IIRC, the remedy for this is to soak the inside in a solution
containing Alum (white powder used for leavening)

Tom Thomas wrote:
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