Corrosion from perspiration, skin oils, blood, and exposure to the elements are the problems we need to know well. In the case of carbon steel, these culprits can cause severe discoloration and rust very rapidly if neglected. I own swords that literally will rust before your eyes if left un-oiled. During a take giri (bamboo cutting) demonstration my students and I were performing, I had a drop of my perspiration land, unnoticed, on one of my Rapier (thrusting sword) blades. In just a few minutes, I was shocked to see a bright orange spot of rust on my hand-polished sword. This is a very serious problem the martial arts student must know how to combat. Even breathing on an un-oiled sword blade can begin the dreaded process of corrosion. The edge is the thinnest part of a cutting implement and the most vulnerable to neglect. If allowed to rust, a razor-sharp weapon will become dull in a short period of time. Genuine katana [are] famous for their polish and [mirror-like] finish. This is not for merely cosmetic appearance. Steel has microscopic surface irregularities that can collect moisture and corrosive elements. A finely polished blade has smaller irregularities and sheds blood much more easily than an unpolished one. Hence, the more corrosive agents that collect in the pores, the more tarnish and rust will accumulate.
Thank the gods for the Internet Archive Wayback Machine, else I could not have linked to the source blogpost.