CORROSION PREVENTION
To stop metals (especially steel) from corroding we can
try to stop the oxygen and water from coming in
contact with the metal. Areas that are humid (more
moisture in the air) will have more corrosion than areas
that are dry. Corrosion is also faster when there are
more ions present in the water (i.e. houses and cars rust
much faster near the ocean than inland).
To try to stop, or at least slow down, the rate of
corrosion the following techniques can be used:
1.
Using an inert/unreactive coating.
The most common method of protection is simply
painting the metal object. However, coatings such as
concrete, plastic (PVC), fibreglass, rubber and
bitumen can also be used. They all simply stop the
oxygen and/or the water from coming into contact with
the metal. The main problem with this method is if you
scratch the protective coating. The iron will begin to
corrode and will lift off even more of the protective
surface.
2.
Using an inert/unreactive metal coating.
If you coat the iron in an unreactive metal it can protect
the iron underneath. For example, tin cans are steel cans
coated in a layer of tin and tin solder (used on the seams
of the cans). As tin is less reactive than iron it doesn’t
react with the contents of the can (which are often
slightly acidic). However, you should never buy dented
or damaged tin cans as it is possible that the tin layer has
been breached and the steel underneath is now corroding.
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Nickel, gold and silver are often used as decorative
coatings – they look attractive and they are very
unreactive so they will not corrode.
Some metals react with oxygen to form metal oxides
that are tough and protect the metal underneath. The
most common metals that do this are chromium (forms
Cr2O3), aluminium (forms Al2O3) and nickel (forms
NiO). The protective oxide layers that are formed are
very unreactive and so prevent the metal underneath
from corroding. This is why aluminium is such a good
metal to be used for window frames. It is also why
stainless steel (which contains both chromium and
nickel) doesn’t corrode like steel does.
In order to coat a metal (e.g. steel) in another metal to
prevent corrosion, an electrolytic cell must be set-up.
This particular type of electrolytic cell is known as
electroplating. For example, if you wanted to coat a
chair in chromium (i.e. chrome plating) you would need
the following set-up:
+
Chromium
block –
anode
-
Cr3+
Object to be
plated (i.e.
chair) –
cathode
Chromium (i.e. Cr3+) solution
Anode:
Cr(s) → Cr3+(aq) + 3eCathode: Cr3+(aq) + 3e- → Cr(s)
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3.
Using a sacrificial (more reactive) metal.
Iron that is coated in zinc (i.e. galvanised iron) is also
protected, even if the surface is scratched. This is
because of the increased reactivity of zinc. Firstly, it
forms its own zinc oxide (ZnO) coating. Secondly, if
the iron is exposed, the zinc will oxidise in preference
to the iron (because its more reactive), hence giving
electrons to the iron, preventing corrosion. The redox
reactions are as follows:
Anode:
Zn(s) → Zn2+(aq) + 2eCathode: Fe2+(aq) + 2e- → Fe(s)
Overall: Zn(s) + Fe2+(aq) → Zn2+(aq) + Fe(s)
The zinc donates its electrons to the iron, thus
sacrificing itself to ensure that the iron doesn’t
corrode. When all of the zinc is used up it will need to
be replaced with another piece.
This technique will work as long as the metal being used
is more reactive, and hence willing to lose its electrons,
more readily than iron. Magnesium metal blocks are
often attached to the outside of the hull of boats to help
prevent corrosion. The magnesium blocks must be
replaced every 2 – 3 years (depending on the size of the
ship and the size of the piece of Mg attached to it).
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4.
Using electricity.
Most motor vehicles and large sheet-metal objects get
coated in surface rust during manufacture. It is
removed by a quick dip in a tank of sulfuric acid.
The tank of sulfuric acid could be made out of glass (as
it would not react), but it would be very dangerous if it
were damaged and broke. It is actually made out of
steel. To stop the acid eating away at the tank it has a
small voltage (about 4 – 6 V) applied to the walls of the
tank. As electrons are supplied to the steel container, it
is gaining, rather than losing electrons, and hence
doesn’t corrode.
It is a simple electrolytic cell, where the steel tank is
made the cathode of the cell by supplying it with
electrons. The anode can be anything cheap because it is
going to be slowly eaten away in order to stop the tank
from corroding.
+
➀
➂
➀
➁
➂
=
=
=
➁
Sulfuric acid
– H2SO4
Cheap metal anode (+ve terminal)
Iron tank – cathode (-ve terminal)
Any object needing surface rust removed
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