Subject: Does Iron Rust Faster and More in Salt Water or Fresh Water

hello, I'm working on my fifth grade science project to see if salt water 
rusts irons faster than fresh water.  I am using filtered water from my 
refrigerator -- for fresh water, and salt water purchased from my local 
acquarium store.  I am finding that in the earliest stages of my trials, the 
salt water does start to show signs of rusting first; however, by the end of a 
24 hour period, for any given trial, the fresh water has caused much more rust 
than the salt water.  The trials are lasting 72 hours each.  In perspective, 
at the end of 24 hours, the water color for the salt water nail is pale 
yellowish, while the water color for the fresh water nail is orange to dark 
orangish in color.   This tells me that fresh water rusts metal more than salt 
water, which is a different finding from what I'm seeing in the literature.  
Can you assist me with explaining why this is the case.  

Re: Does Iron Rust Faster and More in Salt Water or Fresh Water