Citric acid for mill scale removal

100% this. If simply equating cost and taking shelf life / storage into account, citric beats acetic hands down. It does turn green in the long term, but it also doesn’t STANK up your work area.

If you really want to, you can chelate the acid.

I use a cup or two to a few gallons of water and just let it work, using a shallow cement mixing plastic tub from a certain hardware store.

Rinse the parts well in clean water afterwards and do whatever work you need to, or coat in oil, or they will flash oxidize.

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