let's think about cars for a moment the
car that you've had in mind was likely
painted with your favorite color
the paint is not only there to make your
car look pretty
but it has a more important purpose to
prevent rusting when iron and steel are
exposed to oxygen and moisture rusting
will occur
rusting describes the corrosion of iron
and steel
the term corrosion is used to describe
the breakdown of a metal when exposed to
the environment when we say a metal
corrodes we mean that it has reacted
with water and oxygen from the
environment to form their respective
oxides
we can demonstrate this
using a simple experiment
place an iron nail in four test tubes
fill one with regular tap water until
the nail is submerged
fill another with regular tap water and
add half a spoonful of sodium chloride
regular table salt shake from side to
side to ensure that sodium chloride
fully dissolves
fill the third test tube with recently
boiled water and add a thin layer of oil
when we boil the water we remove any
dissolved oxygen and the layer of oil
prevents any oxygen from further
dissolving in the last test tube add a
spoonful of calcium chloride
calcium chloride removes moisture or
water so in the first test tube the nail
is exposed to oxygen and moisture in the
second test tube the nail is exposed to
oxygen moisture and salt
so in the third test tube the nail is
only exposed to water in the fourth test
tube the nail is exposed only to oxygen
stopper all four test tubes with a
rubber bung and let stand for a couple
of days you will find that the nails in
the first and second test tubes have
rested in particular the nail in the
second test tube has rusted more than
the nail in the first test tube and the
rust has likely flaked off you may see a
reddish brown precipitate at the bottom
of these tubes
the nails in the third and fourth test
tubes have not rusted so what has
happened here
the iron has undergone an oxidation
reaction forming hydrated iron three
oxide or rust
salt and acid act as a catalyst for this
reaction which is why the nail in the
second test tube
formed more rust than the nail in the
first test tube
rusting can be very costly if we have to
constantly replace these items made of
iron and steel
rust can be prevented if we paint the
areas exposed to oxygen and moisture
this is done on many bicycles and cars
another method to prevent rust is by
placing a layer of oil or grease this is
done on many bicycle chains it helps
lubricate the moving parts which reduces
friction and slows down the resting
process
another way to prevent rusting is a
sacrificial protection method called
galvanizing have a look at this
reactivity table zinc is more reactive
than iron when exposed to oxygen and
moisture
zinc will corrode faster than iron so if
we put them together
zinc protects iron but is sacrificing
itself
hence the term sacrificial protection
this method is applied to prevent
rusting on ships
the oxidation of zinc can be described
using this following equation
so besides preventing rusting there is
another advantage to this galvanizing
process the formed zinc oxide layer can
be removed and the freshly exposed zinc
can corrode once again thereby further
protecting the iron or steel hull
you