Tubing using acid to dissolve core

Hi all! i have a production problem to submit to the Ganoksin
Community. I am producing gold tube links using chloridric acid to
dissolve the iron inside but the gold surface turns in a dark, very
hard oxidation confusing the yellow and white gold links. Is there a
fast method to eliminate the dark oxidation from the gold tube
surface? I tried utilising the magnetic tumbler but without good
results. Any suggestions? Thanks very much for your help

Alberto