Hi Jurgen,
The oxide layer is so incredibly thin that I really doubt that is does any harm to the boiler.
In fact, if you are not careful it can be wiped of just by rough handling.
It actually happend to me and I have re-done some various times, never noticed any damage by the chemical agents.
I did noticed that the soldered parts do show some reaction with the chemicals, preventing proper oxidation of the brass, but no damage.
Regards,
Maurice.