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.
R ust is the reddish-coloured, flaky or porous deposit that forms on exposed iron and steel; it is hydrated iron oxide , Fe
2
O
3
.xH
2
0 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 oxidise, so we expect gold not to corrode. Again that is our observation: a gold ring or bracelet does not tarnish (corrode).

O
2
( g ) + 2H
2
O( l ) + 4 e-
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 CO 2 and so on) rusting proceeds more quickly in salt water. This migration of ions to preserve electrical neutrality in the galvanic cell moves Fe 2 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 h as 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

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.
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