Stainless steel is just fine in a copper system if it’s not electrically connected to the copper. Even if it is, you are unlikely to have problems unless the wetted area of the stainless steel is comparable to or larger than the copper, in which case you may accelerate corrosion of the copper.
You won’t have the problems you would have with galvanized steel. Copper ions leached out of a copper pipe will react with zinc and non-stainless steel and can very quickly degrade under some conditions even without electrical contact. Stainless steel, not so much.
(Don’t let rainwater that came from a copper gutter or that ran over treated wood hit a galvanized steel flashing. Really, just don’t use galvanized steel flashings at all.)
Stainless in (or near) contact with copper with moisture between them forms a battery, which will corrode them.
Metals in a plumbing (i.e. wet) system need to be either isolated from each other or be galvanically compatible.
This applies to roofing, too. Nail the flashing down with galvanically compatible nails, or you'll find the flashing all comes loose after a couple years.
> Stainless in (or near) contact with copper with moisture between them forms a battery, which will corrode them.
Not really?
A battery has two paired half reactions. Something gets oxidized, something gets reduced, and electrons flow through the conductive bits.
So if you have some metallic zinc, and you have some copper ions, and you have some metallic copper, and you have electrical contact, zinc will be oxidized, copper ions will be reduced and plate into the copper, and current will flow. Once the zinc is gone, if you had galvanized steel to start with, the iron will oxidize.
Now try stainless steel. The half-reaction involving the stainless steel, hmm, doesn’t involve iron — the iron is physically separated from the electrolyte by the passive layer. Chromium? Barely - a tiny bit oxidizes and then it stops. We’re left with 2H+ + 2e- -> H2. Which can happen without the stainless steel too, but with more available surface area, it’s faster.
So a lot of stainless steel will accelerate the corrosion of a small amount of copper. But the reverse has little effect.
There is apparently a real concern the with H2: some grades of stainless steel are apparently rather susceptible to hydrogen embrittlement.
I'm not a chemist, but I believe Boeing when they told me never to place copper in contact with stainless, due to galvanic corrosion. Corrosion is the mortal enemy of all airplanes.
You'll also find the same if you google for copper and stainless galvanic compatibility.
You won’t have the problems you would have with galvanized steel. Copper ions leached out of a copper pipe will react with zinc and non-stainless steel and can very quickly degrade under some conditions even without electrical contact. Stainless steel, not so much.
(Don’t let rainwater that came from a copper gutter or that ran over treated wood hit a galvanized steel flashing. Really, just don’t use galvanized steel flashings at all.)