What is oxidation? (with photos)

A freshly cut apple turns brown, a bicycle bumper rusts, and a copper coin suddenly turns green. What do all these events have in common? All are examples of a process called oxidation.

Oxidized copper often develops a greenish hue or patina.

Oxidation is defined as the interaction between oxygen molecules and all the different substances they can come into contact with, 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. These substances may or may not include oxygen. (By the way, the opposite of oxidation is reduction — the addition of at least one electron when substances come into contact with each other.) Sometimes oxidation isn’t so bad, as in the formation of superdurable anodized aluminum. Other times, oxidation can be destructive, such as car rust or fresh fruit spoiling.

Rusted metal gears.

We often use the words oxidation and rust synonymously, but not all materials that interact with oxygen molecules actually disintegrate into rust. In the case of iron, oxygen creates a slow burning process, which results in the brown brittle substance we call rust. When oxidation occurs on 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.

A rusty I-beam.

When it involves oxygen, the oxidation process depends on the amount of oxygen present in the air and the nature of the material it touches. True oxidation takes place on a molecular level – we only see the large-scale effects as oxygen causes free radicals on the surface to break apart. In the case of fresh fruit, the peel usually provides a barrier against oxidation. That’s why most fruits and vegetables arrive at the supermarket in good condition. Once the skin has broken down, however, the individual cells come into direct contact with the air and the oxygen molecules begin to burn them. The result is a form of rust that we see as spots or brownish spots.

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Rusty tractor.

Oxidation can also be a problem for car owners, as the outermost layers of paint are constantly exposed to air and water. If the car’s exterior finish isn’t protected by a wax or polyurethane coating, the oxygen molecules in the air will eventually start interacting with the paint. As the oxygen burns the free radicals contained in the paint, the finish becomes more and more opaque. Restoration efforts may include removing multiple layers of affected paint and reapplying a new coat of protectant. That’s why professional car detailers recommend that at least one coat of wax or other protectant be used every time the car is washed.

Rusty truck.

The key to 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 coat of paint on metal objects, or a quick spray of an antioxidant such as lemon juice on exposed fruit. Destructive oxidation cannot occur if oxygen cannot penetrate a surface to reach the free radicals it craves.

Galvanized metal, like this bucket, resists oxidation.

That’s why stainless steel doesn’t rust and regular steel does. Stainless steel has a thin layer of another metal that does not contain free radicals. Ordinary steel can be painted to protect against oxidation, but oxygen can still exploit any opening, no matter how small. That’s why you can find a painted metal bike still damaged by rust.

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