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

Are the nails of identical composition?  Are they clean of organic residues 
or other coatings (a steel wool scrub, water or alcohol rinse, and wipe dry 
helps here)?  You used tap water, which already contains various salts 
including chloride (especially if it is softened water).  A 
distilled/deionized water start is a better experiment.  Was the salt 
iodized?  Did you uniformly aerate the water (pour between containers a few 
times)?  Was one nail bent?  Strained metal corrodes much faster.

Pure iron sealed in pure dry oxygen does not corrode.  In the presence of 
moisture (humidity) and oxygen it corrodes. Look up the half potentials for 
iron and water (various reactions).  The "CRC Handbook of Chemistry and 
Physics" (library) has extensive reduction potential tables.

The half-potential for reaction is changed if the resulting ion is 
complexed.  Chloride is a good complexing agent for Fe(II) and Fe(III).  
Chloride is the universal corrodant for almost all metals, forming 
electrolyte films and altering chemistry toward reactivity.

A better corrosion environment is the nail half in and half out of water.  
All sorts of nasty things take place at an air/water or air/electrolyte 
interface.  Why?

Uncle Al!