You probably have a leaking head gasket and coolant has leaked into the hole where the head bolt goes the head and maybe further down into the threads in the block. Depending on which head bolt is stuck can make a bit of a difference. If it's near one of the side covers on the block, you may be able to get a torch on that area and heat it until the bolt starts to turn. I've seen this before and heat was the only thing that broke it loose.
Forget about penetrant, it won't work.