There need be no worries about corroding copper or or brass when pickling with citric acid solution. And the acid is safe - even if a little bitter tasting. Only a short pickle is needed after quenching to remove flux. A long pickle (24hrs) will result in all the oxidation being removed leaving copper a lovely salmon pink - after scrubbing in water. That's how it worked out with my small steam engine boiler. It's now oxidised nicely looking grubby again.
For the carbon removal I believe it wasn't the acid, just the heating to dull / medium red heat then quenching when it had just cooled to black again.