I got this off off the internet:
I use a white vinegar solution 2 parts vinegar 1 part water to clean
the rust off cast iron pans. I collect antique cast iron pans and
after cleaning the old grease off them many times they are rusty
under the old seasoning. I soak the pan overnight in the vinegar
solution and the vinegar just dissolves the rust. Very rusty pans
many take an extra evening. Don't leave it in there too long or you
will not have a pan. The vinegar actually dissolves the metal. This
would work on new cast iron pans also.