Heat, water, and metal. Perfect recipe for corrosion. All you need now is some salt. And for those that say Stainless and Titanium don't rust corrode, I've seen stainless rust overnight in Okinawa if it's not coated, and I can walk outside of my office right this second and look at a titanium aircraft exhaust covered in rust corrosion.
Edit: Before someone goes all metallurgist on me, Titanium doesn't "rust" it oxidizes which actually forms a protective barrier. But it still looks like shit and if you remove that layer of oxidization it loses its corrosion resistance.