According to the University of California Santa Barbara, iron rusts faster in saltwater because of the conductivity of dissociated ions and electrolysis actions accelerating corrosion. In layman's terms, the rusting process requires electrons to move around, and the electrons can more easily do this in saltwater.
The iron borrows electrons from neighboring compounds, charging the metal atoms so that they dissolve in the water. Seawater also contains deep ocean bacteria that eat the iron and produce rust excrement, according to a publication sponsored by the Department of Energy. Iron also corrodes in terrestrial environments, such as desert, tropical and temperate climates, but moisture plays a large role in corrosion, so the aquatic environments of saltwater and freshwater corrode iron at a faster rate.