|Corrosion is possible only as long as certain metals have
oxygen and moisture. The corrosion of metals such as iron is an
process. Very pure samples of iron seem to resist
In contrast, when a piece of iron containing specks of impurities such
as copper are exposed to moist air, the iron becomes pitted with rust
Iron rust is a reddish-brown hydrated compound of varying composition
the formula Fe(OH)3.x H2O.
Therefore, rust spots indicate the location at which pure iron has been
oxidized. These rust spots represent the anode half cells of
|Such impurities as copper serve as the cathode half cells
located in unaffected areas adjacent to the rust spots. The
electrons travel through the iron from the anode to the cathode.
A film of moisture serves as the medium through which ions travel to
|The electrochemical theory of corrosion is supported
observation that iron exposed to perfectly dry air does not
Three half-cell reactions which would present possible reactions in
complex mechanism are:
Fe(s) <====> Fe+2
Cathode 1/2 O2(g) + H2O(l) + 2 e-1 <====> 2 OH-1
Cathode 2 H+1 + 2 e-1 <=====> H2(g)
|The overall cell reaction can be obtained by adding the
Fe(s) + 1/2 O2(g) + H2O(l)
|Further oxidation of Fe(OH)2 yields Fe(OH)3.
Examination of the equation shows that three factors influence the
of corrosion. These are the concentration of oxygen, the
and the presence of oxygen. There is in fact a fourth factor, the
quality of the iron metal itself.
|To minimize corrosion, protective coatings are applied to prevent the direct contact of moisture and oxygen with the metal. Electrochemical principles can also be applied to inhibit corrosion. This is known as cathodic protection. In cathodic protection, iron is made the cathode half-cell so that it will not lose electrons and dissolve. This can be accomplished by attaching a more active metal such as magnesium to the iron or by connecting the iron to the cathode of an external power source. These devices are frequently used to protect underground pipes and tanks and even the hulls of ships. Steel hulls of ships are especially vulnerable to corrosion because the bronze propeller acts as a cathodic, salt water as a salt bridge and the hull as an anode. This problem is resolved by periodically attaching large pieces of magnesium metal to the hull of the ship. Thus the magnesium metal is corroded and dissolved, but the steel hull to which it is attached is protected.|