The combination of moisture, oxygen and salt, especially sodium chloride, damages metal worse than rust does. This combination corrodes, or eats away at, the metal, weakening it and causing it to fall apart. Saltwater corrodes metal five times faster than fresh water does and the salty, humid ocean air causes metal to corrode 10 times faster than air with normal humidity. Bacteria in ocean water also consumes iron and their excretions turn to rust. here