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.
