Chemistry Reference
In-Depth Information
Corrosion
Rusting is the most common form of corrosion but
this term is confined to iron and steel.
Corrosion
is the general name given to the process which
takes place when metals and alloys are chemically
attacked by oxygen, water or any other substances
found in their immediate environment. The metals
in the reactivity series will corrode to a greater or
lesser extent. Generally, the higher the metal is in
the reactivity series, the more rapidly it will corrode.
If sodium and potassium were not stored under oil
they would corrode very rapidly indeed. Magnesium,
calcium and aluminium are usually covered by a thin
coating of oxide after initial reaction with oxygen
in the air. Freshly produced copper is pink in colour
(Figure 5.16a on p. 81). However, on exposure to
air, it soon turns brown due to the formation of
copper(
ii
) oxide on the surface of the metal.
In more exposed environments, copper roofs and
pipes quickly become covered in verdigris. Verdigris
is green in colour (Figure 10.26) and is composed
of copper salts formed on copper. The composition
of verdigris varies depending on the atmospheric
conditions, but includes mixed copper(ii)
ii
) carbonate
and copper(
ii
) hydroxide (CuCO
3
.Cu(OH)
2
).
Gold and platinum are unreactive and do not
corrode, even after thousands of years.
it must be renewed. Gas and water pipes made of
iron and steel are connected by a wire to blocks of
magnesium to obtain the same result. In both cases,
as the more reactive metal corrodes it loses electrons
to the iron and so protects it (Figure 10.25b).
a
Bars of zinc on the hull of a ship.
ship's hull
made of steel
(mainly iron)
zinc bar
water
2e
zinc (Zn)
Zn
2
iron (Fe)
hull
b
The zinc is sacrificed to protect the steel. Electrons released from the
dissolving zinc cause reduction to occur at the surface of the hull.
Figure 10.25
Sacrificial protection.
Figure 10.26
Verdigris soon covers copper roofs in exposed environments.
