CATHODIC PROTECTION CONCEPTS
Corrosion as an Electrochemical Process. Corrosion of metals is a
result of electrochemical reactions. An electrochemical reaction is a
chemical reaction accompanied by a flow of electrical current.
Driving Force. The driving force for the corrosion of metals
through electrochemical reactions is the free energy of the metal atoms in
their metallic form. All chemical systems tend to change so that the free
energy present is at a minimum. This is analogous to the flow of water
downhill to minimize the free energy due to gravity. Most engineering metals
are found in nature in a form with low free energy. These metal ores are
chemical compounds consisting of the metal atoms combined with other atoms
such as oxygen or sulfur. The process of breaking up these ores into their
metallic and non-metallic atoms involves an addition of energy in order to
free the metal atoms from the natural, low energy content chemical compounds.
The corrosion process is driven by the tendency of these metal atoms to revert
to their natural state. If corrosion products are analyzed, their chemical
composition is usually identical to the ore from which the metal was
The Electrochemical Cell. Electrochemical reactions occur through
a combination of chemical reactions and the exchange of electrical charges
(current) between areas where these chemical reactions are occurring. The
entire process is commonly known as an electrochemical cell. This process is
described in the following paragraphs.
Components of the Electrochemical Cell. Every electrochemical cell
consists of an anode, a cathode, an electrolyte and a metallic path for the
flow of electrical current between the anode and cathode. A schematic
electrochemical cell is shown in Figure 1.
Reactions in an Electrochemical Cell. Chemical oxidation occurs at
the anode in an active electrochemical cell. Chemical oxidation is a reaction
where an atom or molecule gives up electrons. The chemical shorthand for a
typical oxidation reaction is:
Mo -> M+ + e
In this reaction the metal atom, which in combination with the other atoms in
a piece of metal has high strength and other metallic properties, is
transformed into a metal ion which usually dissolves. The electron is
available for transfer to another site of lower electrical potential.