MIL-HDBK-1004/10
At the cathode in an active electrochemical cell, chemical
reduction occurs. Chemical reduction is a reaction where an atom or molecule
gains electrons. The chemical shorthand for a typical reduction reaction is:
-
R+ +
-> Ro
EQUATION:
e
(2)
where
R+
=
positive ion in solution
-
e
=
electron
Ro
=
reduced atom
A reduced atom may either be discharged as a gas or may be deposited on the
cathode. The electrolyte in an electrochemical cell serves as a source of
material for the chemical reactions, a medium for the deposition of the
products of the chemical reactions, and a path for the flow of charged ions in
solution. The electron path, usually a metallic connection, is required so
that the electrons produced at the anode can flow from the anode to the sites
at the cathode where they are consumed. The electrochemical cell consists of
an anode where electrons are produced by a chemical reaction, a cathode where
electrons are consumed by a chemical reaction different than the one occurring
at the anode, an electrolyte for the flow of ions, and a metallic path for the
flow of electrons (dc current).
Figure 2 shows an example of a corrosion cell where zinc is
connected to platinum in hydrochloric acid. The zinc corrodes at the anode,
hydrogen gas forms at the cathode, and electric current flows through the
external electron path. This electric current can be made to do useful work.
An ordinary dry cell battery is an electrochemical cell. When in storage, the
electron path is not completed and the electrochemical reaction which produces
the current is only allowed to proceed when the external metallic path is
completed.
The Electrochemical Basis for Cathodic Protection. Cathodic
2.2
protection utilizes a flow of direct current electricity to interfere with the
activity of the electrochemical cell responsible for corrosion. As shown in
Figure 3, corrosion can be prevented by coupling a metal with a more active
metal when both are immersed in an electrolyte and connected with an external
path. In this case the entire surface of the metal being protected becomes a
cathode; thus the term "cathodic protection."
Potentials Required for Cathodic Protection. Every metal immersed
2.2.1
in an electrolyte develops an electrochemical potential due to the free energy
of the atoms in the metal. In order to prevent anodic reactions from
occurring due to electrochemical reactions on that metal, electrons must be
prevented from leaving the metal. Since electrons can only flow from an area
of high (negative) potential to an area with lower (negative) potential,
connection of the metal to be protected to a source of more negative electrons
can effectively prevent the anodic reaction on the metal to be protected and
can thus prevent corrosion. In this case, the flow of electrons is from the
external source to the metal being protected. Conventional current flow is
described by the flow of imaginary positive charges in a direction opposite
the electron flow.
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