Iron does not technically rust. The orange or red layer that we see forming on iron is actually born of a chemical process and is nothing more than iron oxide and corrosion attaching itself to the surface. We say that iron rusts, because if left to its own devices, this corrosion will literally eat away the iron, leaving behind only the fine oxide dust.
Oxidation forms on irons that are exposed for long periods of time to both oxygen and water. Neither element alone will cause iron to rust, and, therefore, iron stored in absolutely pure water or oxygen atmospheres will remain oxidation free. Both elements are necessary and work in tandem when causing the red oxide to form. The electrons in the metal are actually transformed into oxygen gasses, which react to other catalysts such as water or salts to form the iron oxide we call rust. The hydroxide that is created in this process has a very low Ph and is therefore extremely acidic, which is why rust will eat through metal if given enough time.
Rust can be prevented from forming on iron in several ways. A good coat of primer or paint will stop the oxide from appearing, as will a liberal coating of grease or oil. Metals can be purchased that have special chromium and zinc coatings that will not allow the base alloy to rust. This form of metal is known as galvanized and is impervious to the formation of oxides. However, due to the porous makeup of iron, even the galvanization process can sometimes fail, allowing the conditions necessary for rust to form.
Aluminum versus Iron
Maybe you have noticed that aluminum objects never seem to rust, and instead only form an opaque layer of oxidation that is easily buffed out, bringing the aluminum alloy back to its original luster. This is because the oxidation that forms on iron is extremely porous, which allows the process of oxidation to continue occurring, even after the first signs of rust appear. The oxidation that happens on aluminum alloys is very tightly knit and essentially seals the alloy from further damage.
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