You can run into situations where copper can be pulled off the walls, and find it's way back into the boiler. This sets up a galvanic cell that looks very similar to oxygen pitting. This is bad news, and difficult to deal with. I know of one application where copper tube was used for heat tracing. The thermodynamic traps kept blowing through, because they were getting copper plating out on the seats and discs. They changed the tracing tube material to SS, and the problem went away.
Steam in copper is not a problem. Condensate in copper, in and of itself, is not nearly always a problem. But the table is set, and just waiting for another factor like boiler carry-over to get the party started. I would NOT use copper for condensate service. If carbon steel won't stand up effectively (and in most cases, it will), then go to SS
Cooper in steam service is not always a good solution. Cooper is very sensible to corrosion by the typical conditioning chemicals (NH4 at high pH, morpholin ect.) used in steam/condensate system systems.
The best way is to control the conditions of your system (deareation, chemical conditioning) in this way you will not have CO2 or Oxygen that makes condensate very agressive (Low pH).
