-TEN brand for example. They might be rated in terms of specified material loss rate under given weathering conditions, and take on a notably brown patina, but survive well for infrastructure (when you don't mind the staining). Hence many bridges, towers, sculptures, etc. made from the stuff.
Basically, you can imagine passivation, or the lack thereof, being a step on the continuum of such a process. 304, 316, 18-8, etc. are on the "negligible" end of the material loss rate spectrum.
Considered in this framing, even mild steel might be acceptable for certain applications, certainly so when the environmental conditions are more mild, or protection is available (oil, paint, etc.), but also where shortdefined lifetime is acceptable given the cost -- automobiles for example.
Regardless of alloy, you need the usual trifecta for corrosion: water, oxygen, and electrolyte. If this is an indoor application in most regions, I wouldn't worry about it, and something like galvanized rod with paint dabbed on the cut end will last ~forever. If condensing humidity is a common occurrence (as can be in some particularly rainy corners of the world, PNW for example), even the salt from ambient dust can suffice to catalyze corrosion, and galvanizing paint or dip might be desirable, or indeed using stainless.
I forget if you mentioned you're in coastal Boston or what. Sea spray, and, say if it's mostly indoors but like a garage regularly open to outside air, that would be marginal enough I'd probably want stainless, and passivation if it should look good or remain hygienic.
Tim
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