I know you guys know a lot about metals, I hope you don't mind the OT post.
My mother has a pair of small, old metal bowls, I assume they are aluminum. Last night we put canned Salmon cat food in them. When the cats were done, I rinsed the bowls out and put them in the dishwasher. When removed from the dishwasher, the bowls were discolored, a dark grey or black. The discoloration is mostly on the inside- outside is pretty much untouched. We have used the bowls this way many times before, with no problems. I don't understand what hapened. They are 40 years old or more, and I would like to save them. I would really appreciate any suggestions on restoring these bowls. Thanks!!!
Regards, Tom