CORROSION
A sacrificial anode mounted
on a boat hull preserves the
Chemical protection
Cathodic protection is a way of controlling the electrochemical process that causes corrosion,
so that the part is not damaged. This article explains the electrochemical process, how cathodic
protection prevents corrosion, and the applications and advantages of this approach
TBy Jody Muelaner
he word corrosion can be
used to describe any damage
to metal caused by reaction
with its environment.
Rusting refers specifically to
the corrosion of ferrous metals. Almost
any environment can cause corrosion,
including air, water and soil. Research
carried out in the 1970’s and 1980’s found
that the costs of corrosion makes up
approximately 5% of the gross domestic
product of industrialised nations, but this
could be readily reduced to 3% with better
maintenance.
There are three commonly used
methods of preventing corrosion. The
first is to control the environment to make
propeller
cell, some basic concepts are required.
Ions are electrically charged atoms or
molecules. They are charged because
they have either gained electrons
(negatively charged) or lost electrons
(positively charged). Anions are negatively
charged ions, and cations are positively
charged ions.
A redox reaction, short for reductionoxidation,
is a chemical reaction in which
ions are formed. Oxidation is a redox
reaction in which electrons are lost,
creating cations and an ‘oxidised’ material.
Although this often involves oxygen, it
doesn’t have to. Reduction is a redox
reaction in which electrons are gained,
creating anions and the material is said to
be reduced. Electrolytes are substances
that readily separate into cations and
it less corrosive, for example by keeping
steel parts dry. The second method is
closely related to the first, involving using
coatings such as oil, paint and plating
to create a barrier which prevents the
metal from coming into contact with
the corrosive environment. The third
major method of corrosion protection is
cathodic protection.
THE CHEMISTRY
Corrosion is an electrochemical process,
in which metals are consumed by redox
reactions. The metal being corroded
effectively forms a battery, with an
electrical current flowing as free electrons
through the metal and as ions through an
electrolyte. To understand the process
of corrosion, or of any electrochemical
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