I have done this project many times and I get the same answer..it creates an electrochemical reaction.. Normal tap water will only make a thin top layer of rust over the entire surface or until it can't rust anymore.. But the salt water will rust a thin layer over the entire surface and then make rust under that layer and then under that layer ect.. Until there is no more "medal" or whatever you have rusting.. That is why its makes it more brittle in salt water than in tap water.. But tap water will cause rust to form faster but the salt water will continue to eat away at the "medal" when the tap water can only cause a thin top layer.. But this also depends on several factors.. The temp of the water.. And the surface area of the "medal" you are rusting.. How salty the water is.. If you stir the water or leave it to settle.. Ect.. The hotter the water and more salt and if you stir the water to cause oxygen to move around the faster it will cause the "medal" to rust..