Why can't rust form without water?
We could explain it from an electrolytic point of view.

Iron has a certain tendency to dissolve in water according to the following equation:

$\ce{Fe (s) -&gt; Fe^2+ (aq) + 2e^-}$

The release of electrons causes a small current to flow in the metal (Remember that metals conduct electricity). This turns the point where iron dissolves into an anode, and the region around this area becomes rich in electrons, a cathode. Now, we have to remember that water dissociates to small extents according to:

$\ce{H2O (l) &lt;=&gt; H^+ (aq) + OH^- (aq)}$

If it was an acid, the $\ce{H^+}$ would have quickly taken all the electrons liberated from the dissolution of iron earlier, but the concentration of hydrogen ions is not large enough in water, so that we get another reaction taking place at the cathode [The hydrogen ions actually 'gather up' near to the surface but cannot do anything, creating a thin protective layer of hydrogen ions around the cathode, but not strong enough to prevent further reaction]. This reaction involves oxygen and other water molecules:

$\ce{2H2O (l) + O2 (g) + 4e^- -&gt; 4OH^- (aq)}$

Now this is the $\ce{OH^-}$ that reacts with the earlier liberated iron (or ferrous) ions:

$\ce{Fe^2+ (aq) + 2OH^- (aq)-&gt;Fe(OH)2 (aq)}$

Now more oxygen will react to oxidise the iron (II) hydroxide...

$\ce{Fe(OH)2 (aq) \xrightarrow[O] Fe(OH)3 (aq) \xrightarrow[O] Fe2O3 (s)}$

You probably can recognise the last product.

As the $\ce{Fe^2+}$ are consumed, more iron will get dissolved (Le Chatelier's Principle) and keep the whole process going.

Maybe a little picture to go along with this :)