CORROSION
anions, enabling current to ow through
the movement of these ions in opposite
directions. Electrodes are the sites in an
electrochemical cell where the metallic
part of the electrical circuit makes contact
with the electrolyte. Anodes are the
electrodes where the oxidation reaction
occurs, producing anions and releasing
electrons which ow through the metallic
conductor. Cathodes are the electrodes
where the reduction reaction occurs,
taking up electrons from the metallic
conductor and releasing cations into the
electrolyte.
Typically, when a metal is corroding,
some sites act as anodes, where oxidation
occurs, and other sites act as cathodes,
where reduction reactions take place. The
actual damage of the corrosive process
occurs at the anodes, with metal being
consumed by oxidation. This may, for
example, appear as rust spots.
In electrochemical corrosion, the
anodic and cathodic sites can be clearly
distinguished. This can occur in a single
piece of metal but is most clear where two
di erent metal objects are present, with
one acting as the anode and one as the
cathode, causing the anode to corrode at
an accelerated rate.
In chemical corrosion, individual
electrochemical sites may be microscopic,
and it is therefore not possible to
clearly identify the components of an
IMPRESSIVE
Dutch rm Corrosion and Amphibious
Energy has launched a power system
for wind turbines under construction
that aims to replace diesel generators
and sacri cial anodes. The ICCP-POD
system comprises a small (3-5kW) wind
turbine and solar panels to generate between 220-1,000W of
minimum continuous load circulating to submerged anodes.
electrochemical cell, but underlying
this, it is fundamentally the same
electrochemical process.
CATHODIC PROTECTION
Cathodic protection ensures that the
entire surface of a metal component acts
as a cathode, eliminating anodic sites.
This can be achieved in one of two ways.
Cathodic protection requires an
electrical circuit both through the metals
involved, and externally. It can therefore
only be used for components which are
completely submersed in a conductive
uid. Submerged components in marine
environments or structures buried in soil
are commonly protected in this way.
The material used for the sacri cial
anode in a cathodic protection system
must be more corrodible that the material
being protected. Magnesium, zinc and
aluminium are typically used. Passivation
is a process by which, as a metal starts to
corrode, it forms a thin uniform layer of
corrosion product, which acts as a barrier
to prevent further corrosion. Aluminium
passivates, but can be alloyed with zinc
to prevent passivation, allowing its use
as a sacri cial anode, particularly for
structures buried in soil.
In an impressed-current cathodic
protection system (ICCP), the anodes do
not need to be more corrodible; in fact,
it is actually advantageous if they do not
corrode, so that they are long-lasting. If
a corrodible material was used, the much
greater current than would be normally
encountered in corrosion would cause the
anode to corrode very rapidly. Anodes are
therefore made from materials such as
graphite or titanium. The anode reaction
is then not corrosion of the anode, but
oxidation of the environment.
Coatings prevent corrosion by
providing a physical barrier which prevents
moisture reaching the surface of the
metal. However, some coatings also
provide cathodic protection. Zinc coating,
also known as galvanising, provides a more
reactive zinc surface which acts as an
anode. Chrome also more readily supplies
positively charged ions and electrons. It
therefore acts as the anode, preventing
corrosion of iron or steel. For this to work,
the coating must be in good electrical
contact with the metal being protected.
Thicker chrome plating is also used to
improve surface hardness, reduce friction,
ease cleaning and provide a decorative
nish.
A diagram
of cathodic
protection
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