With a Triumph cutter I just purchased, I was given a spare blade that was apparently just sharpened before the machine was taken out of use a couple years ago. The actual leading edge looked ok, but the rest (including 3/8" onto the angled portion approaching the sharpened edge) was covered in rust. What causes this? (the blade in the machine is rust-free.)

Not knowing what to do, I removed the rust and sprayed it with oil that says "displaces moisture and inhibits rust" on the can hoping to keep it ready for use, but the rust returned in 2 days already

What causes this? Can I fix this and keep the blade from rusting so quickly/badly?