When it comes to protecting major infrastructure, corrosion prevention can get very complicated. Even so, the most basic principles of corrosion are quite simple. This video from The Fuse School explains why certain metals corrode faster than others, and how a metal's reactivity can be used to used as a method of preventing corrosion as well.
VIDEO: The Basics of Corrosion and Protection
Takeaway: Metals are considered corroded when they react with water and/or oxygen in the air.