Why would a metal corrode more quickly in salt water than distilled water?

Answer 1

This is due to salt water containing more ions that engages in significantly more reduction-oxidation reactions with the metal compared to distilled water.

Answer 2

Corrosion of iron is the same as rusting of iron, which is a redox process involving the oxidation of neutral iron to iron(+3) as well as the reduction of oxygen. Redox processes involve electron transfer through solution, which requires the presence of ions to carry the charge. This is why sea water, which is full of salt (ionic) will cause automobiles to rust a lot faster.