Preventing diplation after soldering

Hey there. I hope i do it right. its my first time to ask here. I
have a question. Im working with 14k yellow gold most of the time.
when im soldering/annealing (coated with flux or boric acid) i have
places that get oxidized and when i put the piece in the sulfuric
acid pickle (room temperature) solution, the acid “eats” the copper
oxides and lieave me with a greenish layer of gold. I tryed prips
flux but still have some places that have this problem. I can polish
it off but it can effects crisp edges or textures. There is a method
to prevent it? or maybe an acid that will take off allso the silver
and not only the copper oxide? I hope i manage to explain myself
correct. Thank you all for any help!Great site by the way!