Nuts are often made of inferior metals, not the same stainless or bronze that keel bolts are. And, even with stainless nuts, sometimes plain steel washers are used with them.
So you might want to use a nut cracker, or a Dremel tool, to split off just one keel bolt NUT ONLY, from the center of one of your keels. Then use some rust remover to clean up the keel bolt, and see if the bolt itself is a problem--or just the nut.
If the nuts are the only problem, remove them and replace them one at a time. Check for the size of the nuts and make sure to retorque the new ones to the proper torque range. I'd apply a generous amount of threaed lock (LocTite) to lock and seal the threads, then apply a generous coat of something like beeswax over the whole, to waterproof it. (Easy to remove for inspection, sticks well, durable enough, cheap.)