Rusting can happen quickly or slowly, depending on the material that's
rusting, and the environment. Rust is the oxidation of iron along with
the absorption of water to make Fe2O3 with water molecules attached.
Here are some things that can affect the rusting rate:
1) The iron can have additives to prevent rusting. Stainless steel
has added nickel and chromium which bind to the iron atoms and keep
them from oxidizing. I haven't seen stainless steel rust even over long
periods of time.
2) The iron can be painted or coated with oil, preventing oxygen
and water from coming into contact. This can slow or halt rusting.
3) The air can be devoid of humidity and in some place it doesn't
rain much. Cars last longer out in the desert because it's so dry,
rusting is slowed.
4) Hot iron rusts faster than cold iron -- typically heat speeds up
chemical reactions. This is one reason why mufflers and exhaust
manifolds in cars get rusty very quickly (unless they are coated or
made out of non-rusting materials).
5) Thin iron can rust through (get holes in it) faster than thick
iron. The rusting rate may be the same, but you may notice it sooner in
thin metal sheeting than on a thick piece of iron because the former
will have a hole in it sooner. Some kinds of steel wool also rust
quickly (they are commonly exposed to water so this doesn't help),
although other steel wools are made of stainless steel or coated.
6) The rate of rusting or corrosion in water can be affected by the
electrical environment. If you have two different metals in electrical
contact, and both in contact with salty water, then effectively a
battery is made. Current flows, and the energy comes from the corrosion
of the metals. Some companies sell blocks of zinc that you can attach
to boats so that the zinc corrodes first, protecting the other metals.
7) Some questioners on this site have found that rusting rates in
iron submerged in water are affected by dissolved impurities. Vinegar
and bleach have been tried and seem to affect the rusting rate.
Fresh iron exposed to a hot atmosphere with plenty of oxygen and
water will form a thin layer of rust immediately (although if you look
at a very short time after exposing the iron surface, you will have a
very small amount of rust). Any of the above variables can affect the
rusting rate however. Can you think of more?
(published on 10/22/2007)