Handbook of Electrical Installation Practice - BeKnowledge

Cathodic Protection 181
Theory of corrosion
Corrosion of a metallic substrate in contact with an electrolyte is fundamentally an
electrochemical process. The corrosion process involves anodic areas (anodes) at
which metal is converted to positive ions and cathodic areas (cathodes) at which an
oxidising species, usually oxygen, is reduced. An electric current flows from the
anode to the cathode through the electrolyte, and the species formed at the anodic
and cathodic areas combine to form the corrosion product. The circuit is completed
by current flow within the metal between the cathodic areas and the anodic areas.
A particular area of metal is determined as anodic or cathodic by the reactions
taking place on it. The following must be present for corrosion to occur: anode,
cathode, electrolyte and return current path.
Electrochemical and galvanic series
When two different metals are in metallic contact and in the same electrolyte, in
general one metal will have a greater tendency to go into solution than the other.
The electrochemical and galvanic series are attempts to make this predictable. The
two series are the potentials adopted by different metals in an electrolyte. The electrochemical
series comprises the potentials adopted by metals when in a solution
of their own ions, thus it does not have much practical application. The galvanic
series comprises the potentials adopted by metals when in seawater or soil, thus corresponding
to a real situation. It is only possible to measure potential differences,
as absolute values of potential cannot be obtained. In order to standardise potential
measurements a particular potential value is taken to be zero. One such standard
is the potential of the standard hydrogen electrode.
When two metals are in metallic contact in the same electrolyte the metal with
the more negative potential in the galvanic series will be made more anodic and
suffer accelerated corrosion; the other metal will become more cathodic and have
reduced corrosion. Tables 8.1 and 8.2 show the electrochemical and galvanic series
of some metals of interest in corrosion control and cathodic protection work.
The two series do not indicate the magnitude of the change in corrosion rate
which will occur when two metals are connected together in the same electrolyte,
but only the direction of any change.
Corrosion cell
There are two basic cell arrangements in which corrosion takes place, known as
galvanic and electrolytic cells.
Galvanic cell
A galvanic cell comprises anode, cathode, electrolyte and return current path where
the corrosion current flows spontaneously. The anode is negative with respect to the
cathode. Galvanic cells exist not only when dissimilar metals are in contact but may
be caused by one or more of the following conditions: dissimilar surface conditions;
differences in heat treatment; differences in local stresses within metal; differences
in concentrations of ionic species; differences in oxygen concentrations; differences