Metals containing iron, such as most kinds of steel, will rust when
exposed to air and water. Rust is just iron oxide, usually with water
molecules incorporated in it too. It usually takes the form of an
orange powder on the surface of the metal. Other metals oxidize or form
other compounds in a similar way, although we usually say they corrode.
Rusting is a specific kind of corrosion which applies to
Rusting has a number of effects on metal objects. It makes them
look orange and rough. It makes them weaker, by replacing the strong
iron or steel with flaky powder. Some oxides on some metals such as
aluminum form just a thin layer on top which slows down further
corrosion, but rust can slowly eat away at even the biggest piece of
iron. If a piece of iron's strength is important for safety, such as a
bridge support or a car's brake caliper, it is a good idea to inspect
it for rust damage every now and then.
Rust also can cause metal parts that are supposed to slide over
one another to become stuck. Just ask someone who has tried to get a
rusty nut unstuck from a rusty bolt.
Rust can make holes in sheet metal. Rusty car mufflers sometimes
develop holes in them, and the sheet steel making the outer bodies of
cars will often rust through, making holes.
Rust is a lot less magnetic than iron. An iron magnet will probably
still work almost as well when it has a thin coating of rust on it, but
if it has rusted so badly that most of the metal is gone, then it will
not work very well as a magnet.
Rust is an insulator, meaning that it doesn't conduct electricity
easily, unlike iron, which is a metallic conductor. So if some
electrical connection is made with iron, it's likely to go bad when the
iron surface rusts.
Tom J. (and mike)
(published on 10/22/2007)