Subject: Why does iron rust more in fresh water than in salt water

I am working on a project to test the rusting in different types of water. I 
thought that iron will rust in salt water more than in Fresh water. I put a 
nail in a cup containg 3/4 of tap water and put another nail in a cup containg 
3/4 of tap water plus 1 teaspoon of salt. The nail was fully submerged within 
the water in both cases. The nails used were of the same type. 

I did another experiment. I put 2 nails in the salt water for about 3/4 hours. 
There was no rust formed. Then I moved one of the nails into a cup containing 
tap water. Within couple of hours, the nail in the fresh water started rusting 
while the nail in salt water remained same.

I hope you can help me understand this issue ( I saw similar question asked in 
1998 (ID: 887856161.Ch )in this forum but the answer was not clear to me, so I 
am asking again - sorry for the inconvenience)
thanks
Ram

Re: Why does iron rust more in fresh water than in salt water