Any tips for cleaning this iron key-shaped item?

[HollandsBrook]

Dug this iron object near an old cellar hole last weekend and would really love to clean it up. I'm assuming it's a key but I think it's made of iron since it's magnetic. Tried about 20 minutes of electrolysis and didnt really seem to help much. Any help with cleaning tips would be much appreciated!

Thanks!
Marc

 

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