A freshly-cut apple turns brown, a bicycle fender becomes rusty and a copper penny suddenly turns green. What do all of these events have in common? They are all examples of a process called oxidation.
Oxidation is defined as the interaction between oxygen molecules and all the different substances they may contact, from metal to living tissue. Technically, however, with the discovery of electrons, oxidation came to be more precisely defined as the loss of at least one electron when two or more substances interact. Those substances may or may not include oxygen. (Incidentally, the opposite of oxidation is reduction — the addition of at least one electron when substances come into contact with each other.) Sometimes oxidation is not such a bad thing, as in the formation of super-durable anodized aluminum. Other times, oxidation can be destructive, such as the rusting of an automobile or the spoiling of fresh fruit.
We often used the words oxidation and rust interchangeably, but not all materials which interact with oxygen molecules actually disintegrate into rust. In the case of iron, the oxygen creates a slow burning process, which results in the brittle brown substance we call rust. When oxidation occurs in copper, on the other hand, the result is a greenish coating called copper oxide. The metal itself is not weakened by oxidation, but the surface develops a patina after years of exposure to air and water.
When it involves oxygen, the process of oxidation depends on the amount of oxygen present in the air and the nature of the material it touches. True oxidation happens on a molecular level — we only see the large-scale effects as the oxygen causes free radicals on the surface to break away. In the case of fresh fruit, the skin usually provides a barrier against oxidation. This is why most fruits and vegetables arrive in good condition at the grocery store. Once the skin has been broken, however, the individual cells come in direct contact with air and the oxygen molecules start burning them. The result is a form of rust we see as brownish spots or blemishes.
Oxidation can also be a problem for car owners, since the outermost layers of paint are constantly exposed to air and water. If the car's outer finish is not protected by a wax coating or polyurethane, the oxygen molecules in the air will eventually start interacting with the paint. As the oxygen burns up the free radicals contained in the paint, the finish becomes duller and duller. Restoration efforts may include removing several layers of affected paint and reapplying a new layer of protectant. This is why professional car detailers recommend at least one layer of wax or other protectant be used every time the car is washed.
The secret of preventing oxidation caused by oxygen is to provide a layer of protection between the exposed material and the air. This could mean a wax or polyurethane coating on a car, a layer of paint on metal objects or a quick spray of an anti-oxidant, like lemon juice, on exposed fruit. Destructive oxidation cannot occur if the oxygen cannot penetrate a surface to reach the free radicals it craves.
This is why stainless steel doesn't rust and ordinary steel does. The stainless steel has a thin coating of another metal which does not contain free radicals. Regular steel may be painted for protection against oxidation, but oxygen can still exploit any opening, no matter how small. This is why you may find a painted metal bicycle still damaged by rust.