My thoughts are regardless of corrosion potential, you’ve got a chipped new frame. Are you chasing for at least a partial refund?
Otherwise, back it goes and a new one please
I think it would be rather unfair to accept the sellers partial refund to only go back and say it's not good enough. If I sold something and it arrived damaged I would offer a part or full refund but once that was accepted I would consider the matter closed.
My thoughts are regardless of corrosion potential, you’ve got a chipped new frame. Are you chasing for at least a partial refund?
Otherwise, back it goes and a new one please