Corrosion is an oxidation process of metals in the presence of
water. For iron this process is known as rusting, but it
is important to note that many other metals also corrode.
The effects of corrosion are often (ok, essentially always)
undesirable. Rust is generally formed at the surface of the metal, and
doesn't adhere to the surface. When rust flakes off, the underlying
metal is exposed to further corrosion (a process called pitting), and
eventually the structural integrity of the metal disintegrates.
The costs associated with corrosion damage are enormous. For
example, in the US they add up to approximately 4.2 percent of the
GNP (well over 400 billion dollars per
year). These costs can be attributed to failure or deterioration of
equipment, and the associated downtime, replacement and maintenance.
The overall reaction for the corrosion of iron is as follows:
H2O
4 Fe (s) + 3 O2 (aq) -> 2 Fe2O3 (s)
The preceding reaction balance does not show what is actually
happening to the metal (i.e. the reaction mechanism). The
reaction does not occur by a direct reaction between oxygen and iron
atoms such as in a combustion reaction, but by electron transfer
from the metal to water. This type of reaction is called a redox
reaction. In this reaction, one of the reactants is
reduced (gains an electron), and another one is oxidized.
It's the same process that drives those digital clocks that run on a
potato with an iron and copper electrode stuck in
them.
The half-reactions of the redox reaction give a more accurate
description of the reaction mechanism. The iron is first oxidized to
Fe2+ (ferrous ion):
Fe -> Fe2+ + 2 e-
The ferrous ion is further oxidized to Fe3+, ferric
ion:
Fe2+ -> Fe3+ + e-
The electrons that were produced by the oxidation steps are used in the
reduction reaction of water:
O2 + 2 H2O + 4 e- -> 4 OH-
Combining the above reactions, and balancing them for the number of
electrons being transferred, we get:
4 Fe + 3 O2 + 6 H2O -> 4 Fe3+ + 12 OH-
however, these free ferric and hydroxide ions are not actual
intermediates for the reaction, but directly form a more stable
complex, under the liberation of water:
-> 2 Fe2O3 + 6 H2O
Thus summarizing, for a corrosion reaction we need (1) a metal that
can be oxidized, (2) oxygen, and (3) water. The reaction can be
facilitated by adding a salt to the water (such as sodium chloride,
sea salt). The salt increases the conductivity of the water, and thus
enhances the electron transfer. This is the reason why cars rust so much
faster in the winter (plenty of water, salt on the roads). Another way
to enhance the corrosion of metals is to increase the acidity of the
solution; the increased availability of H+ ions not only
increases conductivity, but also promotes the reduction reaction.
Finally, temperature is also a factor: at higher temperatures metals
corrode faster.
Some metals corrode more easily than others. This is a function of
their redox potential, or their ability to donate electrons. Whether
they have a high redox potential because they can give up electrons
easily or give up electrons easily because of their redox potential is a
chicken and egg question. In any case, materials such as magnesium,
zinc, aluminum, and chrome corrode more easily than iron.
Materials such as tin, lead, copper, silver and gold corrode
less easily than iron.
In case you noticed that aluminum was in the list of easily
corroding materials, this is no mistake. However, this material has the
pleasant property that its oxide (aluminum oxide) forms a protective
layer around the aluminum, so that no more oxygen can permeate. Hence,
many people are under the somewhat incorrect impression that aluminum
doesn't "rust"...
... which brings us to the final point: how to control corrosion. One
option would be to keep the metal dry and/or free from oxygen. This is
not always an option, so metals are often coated with paint,
plastic, grease, or any other barrier that keeps out water or oxygen.
This is often a problem when the barrier gets scratched or wears off; in
this case water can get trapped underneath, and corrode the metal even
faster.
One successful method for corrosion control is by using a sacrificial
metal, which is a metal that corrodes more easily. For instance, the
redox potentials for iron and zinc, and tin are:
Fe -> Fe2+ + 2 e- 0.44 V
Zn -> Zn2+ + 2 e- 0.76 V
Sn -> Sn2+ + 2 e- 0.14 V
Because zinc (Zn) has a higher redox potential than iron (Fe), this
material will corrode if both materials are submerged in water. Thus,
zinc is a good coating material for iron, and even inhibits rusting of
iron when the coating is damaged. Often, iron and aluminum ships have
blocks of zinc or magnesium tied to the hull to act as sacrificial
metal. Another name for this method is cathodic protection.
Note that tin (Sn) has a lower redox potential than iron.
This material is also often used as an oxygen barrier for iron. However,
if this coating is damaged, the iron is preferentially oxidized over the
tin. As a result, the iron rusts more readily. Thus, if you want to keep
that 1980 Chevy Impala running throughout the harsh New England winters,
it's better to coat the bottom with zinc spray than with tin.
Sources:
chemistry, years of it.
http://www.terrific-scientific.co.uk/Topics/Corrosion
http://www.corrosioncontrol.biz/Rust.htm
http://www.corrosionsource.com/cost/CorrosionCostUS.htm
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