Why do salt make iron rust?

Rusting is a type of corrosion process and is commonly used to refer to corrosion of iron by oxygen. When iron surfaces are exposed to oxygen, oxidation of iron takes place and metal is consumed. This is the reason we hardly ever see a piece of pure iron in the natural environment. Water speeds up this process by facilitating the movement of electrons. Addition of salt provides free ions (common salt, sodium chloride provides `Na^+` and `Cl^-` ) capable of moving around and further speeding up the electrolysis reaction of rusting. This is one of the reasons metal rusts faster in coastal regions. Places where salt is used to melt ice, such as the east coast of the United States, generally observe a higher rate of vehicular rusting compared to places where salt is not used.

 Hope this helps.