I have tried to remove common red iron rust from a small piece of metal that looks like steel.
I submerged it into hot water with a bit of citric acid, and let it sit overnight. In the morning I have no rust, but the thing is now covered in dark-black oxide.
Was that possibly steel that I was cleaning or something else? How do I remove that black oxide?
How should remove rust in situations like this in the future?!
left - piece from the same metal that has no rust, and was not cleaned, right - piece that was cleaned