It was found on land that was suppose to have a plantation home on it at one time. It burned down some years ago, I found an old smoke house that was still there. I thought I could see a y and a 0 on it before I tried to clean it but can't find it now. Any insight would be appreciated. Thanks.
ALL "slave tags" were made of thin copperbrass sheetmetal -- so your iron disc absolutely cannot be a "slave tag."
The very-few actual genuine "slave tags" which exist were manufactured in Charleston South Carolina, for use in that city's "slave hire system," and have that city's name stamped into the copperbrass sheetmetal tag.
Unfortunately, fake ones are all over the place, particularly seen for sale on Ebay. See the photo below, which shows a few of the many varieties of fake ones.
You asked about how to clean the rust-encrustation off of your iron disc. I know TreasureNet contains info on cleaning iron relics, but I do not know where. Perhaps somebody else here can provide a link to that info at TreasureNet. The two iron-cleaning methods I use are Electrolysis and Zinc-&-Lye Bath.