Oxidation is the interaction between oxygen molecules and substances, causing loss of at least one electron. It can be beneficial, as in anodized aluminum, or destructive, such as rusting. Protection, like wax or paint, can prevent oxidation, and stainless steel doesn’t rust due to its coating.
A freshly cut apple turns brown, a bicycle fender turns rusty, and a copper penny suddenly turns green. What do all 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 come into contact with, from metal to living tissue. Technically, however, with the discovery of electrons, oxidation was more precisely defined as the loss of at least one electron when two or more substances interact. Such substances may or may not include oxygen. (Incidentally, the opposite of oxidation is reduction: the addition of at least one electron when substances contact each other.) Sometimes oxidation isn’t such a bad thing, as in the formation of super anodized aluminum. resistant. Other times, oxidation can be destructive, such as rusting an automobile or spoiling fresh fruit.
We’ve often used the words oxidation and rust interchangeably, but not all materials that 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 however, the result is a greenish coating called copper oxide. The metal itself is not weakened by oxidation, but the surface does develop a patina after years of exposure to air and water.
When it comes to oxygen, the oxidation process depends on the amount of oxygen in the air and the nature of the material it comes into contact with. True oxidation occurs at a molecular level – we only see the effects on a large scale as oxygen causes free radicals to break down on the surface. In the case of fresh fruit, the peel usually forms a barrier against oxidation. This is why most fruits and vegetables arrive in good condition at the supermarket. Once the skin is broken, 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 brownish spots or patches.
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 begin to interact with the paint. As the oxygen burns off the free radicals in the paint, the finish becomes increasingly dull. Restoration efforts may include removing several layers of affected paint and reapplying a fresh coat of protectant. This is why automotive professionals recommend using at least one coat of wax or other preservative every time your car is washed.
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 needs.
This is why stainless steel doesn’t rust and plain steel does. Stainless steel has a thin coating of another metal that doesn’t contain free radicals. Plain steel can be painted to protect it from 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.
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