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How does rust work?

Rust is the common name for a very common compound, iron oxide. Iron oxide, the chemical Fe2O3,is common because iron combines very readily with oxygen -- so readily,in fact, that pure iron is only rarely found in nature. Iron (or steel)rusting is an example of corrosion -- an electrochemicalprocess involving an anode (a piece of metal that readily gives upelectrons), an electrolyte (a liquid that helps electrons move) and acathode (a piece of metal that readily accepts electrons). When a pieceof metal corrodes, the electrolyte helps provide oxygen to the anode.As oxygen combines with the metal, electrons are liberated. When theyflow through the electrolyte to the cathode, the metal of the anodedisappears, swept away by the electrical flow or converted into metalcations in a form such as rust.For iron to become iron oxide, three things are required: iron,water and oxygen. Here's what happens when the three get together:
When a drop of water hits an iron object, two things begin to happenalmost immediately. First, the water, a good electrolyte, combines withcarbon dioxide in the air to form a weak carbonic acid, an even betterelectrolyte. As the acid is formed and the iron dissolved, some of thewater will begin to break down into its component pieces -- hydrogenand oxygen. The free oxygen and dissolved iron bond into iron oxide, inthe process freeing electrons. The electrons liberated from the anodeportion of the iron flow to the cathode, which may be a piece of ametal less electrically reactive than iron, or another point on thepiece of iron itself.
The chemical compounds found in liquids like acid rain, seawater andthe salt-loaded spray from snow-belt roads make them betterelectrolytes than pure water, allowing their presence to speed theprocess of rusting on iron and other forms of corrosion on other metals.