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Question Video: Identifying Differences between the Oxidation of Iron and Aluminum Chemistry • 10th Grade

Why does rusting affect iron more than aluminum?

02:31

Video Transcript

Why does rusting affect iron more than aluminum? (A) Aluminum is less reactive than iron. (B) Aluminum is protected by a surface oxide layer. (C) Aluminum binds to water less strongly. (D) Aluminum oxides are less soluble than iron oxides. Or (E) aluminum oxides are less stable than iron oxides.

Let’s start by looking at the process of rusting and how it relates to iron.

Rusting is a type of corrosion that affects iron and some iron-containing alloys. Rusting occurs when iron is exposed to oxygen and water. Rusting forms rust, which in chemical terms is a hydrated iron(III) oxide. Iron is a strong and durable metal, but when rust forms, it affects the iron negatively, making the piece of iron weak and brittle. And this is due to the physical properties of rust.

Rust has a tendency to flake off the surface of the metal. When the rust flakes off, it can expose more of the metal underneath to oxygen and water, which will cause that part of the metal to rust as well. The rust can flake off the surface of the metal again, exposing more of the metal to oxygen and water. And so the pattern repeats and can cause the metal to degrade over time.

In addition, rust has a greater volume than the same mass of iron will, so iron expands when it rusts. An iron part that expands when it rusts could cause damage to surrounding parts.

Now let’s look at the corrosion of aluminum. Aluminum corrodes in the presence of oxygen, forming aluminum oxide. Aluminum oxide forms a protective layer on the aluminum, and it does not chip away easily. The oxide coating on aluminum forms very quickly, resealing the aluminum if the surface is scratched or chipped. This prevents further corrosion or other chemical reactions from taking place.

Aluminum and aluminum oxide also have more similar volumes than iron and rust do, which means that when aluminum corrodes, there won’t be damage caused to surrounding parts due to expansion caused by corrosion.

If we apply this understanding to the choices in this question, we can see that the correct answer to the question “Why does rusting affect iron more than aluminum?” is option (B); aluminum is protected by a surface oxide layer.

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