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.
Iron (or steel) rusting is an example of corrosion -- an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons).
When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations 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 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.
Images of rusting
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