Vinegar has been known for a long time to dissolve rust. I've used it to get a rust stain out of vintage fabric and it worked (needs to be dabbed on with cotton bud and left in the sun). Lemon juice is a slightly less acidic (and less effective of course) alternative. The black wouldn't bother me: Looks like copper oxide. Is the bobbin case (nickel or chrome) plated copper? It's definitely an oxidation, but like many others have said you should be able to get rid of the oxide layer.

it is stainless steel i assume. i clean it with sand paper, buffed it, sanded again, buffed again and didn't make it shiny. the oxidation is still there. used brass so many times but nothing happened.

Consider me warned then :shock:

The black is a form of iron oxide. The black stuff is stable, unlike red rust. Basically you have a parkerized finish on your bobbin case now. It should polish off with no harm done. Most chemical rust removal methods will convert the rust to the black, stable form of iron oxide.
Rodney

I once put my wedding rings and two others in jewelry cleaner and forgot them for about 2 weeks. To my horror when I pulled the out they were the same color as your bobbin case. I was devastated!!! I ran them to the jewelers, they told me that the cleaning solution had eaten the rhodium plating off of the gold. I had no idea that gold rings were plated with rhodium to give them the finish that mine had. They had them refinished, I learned an expensive lesson (to this day I have NEVER told my husband what I did!)

I'm more surprised your husband didn't notice you weren't wearing your wedding ring for two weeks.

He knows I have finger swelling issues and that I sometimes can not wear my rings for days. He just never asked. So I kept quiet. He would have scolded.

hard lesson for me also. i kept them deep in the vinegar for couple days thinking that it will help and ease up the removal of rust but no can do. now i know better. thanks.

@rodney - hmmm accidental discovery of how to oxidize your metal and give it a patina look. problem with this is the metal feels so rough. i will see how i can salvage this case with brass. i will never use vinegar again. alcohol, kerosene or metal polish it is for me now. thanks a lot.

Vinegar soaking has been mentioned on some home remodeling sites for "aging" shiny brass hinges and knobs to give them patina. I never thought about it for loosening rust.

Would tomato juice/paste restore the metal, works on copper?