Background boosters for elementary teachers
Q: What causes things to rust?
By Bill Robertson
Rules of Attraction
The red rust you find on iron and steel
is a chemical called iron oxide. The
basic chemistry is that iron combines
with oxygen to form iron oxide (chemical formula Fe2O3). The process by
which this happens, though, isn’t as
simple as bringing iron and oxygen together. If that were the case, every bit
58 Science and Children
brian diskin
A:
The simple answer is that
things rust when you leave
them out in the rain. But maybe
you want a more thorough answer.
Rust is a form of corrosion, which is
broadly defined as the wearing away
of materials due to chemical reactions. So corrosion applies to materials other than metals. Metal corrosion is the one we commonly think
of, though, such as tarnish on silver
and that filmy covering (aluminum
oxide) that forms on aluminum.
Rusting occurs on iron and any
metal that contains iron, such as
steel. You probably already know the
conditions that lead to rust—humid
environments, rain, or salty environments. Rusting isn’t a huge problem
in arid climates. Salt air causes things
to rust faster, and anyone who lives
where they salt the roads in winter
knows that this salt causes things to
rust faster. My basement is more humid than the rest of my house, so the
fact that I’m keeping my model train
stuff (a project for retirement!) in the
basement is no doubt a bad thing.
of iron and steel exposed to air (which
contains oxygen, by the way) would
rust. For rust to form on iron, you need
iron, water, and oxygen dissolved in
the water. This oxygen that’s dissolved
in the water is not the oxygen that’s
part of the water molecule (H2O).
Oxygen in the air dissolves in water in
the same way that you have carbon dioxide dissolved in carbonated drinks.
Any drastic movement of the water
encourages oxygen from the air to dissolve in it. Rain falling through the air
does this nicely. Salt isn’t necessary, but
it speeds up the rusting process. So, to
understand how rust forms, consider a
drop of water resting on a piece of iron
metal, shown in Figure 1.
Notice that part of the water drop
is on top of a “pitted” part of the iron.
Rust tends to form near pits in iron, but
not directly on them. It might not look
like it, but the setup in Figure 1 actually
makes a complete electric circuit. No,
not wires and batteries and such, but
there’s movement of electrons and other
charged particles. I’ll explain what’s
going on, but first I have to make sure
you know what a few symbols mean. Fe
is the symbol for the element iron. Fe+
represents an iron atom that’s missing
an electron, Fe2+ represents an iron atom
that’s missing two electrons, and Fe3+
represents an iron atom that’s missing
three electrons. Any atom such as this
that is charged either plus or minus is
called an ion. The symbol for an electron
is e-, the symbol for an oxygen molecule
is O2, and the symbol for water is H2O
(the H is hydrogen).
At the pitted region of the iron, iron
atoms tend to lose electrons. At these
pitted regions, the irregular surface and
the uneven composition of the metal
favor this chemical reaction. This is
represented by the following chemical
equation, in which you can interpret the
arrow as meaning “becomes.”
iron -----> charged iron + electrons
In symbols, this is
Figure 1.
Fe -----> Fe2+ + 2 eThe Fe2+ ions move through the
water droplet and encounter oxygen
molecules dissolved in the water.
These molecules strip another electron from the Fe2+ ions and create
Fe3+ ions, which travel on through
the water droplet. When the Fe3+ ions
reach another place where the water
droplet and the iron meet, Fe3+ ions
combine with oxygen. Check out
Figure 2, and don’t be intimidated
by all the symbols I’ve written. It’s
just certain kinds of atoms and ions
gaining or losing electrons and getting together with other atoms.
In Figure 2, you might be wondering why in the world those charged
iron ions would move through the water droplet to the other place where the
water droplet and the iron meet. Well,
while this is happening, the original
negatively charged electrons that left
the iron are throughout the metal and
provide various random places of
negative charge that attract the iron
ions. There’s an electric attraction.
This is also shown in Figure 2.
Now, assuming the editors let me
get away with all that chemistry talk
above, I’ll now explain what it has to
do with your basic rust formation.
First, Figure 2 shows a complete
electric circuit. Charges are moving
from place to place. You need conductors—materials that allow charges to
flow—in electric circuits. The iron
metal is obviously a conductor that
allows electrons to flow freely. The
water droplet is the other conductor,
but in the water it is ions rather than
Figure 2.
electrons that move from one place to
another. If you didn’t have water, you
wouldn’t have a complete circuit and
nothing would happen. When you
add salt to water it becomes a better
conductor, which explains why salt
air and salt on roads speeds up the
formation of rust.
Hopefully you gathered from the
diagrams and the chemical talk that oxygen plays an important role in rust formation. This is oxygen that’s dissolved
in the water, not oxygen in the air. So,
water without much dissolved oxygen
doesn’t help rust form. Water that is at
high temperatures and low pressures
contains less dissolved oxygen, so for the
most part you have to work at removing
oxygen from water. Finally, you should
notice in Figure 2 that the rust forms on
a different part of the iron from where
the pits are (look closely at rusted metal
and you’ll see this effect). Because iron
atoms are “lost” from the pitted part of
the metal (they aren’t really lost; they
just become part of the rust), the pit just
December 2010 59
keeps getting larger. That means that
the process of rusting slowly corrodes
the metal, which is why rust isn’t a good
thing. See Figure 3.
To summarize, rust forms when you
have iron, water, and dissolved oxygen
in the water. These three things set up a
chemical reaction that forms rust. Adding salt to the water speeds things up
because it helps charged particles move
more efficiently. So, to avoid rust, simply avoid water, dissolved oxygen, salt,
and most of all iron. Better yet, just keep
the iron away from the other stuff.
Fun Facts
I’ll end with a few fun facts to know
about rust. First, steel rusts faster
than iron. Using two different metals makes for an efficient transfer
of ions and electrons and, steel is a
combination of iron and other metals. Whenever you have iron combined with another metal, you have
a more efficient chemical equation,
and the rust forms more rapidly.
The second fun fact is that not all
rust is bad. It’s possible to form iron
oxide when there’s little or no dissolved
oxygen in the water (you subject the
water to high temperatures and low
Figure 3.
pressures to remove the dissolved
oxygen), and no, I won’t go into the
chemical reaction. The iron oxide
formed under such conditions has a
different chemical formula from red
rust—and a different name, magnetite.
Magnetite actually forms a protective
layer on iron and protects it from the
bad rust (booooooo!). Coating iron
with magnetite is known as bluing, and
it’s used on guns and drill bits.
The third fun fact is that stainless
steel doesn’t rust. Well, you probably
already knew that but maybe you don’t
know why. Stainless steel contains the
element chromium, which reacts with
oxygen to form another “good rust”
layer on the metal. This layer keeps the
essential rust ingredients water and
oxygen away from the iron, much as
aluminum oxide layers do for aluminum. A very important application for
all of you people in St. Louis, because
the Gateway Arch is made of stainless
steel and the last time I checked, it can
get a wee bit humid there. n
Bill Robertson ([email protected].
com) is the author of the NSTA Press
book series, Stop Faking It! Finally
Understanding Science So You Can
Teach It.
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60 Science and Children