Let me start off by saying I realize this is probably a dumb question, but what actually causes rust? Water alone does not cause rust, correct? if water is allowed to collect and pool up with dirt/mud etc. I know it can promote rust, but water on its own that's allowed to drain off does not promote rust, correct? or no?

I know other factors are at play such as road salt, de-icers, chemicals, etc., but I am specifically referring to waters role in promoting rust. Thanks.

 

When a drop of water hits an iron object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.
The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other forms of corrosion on other metals.
From how stuff works search on google

 

Good old oxygen.

If you think about it, maybe we shouldn't be so surprised that they found life in pools of sulfuric acid on Mars or where ever.

We think of sulfuric acid as nasty corrosive stuff. BUT oxygen, on which we depend for life, corrodes all sorts of substances as well!

 

Yep it's mainly the "galvanic reaction" between different areas of the metal when it's touched by water. That reaction causes some of the metal to lose electrons, and bind with oxygen, etc, you get iron oxide (rust). The main problem with iron oxide is it's a molecule that is larger than the original iron atom - this causes it to "pull away". That's why rust comes "off", it's too large to stay in place. This is why for example Aluminum, even though it oxides even faster than iron does, doesn't corrode or fall apart from the oxidation - because aluminum oxide is the same size, so it doesn't "fall away". that's the basic explanation, anyway of why rust always falls apart. Probably also due to the iron atom not sharing electrons with the other iron atoms around it anymore . No more ionic/covalent bonding so the structure falls apart . And problem with that is when that oxide falls away, it exposed new fresh iron to get oxidized. So it just keeps going. Whereas in Aluminum, once the oxide layer is formed, it "seals" in the rest of the aluminum and prevents the oxidation from going further. Interestingly, Anodized Alumimun is basially a thick layer of Aluminum oxide that was prompted with some help with electricity