Salt or more specifically salt solution can speed up the rusting process mainly because it acts as an electrolyte wherein it will allow the metal (iron) to lose its electrons more easily.

How do you speed up rust?

Direct sunlight helps speed up the rusting process. Plus, hydrogen peroxide and vinegar can give off a moderate level of fumes, so you'll want to work in a well-ventilated space anyway.

Why Does salt cause more rust?

In a sense, salt turbocharges water's effect on metal.

All this causes plain old rust, or iron oxide, whose molecules take up more space than regular iron molecules, so the oxidation process spreads.