The most fundamental reason that steel rusts
is that the product of rusting, rust aka iron
oxide, has a lower energy than the combination of
steel and oxygen-containing air. This is very
evident in an experiment that is often used to
understand oxidation in the sense that burning
something actually adds something rather than
taking it away: if you take fine steel wool and
make it very fluffy (allow lots of air to surround
it), it will burn (and gain weight in the process,
which comes from the oxygen).
While the fact that rust has lower energy than
iron and oxygen means that in principle
("thermodynamically"), that is where a combination
of those materials is headed, it does not mean
that this transformation occurs easily or quickly.
Some added components such as water or salt can
accelerate the process, and there are also
ways to slow it down, such as coatings or making
alloys (such as stainless steel). This is the
reason why the speed with which "steel" rusts
depends so much on the exact type of steel and its
Hope this helps out,
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