Where does rust come from? How does it begin to form and why only on certain metals?
In: Chemistry
Rust forms on metals that contain iron. Iron is a major component of steel, and steel is a very common building material because it is light and strong.
Rust is a special name we give to iron and iron alloys (like steel) that have undergone a process called oxidization. Most metals oxidize, but iron oxidizes in a way that is particularly troublesome for its integrity.
When iron oxidizes, it pulls in water and this results in significant structural changes. When some metals oxidize, they simply combine with oxygen to form a new molecule. For example aluminum oxide is Al2O3. That’s two aluminum atoms and three oxygen atoms. Nice and simple. Aluminum oxidizes, but it does not rust. When aluminum oxidizes, the aluminum oxide forms a thin, stable layer on the metal’s surface. Aluminum oxide is actually very hard, so once a thin layer has formed, the oxidization process can’t reach the pure aluminum underneath. Aluminum’s oxidization process is self-terminating!
Iron, on the other hand, oxidizes differently. Instead of forming a hard surface, iron oxide is loose and not very strong. This allows the corrosive process to follow tiny fissures in the material, burrowing their way into the surface. The result is a flaky mess that we call rust.
Whether or not a metal corrodes like aluminum or iron depends on the specific chemical oxidization process. Most metals are elemental (composed of a single element) or alloys of a single element (mostly a single element with small parts of others), so their corrosive process is distinct to their elemental characteristics. That is why aluminum oxidization is almost unnoticeable, copper oxidization develops a pretty green patina, and iron turns into flakey dust. These are each the result of their elemental uniqueness.
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