There are many paths, and some depend on what your final target is.
Many would say that a degree in electrochemistry may be best, as most corrosion mechanisms are electochemical in nature.
Another may suggest a degree in corrosion science and/or engineering.
I came to my present "state" in a very indirect manner; my BS was in chemistry, my masters 5 years later was in mechanical engineering where I specialized in materials and corrosion. Can't say I would go so far as to actually recommend this path, but I've found there arte certain aspects, or perhaps insights, associated with a science degree you won't get with an engineering degree, and vice-versa.