the only acid that would harm your brass would be nitric which is also a oxidizer. It would eat the patina the oxidize the brass underneath then eat that patina etc etc. Citric acid will just eat the oxidation off. Same for acetic acid (vinegar). I heard, but have not verified, that citric acid will leave a coating on your brass which will prevent oxidation reoccurring
I think on another pin tumbler thread the guy gives his recipe for how much lemishine/detergent. I have never tried this but I bet you could take a case then let it sit in pure lemon juice a week and it would be fine.
By the way ammonia is a base not an acid and it reacts with the copper in the brass. A bit of trivia here but if you ever want to remove a copper plating from something for some odd reason soak it in ammonia. Want to make your plated bullets turn blue?
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