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CORROSION
Rusting of iron is the commonest form of corrosion. It is the process that gradually
destroys motor car bodies, steel bridges and other structures and iron roofing and
guttering.
Rust is the reddish-coloured, flaky or porous deposit that forms on exposed
iron and steel; it is hydrated iron oxide, Fe2O3.xH20 where x can vary from about
0.5 to 2. This means that the composition of rust varies somewhat, depending upon
the conditions under which it is formed.
Corrosion of metals involves oxidation
This means that the more easily a metal is oxidised, the more easily it will corrode.
Magnesium is oxidised more easily than iron, so we expect magnesium to corrode
more easily than iron. That is what we observe: a piece of cleaned shiny
magnesium
ribbon
tarnishes more rapidly than a clean shiny iron nail. Gold is very difficult to
oxidise, so we expect gold not to corrode. Again that is our observation: a gold ring
or bracelet does not tarnish (corrode).
Predicting the tendency of metals to corrode
HOW RUSTING OCCURS
At some spot on the iron surface (often a spot under stress),
iron atoms lose electrons to form
Fe
moisture on the iron surface or in the water touching the iron if the iron object
is
completely submerged:
O2(g) + 2H2O(l ) + 4e- → 4OH-(aq)
Sites where this reduction of oxygen occurs are called cathodic sites.
In order for this galvanic cell to continue to operate, there has to be a migration
of ions through the moisture layer from one location to the other. Because salt
water is a better conductor than fresh water
(which nevertheless is slightly conducting because of dissolved CO2 and so on)
rusting proceeds more quickly in salt water. This migration of ions to preserve
electrical neutrality in the galvanic cell moves Fe2 and OH– towards each other
to form insoluble iron(II) hydroxide:
Instead of a well-preserved steel hull they found it badly corroded. The most obvious
product of corrosion was stalactite-like structures called rusticles. Scientists found to
their surprise that much of the corrosion was caused by certain bacteria. The Titanic
has provided scientists with a new understanding of deep sea corrosion processes.
http://www.msichicago.org/onlinescience/
The Titanic will eventually be destroyed by nature as the iron-eating bacteria continue
to corrode the structure of the ship. It is estimated that total corrosion will occur within
the next 75–90 years. Although the wreck will never be raised due its poor condition
many artefacts have already been recovered and restored
How to Prevent Corrosion
When metal is exposed to moisture in the air, damage can occur. Most metals
react with oxygen creating rust that can lead to corrosion. When water from rain,
humidity or other sources comes in contact with a metal surface, oxygen is
dissolved causing a reaction. However, there are ways that you can protect
metal surfaces from corrosion, thereby increasing durability and wear of a
product with metal parts
Apply coatings like inorganic paints. This extra layer of coating between the metal
and the atmosphere helps to prevent against corrosion. These special purpose
paints combine tiny zinc or aluminum metals foils with a silicone resin that provides
protection similar to metal plating. However, unlike the extremely high temperatures
needed to apply metal plating, anti-corrosive paints can be applied in air
temperatures just like other paints.
http://www.ehow.com/how_5017220_preventcorrosion.html