|MadSci Network: Chemistry|
This is really a very complicated question. I hope I have produced something you can use.
You will note that while the salt is important, it does not appear in any of the net reactions.
How does rust work?
From your note I believe you want to know how mild steel corrodes and why salt accelerates that corrosion. (Mild steel is the class of iron alloys used in the manufacture of automobiles and many other products of commerce. One estimate I remember hearing is that corrosion of mild steel costs us about 5% of our gross national product.)
Rust (ferric oxide, Fe2O3, or hematite) is formed when iron metal reduces water. That reaction forms hydrogen (H2) and Fe2O3:
2Fe + 3H2O = Fe2O3 + 3H2
You wrote an equation:
4Fe + 3O2 = 2Fe2O3
This also produces Fe2O3 but it is a very slow reaction. If it weren't slow a lot of buildings would fall down because steel is used in the structure of most of them.
In pure water the first reaction is also very slow. Even at elevated temperatures, as in steam turbine generating plants, if the water is pure enough it is very slow.
The reaction is slow in pure water because there are actually two reactions taking place at separate sites. (These sites may be only microns apart, but at atomic dimensions that is a long distance.)
At one site the reaction is the oxidation of the iron to produce ferric oxide and electrons that enter the metal.
At the other site the reaction is the reaction of these electrons to reduce water to hydrogen gas.
The electrons move rapidly through the metal, but if the reaction is to proceed rapidly electrolytic charge must also move rapidly through the water and in pure water it doesn't.
However, when a salt is dissolved in water it forms an electrolytic solution that will conduct electricity. (The salt is an electrolyte. Many acids and bases form conductive solutions in water and are also electrolytes.) The electrolyte is important because it carries electric current through the solution from one site where the iron is oxidized to ferric oxide to another site where hydrogen gas (H2) is produced.
These sites may be only a few microns apart, but that is a long way for a charge to jump without some help.
Salts that are frequently used for melting snow on highways in the Snow Belt include sodium chloride, calcium chloride, and magnesium acetate. Sodium chloride and calcium chloride simply melt the ice. Magnesium acetate prevents the ice from adhering to the pavement so that it can be plowed off. This lets you use less salt.
Urea is not an electrolyte and is used for situations where any corrosion would be a serious problem.
Paint and other coatings are used to protect steel from the corrosive effects of salts.
The reinforcing rods ("rebars") used in concrete are exposed to salt and their corrosion has led to premature highway failures. In many applications the rebars are coated with an epoxy to lessen that corrosion.
A Google search pointed me to: http://science.howstuffworks.com/question445.htm
(Note - The discussion given at this link is inadequate for indicating what occurs chemically.)
Try the links in the MadSci Library for more information on Chemistry.